MANAGEMENT
The term "management," broadly speaking, includes not only the
actual skilled attention necessary for the proper operation of the
machines, after the plant is built, but also other duties which must
be performed from its inception to completion, and which may be
classified as
1. Selection;
2. Location;
3. Erection;
4. Testing;
5. Running;
6. Care;
7. Repair.
That is to say, someone must select the machinery, determine where
each machine is to be located, install them, and then attend to the
running of the machines and make any necessary repairs due to the
ordinary mishaps likely to occur in operation.
These various duties are usually entrusted to more than one
individual; thus, the selection and location of the machinery is done
by the designer of the plant, and requires for its proper execution
the services of an electrical engineer, or one possessing more than
simply a practical knowledge of power plants.
The erection of the machines is best accomplished by those making a
specialty of this line of work, who by the nature of the undertaking
acquire proficiency in methods of precision and an appreciation of
the value of accuracy which is so essential in the work of aligning
the machines, and which if poorly done will prove a constant source
of annoyance afterward.
The attention required for the operation of the machines, embracing
the running care and repair, is left to the "man in charge," who
in most cases of small and medium size plants is the chief steam
engineer. He must therefore, not only understand the steam apparatus,
but possess sufficient knowledge of electrical machinery to operate
and maintain it in proper working order.
The present chapter deals chiefly with alternating current machinery,
the management of direct current machines having been fully explained
in Guide No. 3, however, some of the matter here presented is common
to both classes of apparatus.
* * * * *
=Selection.=--In order to intelligently select a machine so that it
will properly harmonize with the conditions under which it is to
operate, there are several things to be considered.
1. Type;
2. Capacity;
3. Efficiency;
4. Construction.
The general type of machine to be used is, of course, dependent on
the system employed, that is, whether it be direct or alternating,
single or polyphase.
Thus, the voltage in most cases is fixed except on transformer
systems where a choice of voltage may be had by selecting a
transformer to suit.
In alternating current constant pressure transmission circuits, an
average voltage of 2,200 volts with step down transformer ratios of
1/10 and 1/20 is in general use, and is recommended.
For long distance, the following average voltages are
recommended 6,000; 11,000; 22,000; 33,000; 44,000; 66,000; 88,000;
and higher, depending on the length of the line and degree of
economy desired.
In alternating circuits the standard frequencies are 25, and 60
cycles. These frequencies are already in extensive use and it is
recommended to adhere to them as closely as possible.
[Illustration: FIG. 2,784.--Diagram of connections for testing to
obtain the saturation curve of an alternator. The saturation curve
shows the relation between the volts generated in the armature and
the amperes of field current (or ampere turns of the field) for a
constant armature current. The armature current may be zero, in which
case the curve is called _no load saturation curve_, or sometimes the
_open circuit characteristic curve_. A saturation curve may be taken
with full load current in the armature; but this is rarely done,
except in alternators of comparatively small output. If a full load
saturation curve be desired, it can be approximately calculated from
the no load saturation curve. The figure shows the connections. If
the voltage generated is greater than the capacity of the voltmeter,
a multiplying coil or a step down pressure transformer may be used,
as shown. A series of observations of the voltage between the
terminals of one of the phases, is made for different values of the
field current. Eight or nine points along the curve are usually
sufficient, the series extending from zero to about fifty per cent.
above normal rated voltage. The points should be taken more closely
together in the vicinity of normal voltage than at other portions of
the curve. Care must be taken that the alternator is run at its rated
speed, and this speed must be kept constant. Deviations from constant
speed may be most easily detected by the use of a tachometer. If the
machine be two phase or three phase, the voltmeter may be connected
to any one phase throughout a complete series of observations. The
voltage of all the phases should be observed for normal full load
excitation by connecting the voltmeter to each phase successively,
keeping the field current constant at normal voltage. This is done in
order to see how closely the voltage of the different phases agree.]
In fixing the capacity of a machine, _careful consideration
should be given to the conditions of operation both_ =present=
_and_ =future= in order that the resultant efficiency may be
maximum.
Most machines show the best efficiency at or near full load.
If the load be always constant, as for instance, a pump forcing
water to a given head, it would be a simple matter to specify the
proper size of machine, but in nearly all cases, and especially
in electrical plants, the load varies widely, not only the daily
and hourly fluctuations, but the varying demands depending on
the season of the year and growth of the plant's business. All
of these conditions tend to complicate the matter, so that
intelligent selection of capacity of a machine requires not only
calculation but mature judgment, which is only obtained by long
experience.
[Illustration: FIG. 2,785.--Saturation curve taken from a 2,000 kw.,
three phase alternator of the revolving field type, having 16 poles,
and generating 2,000 volts, and 576 amperes per phase when run at 300
R.P.M.]
In selecting a machine, or in fact any item connected with the plant
_its construction should be carefully considered_.
Standard construction should be insisted upon so that in the
event of damage a new part can be obtained with the least possible
delay.
The parts of most machines are _interchangeable_, that is to
say, with the refined methods of machinery a duplicate part
(usually carried in stock) may be obtained at once to replace a
defective or broken part, and made with such precision that little
or no fitting will be required.
The importance of standard construction cannot be better illustrated
than in the matter of steam piping, that is, the kind of fittings
selected for a given installation.
With the exception of the exhaust line from engine to condenser,
where other than standard construction may sometimes be used to
reduce the frictional resistance to the steam, the author would
adhere to standard construction except in very exceptional cases.
Those who have had practical experience in pipe fitting will
appreciate the wisdom of this.
For installations in places remote from large supply houses, the more
usual forms of standard fittings should be employed, such as ordinary
T's, 45° and 90° elbows, etc.
In such locations, where designers specify the less usual forms of
standard fittings such as union fittings, offset reducers, etc.,
or special fittings made to sketch, it simply means, in the first
instance that they usually cannot be obtained of the local dealer,
making it necessary to order from some large supply house and
resulting in vexatious delays.
As a rule, those who specify special fittings have found that their
making requires an unreasonable length of time, and the cost to be
several times that of the equivalent in standard fittings.
An examination of a few installations will usually show numerous
special and odd shape fittings, which are entirely unnecessary.
Moreover, a standard design, in general, is better than a special
design, because the former has been tried out, and any imperfection
or weakness remedied, and where thousands of castings of a kind are
turned out, a better article is usually the result as compared with a
special casting.
In the matter of construction, in addition to the items just
mentioned, it should be considered with respect to
1. Quality;
2. Range;
3. Accessibility;
4. Proportion;
5. Lubrication;
6. Adjustment.
It is poor policy, excepting in very rare instances, to buy a "cheap"
article, as, especially in these days of commercial greed, the best
is none too good.
[Illustration: FIGS. 2,786 and 2,787.--Wheel and roller pipe cutters
illustrating =range=. The illustrations show the comparative
movements necessary with the two types of cutter to perform their
function. The wheel cutter requiring only a small arc of movement
will cut a pipe in an inaccessible place as shown, which with a
roller cutter would be impossible. Accordingly, the wheel cutter is
said to have a greater _range_ than the roller cutter.]
Perhaps next in importance to quality, at least in most cases,
is _range_. This may be defined as _scope of operation_,
_effectiveness_, or _adaptability_. The importance of range is
perhaps most pronounced in the selection of tools, especially for
plants remote from repair shops.
For instance, in selecting a pipe cutter, there are two general
classes: wheel cutters, and roller cutters. A wheel cutter has
three wheels and a roller cutter one wheel and two rollers, the
object of the rollers being to keep the wheel perpendicular to
the pipe in starting the cut and to reduce burning. It must be
evident that in operation, a roller cutter requires sufficient
room around the pipe to permit making a complete revolution of
the cutter, whereas, with a wheel cutter, the work may be done
by moving the cutter back and forth through a small arc, as
illustrated in figs. 2,786 and 2,787. Thus a wheel cutter has a
_greater range_ than a roll cutter.
Range relates not only to ability to operate in inaccessible
places but to the various operations that may be performed by one
tool.
PROPERTIES OF STANDARD WROUGHT IRON PIPE
--------------------------+------+-----------------+
| | |
| | |
| | |
| | |
Diameter |Thick-| Circumference. |
| ness.| |
--------+--------+--------+ +--------+--------+
Nominal| Actual | Actual | |External|Internal|
internal|external|internal| | | |
--------+--------+--------+------+--------+--------+
Inches | Inches | Inches |Inches| Inches | Inches |
--------+--------+--------+------+--------+--------+
⅛ | .405 | .27 | .068 | 1.272 | .848 |
¼ | .54 | .364 | .088 | 1.696 | 1.144 |
⅜ | .675 | .494 | .91 | 2.121 | 1.552 |
½ | .84 | .623 | .109 | 2.639 | 1.957 |
¾ | 1.05 | .824 | .113 | 3.299 | 2.589 |
1 | 1.315 | 1.048 | .134 | 4.131 | 3.292 |
1¼ | 1.66 | 1.38 | .14 | 5.215 | 4.335 |
1½ | 1.9 | 1.611 | .145 | 5.969 | 5.061 |
2 | 2.375 | 2.067 | .154 | 7.461 | 6.494 |
2½ | 2.875 | 2.468 | .204 | 9.032 | 7.753 |
3 | 3.5 | 3.067 | .217 | 10.996 | 9.636 |
3½ | 4. | 3.548 | .226 | 12.566 | 11.146 |
4 | 4.5 | 4.026 | .237 | 14.137 | 12.648 |
4½ | 5. | 4.508 | .246 | 15.708 | 14.162 |
5 | 5.563 | 5.045 | .259 | 17.477 | 15.849 |
6 | 6.625 | 6.065 | .28 | 20.813 | 19.054 |
7 | 7.625 | 7.023 | .301 | 23.955 | 22.063 |
8 | 8.625 | 7.982 | .322 | 27.096 | 25.076 |
9 | 9.625 | 8.937 | .344 | 30.238 | 28.076 |
10 | 10.75 | 10.019 | .366 | 33.772 | 31.477 |
11 | 12. | 11.25 | .375 | 37.699 | 35.343 |
12 | 12.75 | 12. | .375 | 40.055 | 37.7 |
--------+--------+--------+------+--------+--------+
------+-------------------------+---------------+--------+-------+------
| | | | |
| | | | |
| | | Length | |
| |Length of pipe |of pipe |Nominal|
Diam.| Transverse areas. | per square |contain-|weight |Number
| | foot of |ing one | per | of
------+--------+--------+-------+-------+-------+ cubic | foot. |thread
Nom. |External|Internal| Metal |Ext'nal|Int'nal| foot. | | per
intern| | | |surface|surface| | | inch
------+--------+--------+-------+-------+-------+--------+-------+ of
Inches|Sq. ins.|Sq. ins.|Sq.ins.| Feet | Feet | Feet |Pounds |screw
------+--------+--------+-------+-------+-------+--------+-------+-----
⅛ | .129 | .0573| .0717| 9.44 |14.15 |2513. | .241 | 27
¼ | .229 | .1041| .1249| 7.075 |10.49 |1383.3 | .42 | 18
⅜ | .358 | .1917| .1663| 5.657 | 7.73 | 751.2 | .559 | 18
½ | .554 | .3048| .2492| 4.547 | 6.13 | 472.4 | .837 | 14
¾ | .866 | .5333| .3327| 3.637 | 4.635 | 270. | 1.115 | 14
1 | 1.358 | .8626| .4954| 2.904 | 3.645 | 166.9 | 1.668 | 11½
1¼ | 2.164 | 1.496 | .668 | 2.301 | 2.768 | 96.25 | 2.244 | 11½
1½ | 2.835 | 2.038 | .797 | 2.01 | 2.371 | 70.66 | 2.678 | 11½
2 | 4.43 | 3.356 | 1.074 | 1.608 | 1.848 | 42.91 | 3.609 | 11½
2½ | 6.492 | 4.784 | 1.708 | 1.328 | 1.547 | 30.1 | 5.739 | 8
3 | 9.621 | 7.388 | 2.243 | 1.091 | 1.245 | 19.5 | 7.536 | 8
3½ | 12.566 | 9.887 | 2.679 | .955 | 1.077 | 14.57 | 9.001 | 8
4 | 15.904 | 12.73 | 3.174 | .849 | .949 | 11.31 |10.665 | 8
4½ | 19.635 | 15.961 | 3.674 | .764 | .848 | 9.02 |12.34 | 8
5 | 24.306 | 19.99 | 4.316 | .687 | .757 | 7.2 |14.502 | 8
6 | 34.472 | 28.888 | 5.584 | .577 | .63 | 4.98 |18.762 | 8
7 | 45.664 | 38.738 | 6.926 | .501 | .544 | 3.72 |23.271 | 8
8 | 58.426 | 50.04 | 8.386 | .443 | .478 | 2.88 |28.177 | 8
9 | 72.76 | 62.73 |10.03 | .397 | .427 | 2.29 |33.701 | 8
10 | 90.763 | 78.839 |11.924 | .355 | .382 | 1.82 |40.065 | 8
11 |113.098 | 99.402 |13.696 | .318 | .339 | 1.450|45.95 | 8
12 |127.677 |113.098 |14.579 | .299 | .319 | 1.27 |48.985 | 8
------+--------+--------+-------+-------+-------+--------+-------+----
Open construction should be employed, wherever possible, so that
all parts of a machine that require attention, or that may become
deranged in operation, may be accessible for adjustment or repair.
The design should be such that there is ample strength, and the
bearings for moving parts should be of liberal proportions to
avoid heating with minimum attention.
A comparison of the proportions used by different manufacturers
for a machine of given size might profitably be made before a
selection is made.
The matter of lubrication is important.
Fast running machines, such as generators and motors, should be
provided with ring oilers and oil reservoirs of ample capacity, as
shown in figs. 2,788 to 2,794.
[Illustration: FIG. 2,788.--Sectional view showing a ring oiler or
self oiling bearing. As shown the pedestal or bearing standard is
cored out to form a reservoir for the oil. The rings are in rolling
contact with the shaft, and dip at their lower part into the oil.
In operation, oil is brought up by the rings which revolve because
of the frictional contacts with the shaft. The oil is in this way
brought up to the top of the bearing and distributed along the shaft
gradually descending by gravity to the reservoir, being thus used
over and over. A drain cock, is provided in the base so that the oil
may be periodically removed from the reservoir and strained to remove
the accumulation of foreign matter. This should be frequently done to
minimize the wear of the bearing.]
All bearings subject to appreciable wear should be made adjustable so
that lost motion may be taken up from time to time and thus keep the
vibration and noise of operation within proper limits.
=Selection of Generators.=--This is governed by the class of work
to be done and by certain local conditions which are liable to vary
considerably for different stations.
These variable factors determine whether the generators must be of
the direct or alternating current type, whether they must be wound
to develop a high or a low voltage, and whether their outputs in
amperes must be large or small. Sufficient information has already
been given to cover these various cases; there are, however, certain
general rules that may advantageously be observed in the selection of
generators designed to fill any of the aforementioned conditions, and
it is well to possess certain facts regarding their construction.
[Illustration: FIGS. 2,789 to 2,794.--Self oiling self aligning
bearing open. Views showing oil grooves, rings, bolts etc.]
=Ques. Name an important point to be considered in selecting a
generator.=
Ans. Its efficiency.
=Ques. What are the important points with respect to efficiency?=
Ans. A generator possessing a high efficiency at the average load is
more desirable than a generator showing a high efficiency at full
load.
=Ques. Why?=
Ans. The reason is that in station practice the full load limit is
seldom reached, the usual load carried by a generator ordinarily
lying between the one-half and three-quarter load points.
=Ques. How do the efficiencies of large and small generators compare?=
Ans. There is little difference.
[Illustration: FIG. 2,795.--Rotor of Westinghouse type T turbine
dynamo set. The dynamo is of the commutating pole type either shunt
or compound wound. The turbine is of the single wheel impulse type.
The wheel is mounted directly on the end of the shaft as shown. Steam
is used two or more times on the wheel to secure efficiency. A fly
ball governor is provided with weights hung on hardened steel knife
edges. In case of over speeding, an automatic safety stop throttle
valve is tapped shutting off the steam supply. This type of turbine
dynamo set is especially applicable for exciter service in modern,
superheated steam generating stations where the steam pressure
exceeds 125 pounds. Westinghouse Type T turbines operate directly
(that is, without a reducing valve) on pressures up to 200 pounds per
square inch with steam superheated to 150 degrees Fahrenheit.]
=Ques. How are the sizes and number of generator determined?=
Ans. The sizes and number of generator to be installed should be
such as to permit the engines operating them being worked at nearly
full load, because the efficiencies of the latter machines decrease
rapidly when carrying less than this amount.
=Ques. What is understood by regulation?=
Ans. The accuracy and reliability with which the pressure or current
developed in a machine may be controlled.
It is generally possible if purchasing of a reputable concern,
to obtain access to record sheets on which may be found results
of tests conducted on the generator in question, and as these are
really the only means of ascertaining the values of efficiency and
regulation, the purchaser has a right to inspect them. If, for
some reason or other, he has not been afforded this privilege, he
should order the machine installed in the station on approval, and
test its efficiency and regulation before making the purchase.
[Illustration: FIG. 2,796.--Cross section of electrical station
showing small traveling crane.]
=Installation.=--The installation of machines and apparatus in an
electrical station is a task which increases in difficulty with
the size of the plant. When the parts are small and comparatively
light they may readily be placed in position, either by hand, by
erecting temporary supports which may be moved from place to place
as desired, or by rolling the parts along on the floor upon pieces
of iron pipe. If, however, the parts be large and heavy, a traveling
crane such as shown in fig. 2,797, becomes necessary.
=Ques. What precaution should be taken in moving the parts of
machines?=
Ans. Care should be taken not to injure the bearings and shafts, the
joints in magnetic circuits such as those between frame and pole
pieces, and the windings on the field and armature.
[Illustration: FIG. 2,797.--Cross section of electrical station
showing a traveling crane for the installation or removal of large
and heavy machine parts. A traveling crane consists of an iron beam
which, being supplied with wheels at the ends, can be made to move
either mechanically or electrically upon a track running the entire
length of the station. This track is not supported by the walls
of the building, but rests upon beams specially provided for the
purpose. In addition to the horizontal motion thus obtained, another
horizontal motion at right angles to the former is afforded by means
of the carriage which, being also mounted on wheels, runs upon a
track on the top of the beam. Electrical power is generally used to
move the carriage and also the revolving drums contained thereon,
the latter of which give a vertical motion to the main hoist or the
auxiliary hoist, these hoists being used respectively for raising
or lowering heavy or light loads. In the larger sizes of electric
traveling crane, a cage is attached to the beam for the operator,
who, by means of three controllers mounted in the cage, can move a
load on either the main or auxiliary hoist in any direction.]
The insulations of the windings are perhaps the most vital parts
of a generator, and the most readily injured. The prick of a pin
or tack, a bruise, or a bending of the wires by resting their
weight upon them or by their coming in contact with some hard
substance, will often render a field coil or an armature useless.
Owing to its costly construction, it is advisable when
transporting armatures by means of cranes to use a wooden
spreader, as shown in fig. 2,798 to prevent the supporting rope
bruising the winding.
[Illustration: FIG. 2,798.--View of armature in transit showing use
of a wooden spreader as a protection. If a chain be used in place of
the rope, a padding of cloth should be placed around the armature
shaft and special care taken that the chain does not scratch the
commutator.]
=Ques. If an armature cannot be placed at once in its final position
what should be done?=
Ans. It may be laid temporarily upon the floor, if a sheet of
cardboard or cloth be placed underneath the armature as a protection
for the windings; in case the armature is not to be used for some
time, it is better practice to place it in a horizontal position on
two wooden supports near the shaft ends.
=Ques. What kind of base should be used with a belt driven generator
or motor?=
Ans. The base should be provided with V ways and adjusting screws
for moving the machine horizontally to take up slack in the belt, as
shown in fig. 2,799.
Owing to the normal tension on the belt, there is a moment
exerted equal in amount to the distance from the center of gravity
of the machine to the center of the belt, multiplied by the
effective pull on the belt. This force tends to turn the machine
about its center of gravity. By placing the screws as shown, any
turning moment, as just mentioned, is prevented.
[Illustration: FIG. 2,799.--Plan of belt drive machine showing V ways
and adjusting screws for moving the machine forward from the engine
or counter shaft to take up slack in the belt.]
=Ques. How should a machine be assembled?=
Ans. The assembling should progress by the aid of a blue print, or by
the information obtained from a photograph of the complete machine
as it appears when ready for service. Each part should be perfectly
clean when placed in position, especially those parts between which
there is friction when the machine is in operation, or across which
pass lines of magnetic force; in both cases the surfaces in contact
must be true and slightly oiled before placing in position.
Contact surfaces forming part of electrical circuits must also
be clean and tightly screwed together. An important point to bear
in mind when assembling a machine is, to so place the parts that
it will not be necessary to remove any one of them in order to get
some other part in its proper position. By remembering this simple
rule much time will be saved, and in the majority of instances the
parts will finally be better fitted together than if the task has
to be repeated a number of times.
When there are two or more parts of the machine similarly
shaped, it is often difficult to properly locate them, but in such
cases notice should be taken of the factory marks usually stamped
upon such pieces and their proper places determined from the
instructions sent with the machine.
[Illustration: FIGS. 2,800 to 2,802.--Starrett's improved speed
indicator. In construction, the working parts are enclosed like a
watch. The graduations show every revolution, and with two rows of
figures read both right and left as the shaft may run. While looking
at the watch, each hundred revolutions may be counted by allowing
the oval headed pin on the revolving disc to pass under the thumb as
the instrument is pressed to its work. A late improvement in this
indicator consists in the rotating disc, which, being carried by
friction may be moved to the starting point where the raised knobs
coincide. When the spindle is placed in connection with the revolving
shaft, pressing the raised knob with the thumb will prevent the disc
rotating, while the hand of the watch gets to the right position to
take the time. By releasing the pressure the disc is liberated for
counting the revolutions of the shaft when every 100 may be noted by
feeling the knob pass under the thumb lightly pressed against it,
thus relieving the eye, which has only to look on the watch to note
the time.]
=Ques. What should be noted with respect to speed of generator?=
Ans. Each generator is designed to be run at a certain speed in
order to develop the voltage at which the machine is rated. The
speed, in revolutions per minute, the pressure in volts, and the
capacity or output in watts (volts × amperes) or in kilowatts
(thousands of watts) are generally stamped on a nameplate screwed to
the machine.
This requirement frequently requires calculations to be made by
the erectors to determine the proper size pulleys to employ to
obtain the desired speed.
[Illustration: FIG. 2,803.--Home made belt clamp. It is made with
four pieces of oak of ample size to firmly grip the belt ends where
the bolts are tightened. The figure shows the clamp complete and
in position on the belt and clearly illustrates the details of
construction. In making the long bolts the thread should be cut about
three-quarter length of bolt and deep enough so that the nuts will
easily screw on.]
=Example.=--What diameter of engine pulley is required to run
a dynamo at a speed of 1,450 revolutions per minute the dynamo
pulley being 10 inches in diameter and the speed of engine, 275
revolutions per minute?
The diameter of pulley required on engine is 10 × (1,450 ÷ 275) = 53
inches, nearly.
=Rule.=--To find the diameter of the driving pulley, _multiply
the speed of the driven pulley by its diameter, divide the product
by the speed of the driver and the answer will be the size of the
driver required_.
_Example._--If the speed of an engine be 325 revolutions per
minute, diameter of engine pulley 42 inches, and the speed of
the dynamo 1,400 revolutions per minute, how large a pulley is
required on dynamo?
The size of the dynamo pulley is
42 × (325 ÷ 1,400) = 9¾ inches.
=Rule.=--To find the size of dynamo pulley, _multiply the speed
of engine by the diameter of engine wheel and divide the product
by the speed of the dynamo_.
[Illustration: FIGS. 2,804 and 2,805.--A good method of lacing a
belt. The view at the left shows outer side of belt, and at the
right, inner or pulley side.]
_Example._--If a steam engine, running 300 revolutions per
minute, have a belt wheel 48 inches in diameter, and be belted
to a dynamo having a pulley 12 inches in diameter, how many
revolutions per minute will the dynamo make?
The speed of dynamo will be 300 × (48 ÷ 12) = 1,200 rev. per min.
=Rule.=--When the speed of the driving pulley and its diameter
are known, and the diameter of the driven pulley is known, the
speed of the driven pulley is found by _multiplying the speed of
the driver by its diameter in inches and dividing the product by
the diameter of the driven pulley_.
=Example.=--What will be the required speed of an engine
having a belt wheel 46 inches in diameter to run a dynamo 1,500
revolutions per minute, the dynamo pulley being 11 inches in
diameter?
The speed of the engine is 1,500 × (11 ÷ 46) = 359 rev. per min.
nearly.
[Illustration: FIG. 2,806.--Wiring diagram and directions for
operating Holzer-Cabot single phase self-starting motor. =Location:=
The motor should be placed in as clear and dry a location as
possible, away from acid or other fumes which would attack the
metal parts or insulation, and should be located where it is easily
accessible for cleaning and oiling. =Erection:= The motor should be
set so that the shaft is level and parallel with the shaft it is
to drive so that the belt will run in the middle of the pulleys.
Do not use a belt which is too heavy or too tight for the work it
has to do, as it will materially reduce the output of the motor.
The belt should be from one-half to one inch narrower than the
pulley. =Rotation:= In order to reverse the direction of rotation,
interchange leads A and B. =Suspended Motors:= Motors with ring oil
bearings may be used on the wall or ceiling by taking off end caps
and revolving 90 or 180 degrees until the oil wells come directly
below the bearings. =Starting:= Motors are provided with link across
two terminals on the upper right hand bracket at the front of the
motor and with this connection should start considerable overloads.
If the starting current be too great with this connection, it may be
reduced by removing the link. =Temperatures:= At full load the motor
will feel hot to the hand, but this is far below the danger point. If
too hot for touch, measure temperature with a thermometer by placing
bulb against field winding for 10 minutes, covering thermometer
with cloth or waste. The temperature should not exceed 75 degrees
Fahr. above the surrounding air. =Oiling:= Fill the oil wells to the
overflow before starting and keep them full. See that the oil rings
turn freely with shaft. =Care:= The motor must be kept clean. Smooth
collector rings with sandpaper and see that the brushes make good
contact. When brushes become worn they may be reversed. When fitting
new brushes or changing them always sandpaper them down until they
make good contact with the collector rings, by passing a strip of
sandpaper beneath the brush.]
=Rule.=--To find the speed of engine when diameter of both
pulleys, and speed of dynamo are given, _multiply the dynamo speed
by the diameter of its pulley and divide by the diameter of engine
pulley_.
=Ques. How are the diameters and speeds of gear wheels figured?=
Ans. The same as belted wheels, using either the pitch circle
diameters or number of teeth in each gear wheel.
[Illustration: FIGS. 2,807 to 2,809.--Wiring diagrams and directions
for operating Holzer-Cabot slow speed alternating current motors.
=Erecting:= In installing the motor, be sure the transformer and
wiring to the motor are large enough to permit the proper voltage
at the terminals. If too small, the voltage will drop and reduce
the capacity of the motor. =Oiling:= Maintain oil in wells to the
overflow. =Starting: Single phase= motors are started by first
throwing the starting switch down into the starting position, and
when the motor is up to speed, throwing it up into the running
position. _Do not hold the switch in starting position over 10
seconds._ Starter for single phase motors above ½ H.P. are arranged
with an adjusting link at the bottom of the panel. The link is shown
in the position of least starting torque and current. Connect from
W to 2 or W to 3 for starting heavier loads. _Two or three phase_
motors are started simply by closing the switch. These motors start
full load without starters. The motor should start promptly on
closing the switch. It should be started the first time without
being coupled to the line shaft. If the motor start free, but will
not start loaded, it shows either that the load upon the motor is
too great, the line voltage too low, or the frequency too high. The
voltage and frequency with the motor running should be within 5% of
the name plate rating and the voltage with 10 to 15% while starting.
If the motor do not start free, either it is getting no current or
something is wrong with the motor. In either case an electrician
should be consulted. =Solution:= To reverse the direction of rotation
interchange the leads marked "XX" in the diagrams. =Temperature:= At
full load the motor should not heat over 75 degrees Fahr. above the
temperature of the surrounding air; if run in a small enclosed space
with no ventilation, the temperature will be somewhat higher.]
=Ques. What should be noted with respect to generator pulleys?=
Ans. A pulley of certain size is usually supplied with each generator
by its manufacturer, and it is not generally advisable to depart much
from the dimensions of this pulley. Accordingly, the solution of the
pulley problem usually consists in finding the necessary diameter of
the driving pulley relative to that of the pulley on the generator in
order to furnish the required speed.
=Ques. What is the chief objection to belt drive?=
Ans. The large amount of floor space required.
[Illustration: FIG. 2,810.--Tandem drive for economizing floor space
with belt transmission. Belts of different lengths are used, as
shown, each of which passes over the driving wheel _d_ of the engine,
and then over the pulley wheel of one of the generators. In such
an arrangement the belts would be run lengthwise through the room
in which the machines are placed, and it is obvious that since the
width of the room would be governed by the width of the machines thus
installed, this method is a very efficient one for accomplishing the
end in view.]
=Ques. How may the amount of space that would ordinarily be required
for belt drive, be reduced?=
Ans. By driving machines in tandem as in fig. 2,810, or by the double
pulley drive as in fig. 2,811.
=Ques. What is the objection to the tandem method?=
Ans. The most economical distance between centers cannot be employed
for all machines.
=Ques. What is the objectionable tendency in resorting to floor
economy methods with belt transmission?=
Ans. The tendency to place the machines too closely together. This is
poor economy as it makes the cleaning of the machines a difficult and
dangerous task; it is therefore advisable to allow sufficient room
for this purpose regardless of the method of belting employed.
[Illustration: FIG. 2,811.--Double pulley drive for economizing floor
space with belt transmission. Where a center crank engine is used
both pulleys may be employed by belting a machine to each as shown.
Although considerable floor space would be saved by the use of this
scheme if the generators thus belted were placed at M and G yet still
more floor space would be saved by having them occupy the positions
indicated at M and S.]
=Ques. What is the approved location for an alternator exciter?=
Ans. To economize floor space the exciter may be placed between the
alternator and engine at S in fig. 2,811.
=Belts.=--In the selection of a belt, the quality of the leather
should be first under consideration. The leather must be firm, yet
pliable, free from wrinkles on the grain or hair side, and of an even
thickness throughout.
[Illustration: FIG. 2,812.--Separately excited belt driven alternator
showing approved location of exciter. In an electrical station where
alternating current is generated, the alternators for producing
the current generally require separate excitation for their field
windings; that is, it is usually necessary to install in conjunction
with an alternator a small dynamo for supplying current to the
alternator field. The exciter is a comparatively small machine; in
fact, it requires only about 1 per cent. of the capacity of the
alternator which it excites, and so being small is often belted to
an auxiliary pulley mounted on the alternator shaft. Considerable
floor space would be occupied by an installation of this nature if
the exciter be placed at M, and belted to the alternator as indicated
by the dotted lines. By locating the exciter at S, between the
alternator and the engine, much floor space will be saved and the
general appearance of the installation improved.]
If the belt be well selected and properly handled, it should do
service for twenty years, and even then if the worn part be cut off,
the remaining portion may be remade and used again as a narrower and
shorter belt.
Besides leather belts, there are those made of rubber which
withstand moisture much better than leather belts, and which also
possess an excellent grip on the pulley; they are, however, more
costly and much less durable under normal conditions.
In addition to leather and rubber belts, there are belts
composed of cotton, of a combination of cotton and leather, and
of rope. The leather belt, however, is the standard and is to be
recommended.
Equally important with the quality of a belt is its size in order to
transmit the necessary power.
The average strain under which leather will break has been found
by many experiments to be 3,200 pounds per square inch of cross
section. A good quality of leather will sustain a somewhat greater
strain. In use on the pulleys, belts should not be subjected to a
greater strain than one eleventh their tensile strength, or about
290 pounds to the square inch or cross section. This will be about
55 pounds average strain for every inch in width of single belt
three-sixteenths inch thick. The strain allowed for all widths
of belting--single, light double, and heavy double--is in direct
proportion to the thickness of the belt.
=Ques. How much horse power will a belt transmit?=
Ans. The capacity of a belt depends on, its width, speed, and
thickness. _A single belt one inch wide and travelling 1,000 feet per
minute will transmit one horse power; a double belt under the same
conditions, will transmit two horse power._
[Illustration: FIG. 2,813.--One horse power transmitted by belt to
illustrate the rule given above. A pulley is driven by a belt by
means of the friction between the surfaces in contact. Let T be the
tension on the driving side of the belt, and T', the tension on the
loose side; then the driving force = T-T'. In the figure T is taken
at 34 lbs. and T' at 1 lb.; hence driving force = 34-1 = 33 lbs.
Since the belt is travelling at a velocity of 1,000 feet per minute
the power transmitted = 33 lbs. × 1,000 ft. = 33,000 ft. lbs. per
minute = 1 horse power.]
This corresponds to a working pull of 33 and 66 lbs. per inch of
width respectively.
=Example.=--What width double belt will be required to transmit
50 horse power travelling at a speed of 3,000 feet per minute?
The horse power transmitted by each inch width of double belt
travelling at the stated speed is
3,000
1 × ----- × 2 = 6,
1,000
hence the width of belt required to transmit 50 horse power is
50 ÷ 6 = 8.33, say 8 inches.
=Ques. At what velocity should a belt be run?=
Ans. At from 3,000 to 5,000 feet per minute.
=Ques. How may the greatest amount of power transmitting capacity be
obtained from belts?=
Ans. By covering the pulleys with leather.
=Ques. How should belts be run?=
Ans. With the tight side underneath as in fig. 2,814.
[Illustration: FIGS. 2,814 and 2,815.--Right and wrong way to run a
belt. The tight side should be underneath so as to increase the arc
of contact and consequently the adhesion, that is to say, a _better
grip_, is in this way obtained.]
=Ques. What is a good indication of the capacity of a belt in
operation?=
Ans. Its appearance after a few days' run.
If the side of the belt coming in contact with the pulley assume
a mottled appearance, it is an indication that the capacity of
the belt is considerably in excess of the power which it is
transmitting, inasmuch as the spotted portions of the belt do not
touch the pulley; and in consequence of this there is liable to be
more or less slipping.
Small quantities of a mixture of tallow and fish oil which
have previously been melted together in the proportion of two of
the former to one of the latter, will, if applied to the belt
at frequent intervals, do much toward softening it, and thus
by permitting its entire surface to come in contact with the
pulley, prevent any tendency toward slipping. The best results
are obtained when the smooth side of the belt is used next to the
pulley, since tests conducted in the past prove that more power is
thus transmitted, and that the belt lasts longer when used in this
way.
[Illustration: FIG. 2,816.--The Hill friction clutch pulley for power
control. The clutch mechanism will start a load equivalent to the
double belt capacity of the pulley to which the clutch is attached.]
=Ques. What is the comparison between the so called endless belts and
laced belts?=
Ans. With an endless belt there is no uneven or noisy action as with
laced belts, when the laced joint passes over the pulleys, and the
former is free from the liability of breakage at the joint.
=Ques. How should a belt be placed on the pulleys?=
Ans. The belt should first be placed on the pulley at rest, and then
run on the other pulley while the latter is in motion.
The best results are obtained, and the strain on the belt is
less, when the speed at which the moving pulley revolves is
comparatively low. With heavy belts, particular care should be
taken to prevent any portion of the clothing being caught either
by the moving belt or pulleys, as many serious accidents have
resulted in the past from carelessness in regard to this important
detail. The person handling the belt should, therefore, be sure of
a firm footing, and when it is impossible to secure this, it is
advisable to stop the engine and fit the belt around the engine
pulley as well as possible by the aid of a rope looped around the
belt.
[Illustration: FIG. 2,817--Sectional view of Hill clutch mechanism.
In every case the mechanism hub A, and in a clutch coupling the
ring W, is permanently and rigidly secured to the shaft and need
not be disturbed when removing the wearing parts. When erected, the
adjustment should be verified, and always with the clutch and ring
engaged and at rest. If the jaws do not press equally on the ring, or
if the pressure required on the cone be abnormal, loosen the upper
adjusting nuts T´ on eye bolts and set up the lower adjusting nuts
T´´ until each set of jaws is under the same pressure. Should the
clutch then slip when started it is evident that the jaw pressure is
insufficient and a further adjustment will be necessary. All clutches
are equipped throughout with split lock washers. Vibration or shock
will not loosen the nuts if properly set up. The jaws can be removed
parallel to the shaft as follows: Remove the gibs V, and withdraw the
jaw pins P, then pull out the levers D. Do not disturb the eye bolt
nuts T´ and T´´. The outside jaws B can now be taken out. Remove the
bolt nuts I allowing the fulcrum plates R to be taken off. On the
separable hub pattern the clamping bolts must be taken out before
fulcrum plate is removed. The inside jaws C may now be withdrawn.
Always set the clutch operating lever in the position as shown in
fig. 2,816 to avoid interference with mechanism parts. Oil the moving
parts of the clutch. Keep it clean. Examine at regular intervals.]
=Ques. Under what conditions does a belt drive give the best results?=
Ans. When the two pulleys are at the same level.
If the belt must occupy an inclined position it should not form
a greater angle than 45 degrees with the horizontal.
=Ques. What is a characteristic feature in the operation of belts,
and why?=
Ans. Belts in motion will always run to the highest side of a
pulley; this is due partially to the greater speed in feet per minute
developed at that point owing to the greater circumference of the
pulley, and also to the effects of centrifugal force.
If, therefore, the highest sides of both pulleys be in line with
each other, and the shafts of the respective pulleys be parallel
to each other, there will be no tendency for the belt to leave
the pulleys when once in its proper position. In order that these
conditions be maintained, the belt should be no more than tight
enough to prevent slipping, and the distance between the centers
of the pulleys should be approximately 3.5 times the diameter of
the larger one.
[Illustration: FIG. 2,818.--Hill clutch mechanism Smith type. The
friction surfaces are wood to iron, the wood shoes being made from
maple. All parts of the toggle gear are of steel and forgings with
the exception of the connection lever which is of cast iron.]
=Ques. What minor appurtenances should be provided in a station?=
Ans. Apparatus should be installed as a prevention against accidents,
such as fire, and protection of attendants from danger.
In every electrical station there should be a pump, pipes
and hose; the pump may be either directly connected to a small
electric motor or belted to a countershaft, while the pipes
and hose should be so placed that no water can accidentally
reach the generators and electrical circuits. A number of fire
bucket filled with water should be placed on brackets around the
station, and with these there should be an equal number of bucket
containing dry sand, the water being used for extinguishing fire
occurring at a distance from the machines and conductors, and the
sand for extinguishing fire in current carrying circuits where
water would cause more harm than benefit. To prevent the sand
being blown about the station, each sand bucket, when not in use,
should be provided with a cover.
Neat cans and boxes should be mounted in convenient places for
greasy rags, waste, nuts, screws, etc., which are used continually
and which therefore cannot be kept in the storeroom.
While it is important to guard against fire in the station,
it is equally necessary to provide for personal safety. All
passages and dark pits should therefore be thoroughly lighted
both day and night, and obstacles of any nature that are not
absolutely necessary in the operation of the station, should be
removed. Moving belts, and especially those passing through the
floor, should be enclosed in iron railings. If high voltages be
generated, it is well to place a railing about the switchboard to
prevent accidental contact with current carrying circuits, and in
such cases it is also advisable to construct an insulated platform
on the floor in front of the switchboard.
[Illustration: FIG. 2,819.--Method of joining adjacent switchboard
panels.]
=Switchboards.=--The plan of switchboard wiring for alternating
current work depends upon the system in use and this latter may be
either of the single phase, two phase, three phase, or monocyclic
types. The general principles in all these cases, however, are
practically identical.
Fig. 2,820 shows the switchboard wiring for a single phase
alternator. As an aid in reading the diagram, the conductors
carrying alternating current are represented by solid lines, and
those carrying direct current, by dotted lines.
[Illustration: FIG. 2,820.--Switchboard wiring for a single phase
separately excited alternator. The direct current circuits are
represented by dotted lines, and the alternating current circuit, by
solid lines.]
The exciter shown at the right is a shunt wound machine. By
means of the exciter rheostat, the voltage for exciting the field
winding of the alternator is varied; this, in turn, varies the
voltage developed in the alternator since the main leads of the
exciter are connected through a double pole switch G to the field
winding of the alternator.
[Illustration: FIGS. 2,821 to 2,825.--General Electric diagrams of
connections. A, ammeter; C.B, circuit breaker; C.P, candle power;
C.T, current transformer; D.R, discharge resistance; F, fuse; F.S,
field switch; L, lamp; O.C, overload coil; P.P, pressure plug;
P.R, pressure receptacle; R.C, reactance; rheo, rheostat; R.P,
synchronizing plug, running; R.S, resistance; S, switch; S.I,
synchronism indicator; S.P, synchronizing plug, starting; S.R,
synchronizing receptacle; V, voltmeter.]
A rheostat is also introduced in the alternator field winding
circuit to adjust the alternator pressure. It may seem unnecessary
to employ a rheostat in each of two separate field circuits to
regulate the voltage of the alternator, but these rheostats are
not both used to produce the same result. When a considerable
variation of pressure is required, the exciter rheostat is
manipulated, whereas for a fine adjustment of voltage the
alternator rheostat is preferably employed.
Sometimes a direct current ammeter is introduced in the
alternator's field circuit to aid in the adjustment.
The main circuit of alternator after being protected on both
sides by fuses, runs to the double pole switch K. These fuses
serve as a protection to the alternator in case of a short circuit
at the main switch. It will be noticed the fuses are of the single
pole type and are mounted a considerable distance apart; this is
to prevent any liability of a short circuit between them in case
of action. Enclosed fuses are now used entirely for such work,
since in these there is no danger of heated metal being thrown
about and causing damage when the fuse wire is melted. Enclosed
fuses are also more readily and quickly replaced than open fuses,
the containing tube of each being easy to adjust in circuit, and
when the fuse wire within is once melted the tube is discarded for
a new one.
The main circuit after passing through the main switch is
further protected on both sides by circuit breakers. Leaving these
protective devices, the left hand side of the circuit includes
the alternating current ammeter, and then connects with one of
the bus bars. The right hand side of the circuit runs from the
circuit breaker to the other bus bar. As many feeder circuits
may be connected to the bus bars and supplied with current by
the alternator as the capacity of this machine will permit. If,
however, there be more than one feeder circuit, each must be wired
through a double pole switch.
In alternating current work the pressures dealt with are much
greater than those in direct current installations, so that
proportionate care must be taken in the wiring to remove all
possibility of grounds.
To locate such troubles, however, should they occur, a ground
detector is provided. For this class of work the ground detector
must be an instrument especially designed for high pressure
circuits. Two of its terminals should be connected to the line
wires and the third, to ground; in case of a leak on the line, a
current will then flow through the detector and by the position of
the pointer the location and seriousness of the leak may be judged.
A step down transformer is also rendered necessary for the
voltmeter and the pilot lamps, owing to the high voltage in use.
The primary winding of the transformer is connected across the
main circuit of the alternator. This connection should never be
made so that it will be cut out of circuit when the main switch is
open, for it is always advisable to consult the voltmeter before
throwing on the load by closing this switch.
[Illustration: FIGS. 2,826 to 2,829.--General Electric diagrams of
connections. A, ammeter; C.B, circuit breaker; C.P, candle power;
C.T, current transformer; D.R, discharge resistance; F, fuse; F.S,
field switch; L, lamp; O.C, overload coil; P.P, pressure plug;
P.R, pressure receptacle; R.C, reactance; rheo, rheostat; R.P,
synchronizing plug, running; R.S, resistance; S, switch; S.I,
synchronous indicator: S.P, synchronizing plug, starting; S.R,
synchronizing receptacle: V, voltmeter.]
=Ques. How does the switchboard wiring for a two phase system differ
from the single phase arrangement shown in fig. 2,820?=
Ans. It is practically the same, except for the introduction of an
extra ammeter and a compensator in each of the outside wires, and in
the use of a four pole switch in place of the two pole main switch.
The ammeters, of course, are for measuring the alternating
currents in each of the two phases or legs of the system, and the
compensators are two transformers with their primary coils in
series with the outside wires and their secondary coils in series
with each other across the outside wires. The transformers thus
connected are known as compensators or pressure regulators, and
as such compensate for the drop in pressure on either side of the
system.
=Ques. How is the four pole main switch wired?=
Ans. Its two central terminals which connect directly with the line
wires, are joined together by a conductor, and from this point one
wire is led off. This wire, together with the two outside wires, form
the feeders of the system.
=Ques. How many voltmeters are required for the two phase system?=
Ans. One voltmeter is sufficient on the board if a proper switching
device be employed to shift its connections across either of the two
circuits; otherwise, two voltmeters will be necessary, one bridged
across each of these respective circuits.
The same reasoning holds true in regard to ground detectors,
so that one or two of these will be required, depending upon the
aforementioned conditions.
=Ques. What are the essential points of difference between the single
phase switchboard wiring as shown in fig. 2,820, and that required
for a three wire three phase system?=
Ans. The three phase system requires the use of a three pole switch
in place of the two pole switch; the insertion of an ammeter, a
circuit breaker, and a compensator in each of the three wires of the
system; the presence of two ground detectors instead of one, and the
addition of a voltmeter switch if but one voltmeter be provided, or
else the installation of two voltmeters, connected the one between
the middle wire and outer right hand wire, and the other between the
middle wire and outer left hand wire.
[Illustration: FIG. 2,830.--Diagram of switchboard connections for
General Electric automatic voltage regulator with two exciters and
two alternators.]
=Ques. Mention a few points relating to lightning arresters.=
Ans. In most cases where direct current is used they are mounted
on the walls of the station near the place at which the line wires
enter. If they be mounted outside the station at this point, special
precautions should be taken to keep them free from moisture by
enclosing them in iron cases, but no matter where they are located it
is necessary that they be dry in order to work properly.
[Illustration: FIGS. 2,831 and 2,832.--Garton-Daniels alternating
current lightning arrester; diagram showing connections. A lightning
discharge takes the path indicated by the dotted line, across the
upper air gap A, through resistance rod B, C, D, across copper strip
R on the base, thence flowing to ground through the movable plunger
M, lower on gap N, and ground binding post L. The discharge path is
practically straight, contains an air gap, distance of but 3/32 inch,
a series resistance averaging but 225 ohms. The lightning discharge
does not flow through the flexible lead connecting band D on the
lower end of the resistance rod with the top of the movable plunger.
These two points are electrically connected by the heavy copper
strip R, and lightning discharges generally, if not always, take the
path across this copper strip in preference to flowing through the
inductance of the one turn of flexible cable. When a discharge occurs
from line to ground through any lightning arrester, the air gaps arc
over, and so there is offered a path from line to ground for the line
current. This flow of line current following the lightning discharge
to ground may vary anywhere from a small capacity current where
the arrester is installed on an ungrounded circuit, a moderately
heavy flow on a partially grounded circuit, to a very heavy flow on
a grounded circuit--either a circuit operated as a dead grounded
circuit, or a circuit which has become accidentally grounded during
a storm. The path taken by this flow of line current from line to
ground may be traced by following the path shown by the dashed line.
It, as seen, crosses upper air gap A, flows through section B of the
resistance rod to band C. Leaving band C it flows through the magnet
winding H, thence to band D on the resistance rod, through flexible
lead to upper end of movable plunger, through movable plunger, across
lower air gap N, to ground binding post L, thence to ground. The
function of the short length of resistance rod CD is as follows: It
has an ohmic resistance of about 30 ohms but is _non-inductive_.
Magnet winding H, connected to bands C and D on the ends of this
short length of rod has an ohmic resistance of 3 ohms, but is _highly
inductive_. Lightning discharges being of _high frequency_ take the
higher resistance _but non-inductive_ path CD in their passage from
line to ground. The flow of normal current from line to ground being
of a very low frequency, 25 or 60 cycles in ordinary alternating
current circuits, zero in direct current circuits--takes the _low
resistance_ path through coil H in its path to ground. Section CD of
the rod is used therefore simply to shunt the inductance of winding
H to high frequency lightning discharges, leaving the lightning
discharge path in the arrester a _non-inductive_ highly efficient
path. In all Garton-Daniels A. C. lightning arresters operating on
non-grounded or partially grounded circuits, the action of the air
gaps and series resistance are together sufficient to extinguish the
flow of normal current to ground at the zero point of the generator
voltage wave. If, however, as frequently happens, the line grounds
accidentally during a storm, then the arrester does not have to
depend for its proper operation on the arc extinguishing properties
of the air gaps and resistance, but the heavier flow of line current
through the arrester energizes the movable plunger, which raises
upward in the coil, opening the circuit between the discharge point M
and the lower end of the plunger. To limit the flow of line current
to ground the resistance rod B is provided, there being approximately
225 ohms between the discharge point A and clamp C in the 2,500 volt
arrester. This feature is particularly effective where the circuit is
temporarily or accidentally grounded. The series resistance prevents
a heavy short circuit through the arrester and limits the current to
a value that is readily broken by the cut out and is not enough to
impede the passage of the discharge.]
[Illustration: FIG. 2,833.--Diagram of switchboard connections for
General Electric automatic voltage regulator with three exciters and
three alternators.]
If possible, one place should be set aside for them and a marble
or slate panel provided on which they may be mounted.
Wooden supports are undesirable for lightning arresters on
account of the fire risk incurred; this, however, may be reduced
to a minimum by employing skeleton boards and using sheets of
asbestos between the arresters and the wood.
In parts of the country where lightning is of common occurrence
and where overhead circuits are installed which carry high
pressures, heavy currents, and extend over considerable territory,
it is advisable to have the station well equipped with lightning
arresters of the most improved types.
In each side of the main circuit, between the lightning arrester
connections and the switchboard apparatus there should be
connected a choke coil or else each of the main conductors at this
point should be tightly coiled up part of its length to answer the
same purpose.
A quick and effective way of coiling up a wire consists in
wrapping around a cylindrical piece of iron or wood that part
of the conductor in which it is desired to have the coils, the
desired number of times, and then withdrawing the cylindrical
piece. The coils, each of which may contain 50 or 200 turns,
thus inserted in the main circuit introduce a high resistance or
reluctance to a lightning current, and thus prevent it passing
to the generator; there will, however, be an easy path to earth
afforded it through the lightning arrester, and so no damage will
be done. Coils of the nature just mentioned may advantageously be
introduced between the generator and switchboard to take up the
reactive current developed upon the opening of the circuit, and in
the case of suspended conductors, the coils may be used to take up
the slack by the spring-like effect produced by them.
The safety of the operator should be especially considered in
the design of high pressure alternating current switchboards.
Such protection may be secured by screening all the exposed
terminals, or preferably by mounting all the switch mechanism on
the back of the board with simply the switch handle projecting
through to the front; by pushing or pulling the switch handle, the
connections can thus be shifted either to one side of the system
or to the other.
=Ques. Upon what does the work of assembling a switchboard depend?=
Ans. It depends almost entirely upon the size of the plant, varying
from the simple task of mounting a single panel in the case of an
isolated plant, to the more difficult problem of supporting a large
number of panels in a central station.
=Ques. When the material chosen for a switchboard must be shipped a
considerable distance, what form of board should be used?=
Ans. The board units or "slabs" should be of small dimensions, to
avoid the liability of breakage and expense of renewal when a unit
becomes cracked or machine injured.
[Illustration: FIGS. 2,834 and 2,835.--Front and rear views showing
General Electric automatic voltage regulator mounted on switchboard
panel.]
Ordinarily, switchboards vary from five to eight feet in
height and the widths of the panels vary from five to six feet.
In some boards the seams between the slabs run vertically, and
in others horizontally. In order to render the assembling of
the switchboard as simple as possible, and its appearance when
finished the most artistic, these seams should run horizontally
rather than vertically. The edges of each of the slabs should also
be chamfered so that there will be less danger of their breaking
out when being mounted on the framework.
=Ques. In assembling a switchboard, how should the lower slabs be
placed, and why?=
Ans. They should be suspended a little distance from the floor to
prevent contact with any oil, dirt, water or rubbish that might be on
the floor.
=Ques. How are the slabs or panels supported?=
Ans. They are carried on an iron or wooden framework with braces to
give stability.
The braces should be securely fastened at one end to the wall of
the station, and at the other end to the framework of the board,
as shown in fig. 2,836.
To fasten the switchboard end of the brace directly to the
slate, marble or other material composing the board is poor
practice and should never be attempted.
If the station be constructed of iron, these switchboard braces
must be such that they will thoroughly insulate the board and its
contents from the adjoining wall.
[Illustration: FIG. 2,836.--Method of supporting the framework of a
switchboard.]
=Ques. What is the usual equipment of a switchboard?=
Ans. It comprises switching devices, current or pressure limiting
devices, indicating devices, and fuses for protecting the apparatus
and circuits.
[Illustration: FIG. 2,837.--Diagram showing elementary connections of
General Electric automatic regulator for direct current. It consists
essentially of a main control magnet with two independent windings
and a differentially wound relay magnet. One winding, known as the
pressure winding, of the main control magnet is connected across the
dynamo terminals, the other across a shunt in one of the load mains.
The latter is the "compensating winding" and it opposes the action
of the pressure winding so that as the load increases, a higher
pressure at the dynamo is necessary to "over compound" for line drop.
In ordinary practice, the voltage terminals are connected to the bus
bars, and the compensating shunt inserted in one of the principal
feeders of the system. In operation the shunt circuit across the
dynamo field rheostat is first opened by means of a switch provided
for that purpose on the base of the regulator and the rheostat turned
to a point that will reduce the generator voltage 35 per cent below
normal. The main control magnet is at once weakened and allows the
spring to pull out the movable core until the main contacts are
closed. This closes the second circuit of the differential relay,
thus neutralizing its windings. The relay spring then lifts the
armature and closes the relay contacts. The switch in the shunt
circuit across the dynamo field rheostat is now closed, practically
short circuiting the rheostat, and the dynamo voltage at once rises.
As soon as it reaches the point for which the regulator has been
adjusted, the main control magnet is strengthened, which causes the
main contacts to open, which in turn open the relay contacts across
the rheostat. The rheostat is now in the field circuit, the voltage
at once falls off, the main contacts are closed, and relay armature
released, and shunt circuit across the rheostat again completed. The
voltage then starts to rise and this cycle of operation is continued
at a high rate of vibration, maintaining not a constant but a steady
voltage at the bus bars. When neither the compensating winding nor
pressure wires are used, there will be no "over compounding" effect
due to increase of load and a constant voltage will be maintained at
the bus bars. The compensating winding on the control magnet, which
opposes the pressure winding is connected across an adjustable shunt
in the principal feeder circuit. As the load increases the voltage
drop across the shunt increases and the effect of the compensating
winding becomes greater. This will require a higher voltage on the
pressure winding to open the main contacts and the regulator will
therefore cause the dynamo to compensate for line drop, maintaining
at the bus bars a steady voltage without fluctuations, which rises
and falls with a load on the feeders, giving a constant voltage at
the lamps or center of distribution. The compensating shunt may be
adjusted so as to compensate for any desired line drop up to 15 per
cent; it is preferably placed in the principal lighting feeder, but
may be connected to the bus bars so that the total current will pass
through it. The latter method, however, is sometimes desirable, as
large fluctuating power loads on separate feeders might disturb the
regulation of the lighting feeders. Adjustment is made by sliding
the movable contact at the center of the shunt. This contact may be
clamped at any desired point and determines the pressure across the
compensating winding of the regulator's main control magnet. Where
pressure wires are run back to the central station from the center
of distribution they may be connected directly to the pressure
winding of the main control magnet, and it is unnecessary to use the
compensating shunt. The pressure wires take the place of the leads
from the control magnet to the bus bars and maintain a constant
voltage at the center of distribution.]
On some switchboards are also mounted small transformers for
raising or lowering the voltages, and lightning arresters as a
protection from lightning. In addition to the apparatus previously
mentioned nearly all switchboards carry at or near their top two
or more incandescent lamps provided with shades or reflectors, for
lighting the board.
=Ques. What should be done before wiring a switchboard?=
Ans. The electrical connections between the various apparatus
mounted on the face or front of the board, are made on the back of
the board. It is necessary that these connections be properly made
else considerable electrical power will be wasted at this point. The
wiring on the back of the board should therefore be planned out on
paper before commencing the work.
[Illustration: FIG. 2,838.--Diagram showing connections of General
Electric automatic voltage regulator for direct current as connected
for maintaining balanced voltage on both sides of a three wire system
using a balancer set. In operation, should the voltage on the upper
bus bars become greater than that on the lower ones, the middle and
upper contacts on the regulator will close, thus opening the relay
contacts to the left and closing those to the right. This inserts all
the resistance in the field of balancer A, and short circuits the
resistance in the field of balancer B. A will then be running as a
motor, and B as a dynamo, thereby equalizing the two voltages until
that on the lower bus bars becomes greater than that of the upper
ones; then the regulator contacts operate in the opposite direction
and balancer A is run as a dynamo, and balancer B as a motor. This
cycle of operation is repeated at the rate of from three to four
hundred times per minute, thus maintaining a balanced voltage on the
system.]
In laying out the plan of wiring care must be taken to allow
sufficient contact surface at each connection; there should be not
less than one square inch of contact surface allowed for each 160
amperes of current transmitted.
[Illustration: FIG. 2,839.--Diagram of connections of General
Electric voltage regulators for one or more alternators using one
exciter.]
For the bus bars, which, by the way are always of copper, one
square inch per 1,000 amperes is the usual allowance; this is
equal to 1,000 circular mils of cross sectional area per ampere.
Every effort should be made to give the bus bars the greatest
amount of radiation consistent with other conditions, in order
that their resistances may not become excessive owing to the heat
developed by the large currents they are forced to carry. Suppose,
for instance, the number of amperes to be generated is such as
to require bus bars having each a cross sectional area of one
square inch. If the end dimensions of these bars were each 1 inch
by 1 inch, there would be less radiating surface than if their
dimensions were each 2 inches by ½ inch.
=Operation of Alternators.=--The operation of an alternator when run
singly differs but little from that for a dynamo.
As to the preliminaries, the exciter must first be started.
This is done in the same way as for any shunt dynamo. At first
only a small current should be sent through the field winding of
the alternator; then, if the exciter operates satisfactorily and
the field magnetism of the operator show up well, the load may
gradually be thrown on until the normal current is carried, the
same method of procedure being followed as in the similar case of
a dynamo.
[Illustration: FIGS. 2,840 and 2,841.--General Electric equalizer
regulator designed to equalize the load on two machines, and diagram
of connections.]
On loading an alternator, a noticeable drop in voltage occurs
across its terminals. This drop in voltage is caused in part by the
demagnetization of the field magnets due to the armature current,
and so depends in a measure upon the position and form of the pole
pieces as well as upon those of the teeth in the armature core. The
resistance of the armature winding also causes a drop in voltage
under an increase of load.
Another cause which may be mentioned is the inductance of the
armature winding, which is in turn due to the positions of the
armature coils with respect to each other and also with respect to
the field magnets.
[Illustration: FIG. 2,842.--Connection of General Electric equalizing
regulator for equalizing loads on an engine driven dynamo and rotary
converter running in parallel. Should the load on the dynamo become
greater than that on the rotary converter, the middle and upper
contacts on the regulator close, and thus by means of the relay
switch and control motor, cause the feeder regulator to boost the
voltage on the rotary until the loads again become equal. Should
the load on the rotary converter become greater than that on the
generator, the regulator contacts operate in the reverse direction
and the feeder regulator is caused to buck the rotary voltage.]
=Alternators in Parallel.=--When the load on a station increases
beyond that which can conveniently be carried by one alternator, it
becomes necessary to connect other alternators in parallel with it.
To properly switch in a new machine in parallel with one already in
operation and carrying load, requires a complete knowledge of the
situation on the part of the attendant, and also some experience.
The connections for operating alternators in parallel are
shown in fig. 2,843. In the illustration the alternator A is in
operation and is supplying current to the bus bars. The alternator
B is at rest. The main pole switch B' by means of which this
machine can be connected into circuit is therefore open.
[Illustration: FIG. 2,843.--Method of synchronizing with one lamp;
_dark lamp method_. Assuming A to be in operation, B, may be brought
up to approximately the proper speed, and voltage. Then if B, be
run a little slower or faster than A, the synchronizing lamp will
glow for one moment and be dark the next. At the instant when the
pressures are equal and the machines in phase, the lamp will become
dark, but when the phases are in quadrature, the lamp will glow at
its maximum brilliancy. Since the flickering of the lamp is dependent
upon the difference in frequency, the machines should not be thrown
in parallel while this flickering exists. The nearer alternator
approaches synchronism, in adjusting its speed, the slower the
flickering, and when the flickering becomes very slow, the incoming
machine may be thrown in the moment the lamp is dark by closing the
switch. The machines are then in phase and tend to remain so, since
if one slow down, the other will drive it as a motor.]
Now, if the load increase to such extent as to require the
service of the second alternator B, it must be switched in
parallel with A. In order that both machines may operate properly
in parallel, three conditions must be satisfied before they are
connected together, or else the one alternator will be short
circuited through the other, and serious results will undoubtedly
follow.
Accordingly before closing main switch B, it is necessary that
1. The frequencies of both machines be the same;
2. The machines must be in synchronism;
3. The voltages must be the same.
=Ques. How are the frequencies made the same?=
Ans. By speeding up the alternator to be cut in, or change the speed
of both until frequency of both machines is the same.
[Illustration: FIG. 2,844.--Diagram of connections of General
Electric automatic voltage regulator for several alternators running
in parallel with exciters in parallel.]
=Ques. How are the alternators synchronized or brought in phase?=
Ans. The synchronism of the alternators is determined by employing
some form of synchronizer, as by the single lamp method of fig.
2,843, or the two lamp method of fig. 2,845.
=Ques. In synchronizing by the one lamp method, when should the
incoming machine be thrown in?=
Ans. It is advisable to close the switch when the machines are
approaching synchronism rather than when they are receding from it,
that is to say, the instant the lamp becomes dark.
[Illustration: FIG. 2,845.--Method of synchronizing with two lamps;
_dark lamp method_. The two synchronizing lamps are connected as
shown, and each must be designed to supply its rated candle power
at the normal voltage developed by the alternators. Now since the
alternators are both running under normal field excitation the left
hand terminals of each of them will alternately be positive and
negative in polarity, while the right hand terminals are respectively
negative and positive in polarity. If, however, the alternators be
in phase with each other, the left hand terminals of both of them
will be positive while the right hand terminals are negative, and
when the left hand terminals of both machines are negative the right
hand terminals will be positive. Hence, when the machines are in
phase there will be no difference of pressure between the left hand
terminals or between the right hand terminals of the two machines.
Hence, if the synchronizing lamps be connected as shown, both will
be dark. The instant there is a difference of phase, both lamps
will glow attaining full candle power when the difference of phase
has reached a maximum. As the alternators continue to come closer
in step, the red glow will gradually fade away until the lamps
become dark. Then the switch may be closed, thereby throwing the
two machines in parallel. If the intervals between the successive
lighting up of the lamps are of short duration it is advisable to
wait until these become longer even though the other conditions are
satisfied, because where the phases pass each other rapidly there is
a greater possibility of not bringing them together at the proper
instant. An interval of not less than five seconds should therefore
be allowed between the successive lighting up of the lamps, before
closing the switch.]
[Illustration: FIG. 2,846.--Inductor type synchroscope. This type
is especially applicable where pressure transformers are already
installed for use with other meters. As it requires only about ten
apparent watts it may be used on the same transformers with other
meters. There are three stationary coils, N, M and C, and a moving
system, comprising an iron armature, A, rigidly attached to a shaft
suitably pivoted and mounted in bearings. A pointer is also attached
to the shaft. The moving system is balanced and is not subjected
to any restraining force, such as a spring or gravity control. The
axes of the coils N and M are in the same vertical plane, but 90
degrees apart, while the axis of C is in a horizontal plane. The
coils N and M are connected in "split phase" relation through an
inductive resistance P and non-inductive resistance Q, and these two
circuits are parallel across the bus bar terminals 3 and 4 of the
synchroscope. Coil C is connected through a non-inductive resistance
across the upper machine terminals 1 and 2 of the synchroscope. =In
operation=, current in the coil C magnetizes the iron core carried
by the shaft and the two projections, marked A and "iron armature."
There is however, no tendency to rotate the shaft. If current be
passed through one of the other coils, say M, a magnetic field will
be produced parallel with its axis. This will act on the projections
of the iron armature, causing it to turn so that the positive and
negative projections assume their appropriate position in the field
of the coil M. A reversal of the direction in both coils will
obviously not affect the position of the armature, hence alternating
current of the same frequency and phase in the coils C and M cause
the same directional effect upon the armature as if direct current
were passed through the coils. If current lagging 90 degrees behind
that in the coils M and C be passed through the coil N, it will cause
no rotative effect upon the armature, because the maximum value of
the field which it produces will occur at the instant when the pole
strength of the armature is zero. The two currents in the coils M and
N produce a shifting magnetic field which rotates about the shaft as
an axis. As all currents are assumed to be of the same frequency, the
rate of rotation of this field is such that its direction corresponds
with that of the armature projections at the instant when the poles
induced in them by the current in the coil C are at maximum value,
and the field shifts through 180 degrees in the same interval as is
required for reversal of the poles. This is the essential feature of
the instrument, namely, that the armature projections take a position
in the rotating magnetic field which corresponds to the direction of
the field at the instant when the projections are magnetized to their
maximum strength by their current in the coil C. If the frequency
of the currents in the coils which produce the shifting field be
less than that in the coil which magnetized the armature, then the
armature must turn in order that it may be parallel with the field
when its poles are at maximum strength.]
=Ques. What are the objections to the one lamp method?=
Ans. The filament of the lamp may break, and cause darkness, or the
lamp may be dark with considerable voltage as it takes over 20 volts
to cause a 100 volt lamp to glow.
=Ques. What capacity of single lamp must be used?=
Ans. It must be good for twice the voltage of either machine.
[Illustration: FIG. 2,847.--Brilliant lamp method of synchronizing.
The synchronizing lamps are connected as shown, and must be of the
alternator voltage. When the voltages are equal and the machines in
phase, the difference of pressure between _a_ and a given point is
the same as that between _a'_ and the same point; this obtains for
_b_ and _b'_. Accordingly, a lamp connected across _a b'_ will burn
with the same brilliancy as across _a' b_; the same holds for the
other lamp. When the voltages are the same and the phase difference
is 180° the lamps are dark, and as the phase difference is decreased,
the lamps glow with increasing brightness until at synchronism they
glow with maximum brilliancy. Hence the incoming alternator should be
thrown in at the instant of maximum brilliancy.]
=Ques. What modification of the synchronizing methods shown in
the accompanying illustrations is necessary when high pressure
alternators are used?=
Ans. Step down transformers must be used between the alternators and
the lamps to obtain the proper working voltages for the lamps.
[Illustration: FIG. 2,848.--Synchronizing with high pressure
alternators; dark and brilliant lamp methods. In both methods the
primaries of the transformers are connected in the same way across
the terminals of the alternators as shown. In the dark lamp method,
the connections between the secondary coils of the transformers must
be made so that when each is subjected to the same conditions the
action of the one coil opposes that of the other as in the dark lamp
method; then, if the transformers be both of the same design, there
will be no voltage across the lamps when the alternators are in phase
with each other. If the ratio of each transformer is such as to give,
for example, 100 volts across its secondary terminals, then the two
incandescent lamps since they are joined together in series must
each be designed for 100 volts. One 200 volt lamp could be used in
either method in place of the two 100 volt lamps. When, therefore,
the alternators are directly opposite in phase to each other, both
the lamps will burn brightly; as the alternators come together in
phase the lamps will produce less and less light, until when the
machines are exactly in phase no light will be emitted at all, at
which instant the incoming alternator should be thrown in. It must
be evident, if the transformer secondary connections are arranged as
in the brilliant lamp method, so that they do not oppose each other,
the lamps will be at maximum brilliancy when the alternators are in
phase and dark when the phase difference is 180°, assuming of course
equalized voltage.]
=Ques. How is the voltage of an incoming machine adjusted so that it
will be the same as the one already in operation?=
Ans. By varying the field excitation with a rheostat in the
alternator field circuit.
=Ques. How may two or more alternators be started simultaneously?=
Ans. After bringing each of them up to its proper speed so as to
obtain equal frequencies, the main switches may be closed, thereby
joining their armature circuits in parallel. As yet, however, their
respective field windings have not been supplied with current, so
that no harm can result in doing this. The exciters of these machines
after being joined in parallel, should then be made to send direct
current simultaneously through the field windings of the alternators,
and from this stage on the directions previously given may be
followed in detail.
=Ques. What are the conditions when two or more alternators are
directly connected together?=
Ans. If rigidly connected together, or directly connected to the same
engine, they must necessarily run in the same manner at all times.
When machines connected in this way are once properly adjusted
so that they are in phase with each other, their operation in
parallel is even a simpler task than when they are all started
together but are not directly connected.
=Ques. When an alternator is driven by a gas engine, what provision
is sometimes made to insure successful operation in parallel?=
Ans. An amortisseur winding is provided to counteract the tendency to
"hunting."
[Illustration: FIG. 2,849.--Diagram of Lincoln Synchronizer. =In
construction=, a stationary coil F, has suspended within it a coil A,
free to move about an axis in the planes of both coils and including
a diameter of each. If an alternating current be passed through both
coils, A, will take a position with its plane parallel to F. If now
the currents in A and F be reversed with respect to each other, coil
A will take up a position 180° from its former position. Reversal
of the relative directions of currents in A and F is equivalent to
changing their phase relation by 180°, and therefore this change
of 180° in phase relation is followed by a corresponding change of
180° in their mechanical relation. Suppose now, instead of reversing
the relative direction of currents in A and F, the change in phase
relation between them be made gradually and without disturbing the
current strength in either coil. It is evident that when the phase
difference between A and F reaches 90°, the force between A and F
will become reduced to zero, and a movable system, of which A may
be made a part, is in condition to take up any position demanded
by any other force. Let a second number of this movable system
consist of coil B, which may be fastened rigidly to coil A, with its
plane 90° from that of coil A, and the axis of A passing through
diameter of B. Further, suppose a current to circulate through B,
whose difference in phase relation to that in A, is always 90°. It
is evident under these conditions that when the difference in phase
between A and F is 90°, the movable system will take up a position,
such that B is parallel to F, because the force between A and F is
zero, and the force between B and F is a maximum; similarly when the
difference in phase between B and F is 90°, A will be parallel to F.
That is, beginning with a phase difference between A and F of zero
a phase change of 90° will be followed by a mechanical change on a
movable system of 90°, and each successive change of 90° in phase
will be followed by a corresponding mechanical change of 90°. For
intermediate phase relation, it can be proved that under certain
conditions the position of equilibrium assumed by the movable element
will exactly represent the phase relations. That is, with proper
design, the mechanical angle between the plane of F and that of A
and also between the plane of F and that of B, is always equal to
the phase angle between the current flowing in F and those in A and
B respectively. =As commercially constructed= coil F consists of a
small laminated iron field magnet with a winding whose terminals
are connected with binding posts. The coils A and B are windings
practically 90° apart on a laminated iron armature pivoted between
the poles of the magnet. These two windings are joined, and a tap
from the junction is brought out through a slip ring to one of two
other binding posts. The two remaining ends are brought out through
two more slip rings, one of which is connected to the remaining
binding post, through a non-inductive resistance, and the other
to the same binding post through an inductive resistance. A light
aluminum hand attached to the armature shaft marks the position
assumed by the armature.]
=Ques. What is the action of the amortisseur winding?=
Ans. Any sudden change in the speed of the field, generates a current
in the amortisseur winding which resists the change of velocity that
caused the current.
The appearance of an amortisseur winding is shown in the cut
below (fig. 2,850) illustrating the field of a synchronous
condenser equipped with amortisseur winding.
[Illustration: FIG. 2,850.--General Electric field of synchronous
condenser provided with amortisseur winding. Hunting is accompanied
by a shifting of flux across the face of the pole pieces due to the
variation in the effect of armature reaction on the main field flux
as the current varies and the angular displacement between the field
and armature poles is changed. Copper short circuited collars placed
around the pole face have currents induced in them by this shifting
flux, which have such a direction as to exert a torque tending to
oppose any change in the relative position of the field and armature.
This action is similar to that of the running torque of an induction
motor and the damping device has been still further developed until
in its best form it resembles the armature winding of a "squirrel
cage" induction motor. The pole pieces are in ducts, and low
resistance copper bars placed in them with their ends joined by means
of a continuous short circuiting ring extending around the field.
Such a device has proven very effective in damping out oscillations
started from any cause, the same winding doing duty as a damping
device and to assist the starting characteristics.]
=Ques. How are three phase alternators synchronized?=
Ans. In a manner similar to the single phase method.
Thus the synchronizing lamps may be arranged as in fig. 2,581,
which is simply an extension of the single phase method.
=Ques. Are three lamps necessary?=
Ans. Only to insure that the connections are properly made, after
which one lamp is all that is required.
=Ques. How is it known that the connections of fig. 2,851 are
correct?=
Ans. If, in operation, the three lamps become bright or dark
_simultaneously_, the connections are correct; if this action takes
place _successively_, the connections are wrong.
If wrong, transpose the leads of one machine until simultaneous
action of the lamps is secured.
[Illustration: FIG. 2,851.--Method of synchronizing three phase
alternators with, three lamps, being an extension of the single phase
method.]
=Ques. What is the disadvantage of the lamp method of synchronizing?=
Ans. Lack of sensitiveness.
=Ques. Which is the accepted lamp method, dark or brilliant?=
Ans. In the United States it is usual to make the connections for
a dark lamp at synchronism, while in England the opposite practice
obtains.
With the dark lamp method, the breaking of a filament might
cause the machines to be connected with a great phase difference,
whereas, with the brilliant lamp it is difficult to determine the
point of maximum brilliancy. This latter method, therefore may be
called the safer.
=Ques. What may be used in place of lamps for synchronizing?=
Ans. Some form of synchroscopes, or synchronizers.
=Ques. How does the Lincoln synchronizer work?=
Ans. The construction is such that a hand moves around a dial so
that the angle between the hand and the vertical is always the phase
angle between the two sources of electric pressure to which the
synchronizer is connected.
If the incoming alternator be running too slow, the hand
deflects in one direction, if too fast, in the other direction.
When the hand shows no deflection, that is, when it stands
vertical, the machines are in phase. A complete revolution of the
hand indicates a gain or loss of one cycle in the frequency of the
incoming machine, as referred to the bus bars.
=Cutting Out Alternator.=--When it is desired to cut out of circuit
an alternator running in parallel with others, the method of
procedure is as follows:
1. Reduce driving power until the load has been transferred
to the other alternators, adjusting field rheostat to obtain
minimum current;
2. Open main switch;
3. Open field switch.
=Ques. What precaution should be taken?=
Ans. _Never_ open field switch before main switch.
[Illustration: FIG. 2,852.--General Electric 500 kw., horizontal
mixed pressure Curtis turbine connected to a 500 kw. dynamo. In
a Curtis turbine it is not necessary to use the whole periphery
of the first stage for low pressure steam nozzles. A section can
be partitioned off and equipped with special expanding nozzles to
receive steam at high pressure direct from the boilers. Such nozzles
deliver their steam against the same wheel as do the low pressure
nozzles, but occupy only a small portion of its periphery. The steam
is expanded in these nozzles from high pressure all the way down
to the normal pressure of the first stage, and in such expansion
acquires a high velocity and consequently contains a great deal
of energy--much more than does an equal quantity of low pressure
steam. In consequence of this, high pressure steam is used with
a far lower water rate than is obtained with low pressure steam,
or with high pressure steam reduced to low pressure in a reducing
valve. This construction is called "mixed pressure." Its function
is the same as that of the reducing valve, that is, it makes up
for a deficiency of low pressure steam by drawing direct on the
boilers. With this construction, the full power of the turbine can be
developed with: All low-pressure steam, all high pressure steam, or,
any necessary proportion of steam of each pressure. Furthermore, the
transition from all low pressure to all high pressure, through all
the conditions intermediate between these extremes, is provided for
automatically by the turbine governor; a deficiency of low pressure
steam causes the high pressure nozzles to open automatically.]
=Ques. What is the ordinary method of cutting out an alternator?=
Ans. The main switch is usually opened without any preliminaries.
=Ques. What is the objection to this procedure?=
Ans. It suddenly throws all the load on the other alternators, and
causes "hunting."
=Ques. What forms of drive are especially desirable for running
alternators in parallel, and why?=
Ans. Water turbine or steam turbine because of the uniform torque,
thus giving uniform motion of rotation.
With reciprocating engines, the crank effect is very variable
during the revolution, resulting in pulsations driving the
alternator too fast or too slow, and causing cross current between
the alternators.
=Ques. Is a sluggish, or a too sensitive governor preferable on an
engine driving alternators in parallel?=
Ans. A sluggish governor.
=Alternators in Series.=--Alternators are seldom if ever connected
in series, for the reason that the synchronizing tendency peculiar
to these machines causes them to oppose each other and fall out of
phase when they are joined together in this way. If, however, they
be directly connected to each other, or to an engine, so that they
necessarily keep in phase at all times, and thus add their respective
voltages instead of counteracting them, series operation is possible.
NOTE.--According to the practice of the General
Electric Co., 2½ degrees of phase difference
from a mean is the limit allowable in ordinary
cases. It will, in certain cases, be possible to
operate satisfactorily in parallel, or to run
synchronous apparatus from machines whose angular
variation exceeds this amount, and in other cases
it will be easy and desirable to obtain a better
speed control. The 2½ degree limit is intended to
imply that the maximum departure from the mean
position during any revolution shall not exceed
2½ ÷ 360 of an angle corresponding to two poles
of a machine. The angle of circumference which
corresponds to the 2½ degree of phase variation
can be ascertained by dividing 2½ by ½ the
number of pole; thus, in a 20 pole machine, the
allowable angular variation from the mean would
be 2½ ÷ 10 = ¼ of one degree.
[Illustration: FIG. 2,853.--Diagram of connections for synchronizing
two compound wound three phase alternators. A and A' are the
armatures of the two machines, the fields of which are partly
separately excited, the amount of excitation current being controlled
by the series compounding rheostats B and B', which form a stationary
shunt. It is assumed that the alternator A is connected to the
bus bars 1, 2, and 3, by the switch 1S. If an increase make it
necessary to introduce the alternator A', it is first run up to
speed and excited to standard pressure by its exciter, and then
the double plug switch 3S is closed, connecting the primary of the
station transformer T and T' with the bus bars through the secondary
coil, so that the synchronizing lamps light up when the secondary
circuit is closed through the single pole switch 4S. The primary of
the station transformer T is thus excited through the double pole
switch 5S, connecting it with the outer terminals of the armature
A'. The two alternators will now work in opposition to each other
upon the synchronizing lamps, the transformer T being operated by
the new alternator A' through the switch 2S, and the transformer
T' being operated by the working alternator A, from the bus bars.
If the new alternator be not in step with the working alternator,
the synchronizing lamps will glow, growing brighter and dimmer
alternately with greater or lesser rapidity. In this case, the
armature speed of the new alternator must be controlled in such a
manner that the brightening and dimming will occur more and more
slowly, until the lamps cease to glow or remain extinguished for
a decided interval of time. The extinction of the light is due to
the disappearance of the secondary current, and indicates that
the alternators are in step. The switch 2S should now be thrown,
thus coupling the two machines electrically, and both of them will
continue to operate in step. The double pole equalizer switch 6S
should now be closed, connecting the two field windings in parallel
and equalizing the compounding, so that any variations of load will
affect the two alternators equally. After the alternators have been
connected in parallel, the switches 4S and 5S, may be opened leaving
the switch 3S closed, to operate the switchboard lamps K, K, as pilot
lights from the bus bars.]
=Transformers.=--These, as a whole, are simple in construction, high
in efficiency, and comparatively inexpensive. Their principles of
operation are also readily understood.
The efficiency of a transformer, that is, the ratio between full
load primary and full load secondary is greatest when the load on it
is such that the sum of the constant losses equals the sum of the
variable losses.
In general, transformers designed for high frequencies and
large capacities are more efficient than those designed for
low frequencies and small capacities. As a whole, however, a
transformer leaves but little to be desired as regards efficiency,
a modern 60 cycle transformer of 50 kilowatts capacity or more
possesses an efficiency of approximately 98 per cent. at full load
and an efficiency of about 97 per cent. at half load.
=Ques. How should a transformer be selected, with respect to
efficiency?=
Ans. One should be chosen, whose parts are so proportioned that the
point of maximum efficiency occurs at that load which the transformer
usually carries in service.
In many alternating current installations, comparatively light
loads are carried the greater part of the time, the rated full
load or an overload being occurrences of short durations. For such
purposes special attention should be given to the designing or
selecting of transformers having low core losses rather than low
resistance losses, because the latter are then of relatively small
importance.
=Ques. What kind of efficiency is the station manager interested in?=
Ans. The "all day efficiency."
This expression, as commonly met with in practice, denotes
_the percentage that the amount of energy actually used by the
consumer is of the total energy supplied to his transformer during
24 hours_. The formula for calculating the all day efficiency of
a transformer is based upon the supposition that the amount of
energy used by the consumer during 24 hours is equivalent to full
load on his transformer during five hours and is as follows:
5w
E = -------------
24c + 5r + 5w
where
E = the all day efficiency of the transformer,
w = the full load in watts on the primary,
c = the core loss in watts,
r = the resistance loss in watts.
[Illustration: FIG. 2,854.--Performance curves of Westinghouse air
blast 550 kw, 10,500 volt transformer, 3,000 alternations.]
=Ques. What are the usual all day efficiencies?=
Ans. The average is about 85 per cent. for those of 1 kilowatt
capacity, 92 per cent. for those of 5 kilowatts capacity, 94 per
cent. for those of 10 kilowatts capacity, and about 94.5 per cent.
for those of 15 kilowatts capacity.
=Ques. What becomes of the energy lost by a transformer?=
Ans. It reappears as heat in the windings and core.
This heat not only increases the resistances of the windings and
core, producing thereby a further increase of their respective
losses, but in addition causes in time a peculiar effect on the
iron core which is intensified by the reversals of magnetism
constantly going on within it.
After about two years' service, the iron apparently becomes
fatigued or tired, and this phenomenon is called aging of the
iron. Since the life of the transformer depends to a great extent
upon this factor, the conditions responsible for its existence
should as far as possible be removed. Means must therefore be
provided in the construction to radiate the heat as quickly as it
is generated.
=Ques. What kind of oil is used in oil cooled transformers?=
Ans. Mineral oil.
[Illustration: FIG. 2,855.--General arrangement of air blast
transformers and blowers.]
=Ques. How is it obtained?=
Ans. By fractional distillations of petroleum unmixed with any other
substances and without subsequent chemical treatment.
=Ques. What is the important requirement for transformer oil?=
Ans. It should be free from moisture, acid, alkali or sulphur
compounds.
=Ques. How may the presence of moisture be determined?=
Ans. By thrusting a red hot iron rod in the oil; if it "crackle,"
moisture is present.
=Ques. Describe the Westinghouse method of drying oil.=
Ans. It is circulated through a tank containing lime, and afterwards,
through a dry sand filter.
=Ques. What is the objection to heating the oil (raising its
temperature slightly above boiling point of water) to remove the
moisture?=
Ans. The time consumed (several days) is excessive.
[Illustration: FIG. 2,856.--Small Curtis turbine generator set as
made by the General Electric Co., in sizes from 5 kw., to 300 kw.
It can be arranged to operate either condensing or non-condensing,
and at any steam pressure above 80 lbs. for the smaller sizes and
100 lbs. for the larger. There are only two main bearings. A thrust
bearing, consisting of roller bearings and running between hardened
steel face washers located at either end of the main bearings is
provided solely for centering the rotor so as to equalize the
clearance. A centrifugal governor is provided (in the smaller sizes)
completely housed, and mounted directly on the main shaft end. It
controls a balanced poppet valve through a bell crank. In the larger
sizes (75 kw. and above) the governor is mounted on a vertical
secondary shaft geared to the main shaft and controls a cam shaft
which opens or closes a series of valves in rotation, admitting the
steam to different sections of the first stage nozzles. In this
way throttling of the steam is avoided. There is also an emergency
governor which closes the throttle valve in the event of the speed
reaching a predetermined limit. The speeds of operation range from
5,000 R.P.M. for the smallest size to 1,500 R.P.M. for the largest.
The lubrication system is enclosed and is automatic. Air leakage
where the shaft passes through the wheel casing is prevented by steam
seal.]
=Ques. What effect has moisture?=
Ans. It reduces the insulation value of the oil. .06 per cent. of
moisture has been found to reduce the dielectric strength of oil
about 50 per cent. "dry" oil will withstand a pressure of 25,000
volts between two 9½ inch knobs separated .15 inch.
=Ques. What is understood by transformer regulation?=
Ans. It is the difference between the secondary voltage at no load
and at full load, and is generally expressed as a percentage of the
secondary voltage at no load.
=Ques. What governs its value?=
Ans. The resistance and reactance of the windings.
[Illustration: FIG. 2,857.--Cut off coupling for power transmission
by line shafting. It is used to cut off a driving shaft from a driven
shaft. Its use obviates the use of a _quill_, such as is shown in
fig. 2,858.]
=Ques. How may the regulation be improved?=
Ans. By decreasing the resistances of the windings by employing
conductors of greater cross section, or decreasing their reactance by
dividing the coils into sections and closely interspersing those of
the primary between those of the secondary.
NOTE.--_The term_ ="regulation"= as here used
is synonymous with "drop." The _voltage drop_
in a transformer denotes the drop of voltage
occurring across the secondary terminals of a
transformer with load. This drop is due to two
causes: 1, the resistance of the windings; and
2, the reactance or magnetic leakage of the
windings. On non-inductive load, the reactive
drop, being in quadrature, produces but a slight
effect, but on inductive loads it causes the
voltage to drop, and on _leading current loads_
it causes the voltage to rise. As the voltage
drop of a good transformer is very small even
on inductive load, direct accurate measurement
is difficult. It is best to measure the copper
loss with short circuited secondary by means of
a wattmeter, and at the same time the voltage
required to drive full load current through. From
the watts, the resistance drop can be found, and
from this and the impedance voltage, the reactive
drop may be calculated. From these data a simple
vector diagram will give, near enough for all
practical purposes, the drop for any power
factor, or the following formula may be used
which has been deduced from the vector diagram.
_________________________
D = √(W + X)^{2} + (R + P)^{2} - 100 where R
= % resistance drop; X = % reactive drop; P = %
power factor of load; W = % wattless factor of
________
load (√1 - P^{2}); D = % resultant secondary
drop. For non-inductive loads where P = 100 and W
_____________________
= 0, D = √X^{2} + (100 + R)^{2} - 100. In the
case of leading currents it should be considered
negative.
In transformers where there is a great difference in voltage
between the primary and secondary windings, however, this remedy
has its limitations on account of the great amount of insulation
which must necessarily be used between the windings, and which
therefore causes the distances between them to become such as to
cause considerable leakage of the lines of force.
=Ques. How does the regulation vary for different transformers, and
what should be the limit?=
Ans. Those of large capacity usually have a better regulation than
those of small capacity, but in no case should its value exceed 2 per
cent.
[Illustration: FIG. 2,858.--Quill drive. This is the proper
transmission arrangement substitute for heavy service, requiring
large pulleys, sheaves, gears, rotors, etc. It is a hollow shaft
supported by independent bearings. The main driving shaft running
through the quill is thus relieved of all transverse stresses. The
power is transmitted to the quill by means of a friction or jaw
clutch. When the clutch is thrown out the pulley or sheave stands
idle and the driving shaft revolves freely within the quill. As there
is no contact between moving parts there is no wear. Jaw clutches
should be used for drives demanding positive angular displacement.
They can only be thrown in and out of engagement when at rest. All
very large clutch pulleys, sheaves, or gears designed to run loose on
the line shaft are preferably mounted on quills. The letters A, B, C,
etc., indicate the dimensions to be specified in ordering a quill.]
=Ques. What advantages have shell type transformers over those of the
core type?=
Ans. They have a larger proportion of core surface exposed for
radiation of heat, and a shorter magnetic circuit which reduces the
tendency for a leakage of the lines of force into the air.
Both types have advantages and disadvantages as compared with
the other. In the shell type, there is less magnetic leakage, but
also less surface exposed for radiation, and greater difficulty in
providing efficient insulation between the two circuits; in the
core type there is more surface exposed for radiation and less
difficulty in insulating the windings, but there is also a great
leakage of the lines of magnetic force into the outer air.
=Ques. How are the windings usually arranged?=
Ans. As a rule, there is only one primary winding but the secondary
winding is generally divided into two equal sections, the four
terminals of which are permanently wired to four connection blocks
which may be connected so as to throw the secondary sections either
in parallel or in series with each other at will.
=Ques. What is necessary for satisfactory operation of transformers
in parallel?=
Ans. They must be designed for the same pressures and capacities,
their percentages of regulation should be the same and they must have
the same polarity at a given instant.
One may satisfy himself as to the first of these conditions by
examining the name plates fastened to the transformers, whereon
are stamped the values of the respective pressures and capacities
of each.
Although equal values of regulation is given as one of the
conditions to be satisfied, transformers may be operated in
parallel when their percentages of regulation are not the same.
Ideal operation, however, can be attained only under the former
state of affairs. Suppose, for instance, a transformer having a
regulation of two per cent. be operated in parallel with another
of similar size and design but having a regulation of one per
cent. The secondary pressures of these transformers at no load
will of course be the same, but at full load if the secondary
pressure of the one be 98 volts, that of the other will be 99
volts. There will, therefore, be a difference of pressure of one
volt between them which will tend to force a current backward
through the secondary winding of the transformer delivering 98
volts. This reversed current, although comparatively small in
value, lowers the efficiency of the installation by causing a
displacement of phase and a decrease in the combined power factor
of the transformers.
=Ques. Describe the polarity test.=
Ans. The test for polarity consists in joining together by means of a
fuse wire, a terminal of the secondary winding of each transformer,
and then with the primary windings supplied with normal voltage,
connecting temporarily the remaining terminals of the secondary
windings. The melting of the fuse wire thus connected indicates that
the secondary terminals joined together are of opposite polarities,
and that the connections must therefore be reversed, whereas if the
fuse wire do not melt, it shows that the proper terminals have been
joined and that the connections may be made permanent.
[Illustration: FIG. 2,859.--Single overhung tangential water wheel
equipped with Doble ellipsoidal buckets. The central position of the
front entering wedge or lip of the bucket is cut away in the form of
a semi-circular notch, which allows a solid circular water jet to
discharge upon the central dividing wedge of the bucket without being
split in a horizontal plane.]
The object of this test is, obviously, not to determine the
exact polarity of each secondary terminal, but merely to indicate
which of them are of the same polarity.
[Illustration: FIG. 2,860.--Motor generator exciter set driven by
a Pelton-Doble tangential water wheel. The water wheel runner is
mounted on the shaft overhung and the jet is regulated by either a
hand actuated or governor controlled needle nozzle. The speed of the
water wheel is equivalent to the synchronous speed of the induction
motor, hence, the latter floats on the line, and under certain
conditions may perform the functions of an alternator by feeding into
the circuit, should the water wheel tend to operate above synchronous
speed. Should any interruption to the operation of the wheel occur,
causing a diminution of speed, the induction motor would drop back
to full load speed and take up the exciter load, resulting in no
appreciable drop of exciter voltage. The only variation of speed
possible is dependent upon the "slip" of the motor. Where two or more
exciter sets are employed in the station, an advantageous arrangement
embraces the installation of a water wheel driven motor generator set
and an exciter set, consisting of merely the direct current generator
and water wheel. The induction motor being electrically tied into
the circuit, the possibility of a runaway of the water wheel is
eliminated, since its speed can only slightly exceed the synchronous
speed of the system.]
=Motor Generators.=--In motor generator sets, either the shunt or
series wound type of motor may be employed at the power producing
end of the set, but the field of the generator is either shunt
or compound wound, depending upon whether or not it is desired
to maintain or to raise the secondary voltage near full load. In
either case a rheostat introduced in the shunt field winding of the
generator will be found very essential. Both generator and motor are
so mounted on the base that their respective commutators are at the
outer ends of the set. By this means ample space surrounds all of the
working parts, and repairs can readily be made.
Motor generators are frequently used as boosters to raise or boost
the voltage near the extremities of long distance, direct current
transmission lines. Of these, electric railway systems in which it is
desired to extend certain of the longer lines, form a typical example.
[Illustration: FIG. 2,861.--Automatically governed Pelton-Doble
tangential water wheel driving exciter dynamo. The water wheel is
mounted on one end of the shaft, while the opposite end is extended
to carry a fly wheel of suitable design to compensate for the low fly
wheel effect of the direct current armature. Two bearings support
the shaft which carries the rotating elements of the unit. A needle
nozzle actuated by a direct motion Pelton-Doble governor (designed
for operation by either oil or water pressure) maintains constant
speed.]
Owing to the great cost of changing such a system over to one
employing alternating current, or storage batteries, or of
constructing an additional power station, these solutions of the
problem are usually at variance with good judgment and the amount
of money at hand. The choice then remains between the purchase of
additional wire for feeders, the connection of a booster in the old
feeders, or the installation of both larger feeders and a booster.
Of these, it is generally found that either the second or the third
mentioned alternative meets the conditions most satisfactorily.
A booster installed in a railway system for the purpose just
mentioned, would have a series wound motor, and the conditions
to which it must conform would be as follows: The motor having a
series winding must provide for the full feeder current passing
through both armature and field windings.
Owing to the varying loads on a railway system, due to the
frequent starting and stopping of cars, the feeder current varies
between zero and some such value as 150 amperes. This fluctuation
of current through the field winding will, in ordinary cases, vary
the magnetization of the pole pieces from zero almost to the point
of saturation; that is, the maximum feeder current will so nearly
fill the magnet cores with lines of force that it would be quite
difficult to cause more lines of magnetic force to pass through
them.
So long as the point of saturation is not reached, however, the
proportion of current to field strength remains constant, and
therefore the ratio of amperes to volts will not vary.