SELECTION AND INSTALLATION OF DYNAMOS AND MOTORS
=General Conditions Governing Selection.=--In any particular
case, the voltage, current capacity, and type of dynamo selected will
depend upon the system of transmission or distribution to which it is
to be connected, and the character of the work which it is required to
perform. The suitability of the different types of dynamo for various
kinds of work has already been considered to some extent, but there are
certain general conditions which are applicable to almost all cases,
such as:
1. Construction;
2. Operation;
3. Cost;
4. Number and size of units.
=Construction.=--This should be as _simple_ as possible and of the
most solid character. All parts should be interchangeable, and have a
good finish. All machines should be provided with eye bolts or other
means by which they can be lifted or moved, as a whole or in parts,
easily and without injury. These features are so carefully attended to
and guaranteed by the manufacturers as to leave little choice in this
direction.
=Operation.=--The considerations relating to the operation of a
machine involve an examination of the details of its construction,
in order to determine the amount of attention it will require, the
character of its regulating device, its _capacity_, _form_, and
_weight_.
=Ques. What may be said regarding capacity?=
Ans. Dynamos and motors should not be overloaded, because the
efficiency is greater when the working load does not exceed the rated
capacity of the machine.
=Form.=--As a rule, there is not much choice in the matter of form
between standard machines, as they are uniformly symmetrical, well
proportioned and compact. It is a mistake, however, to select a light
machine for stationary use, as the weight of a machine increases its
strength, stability and durability.
=Cost.=--In some cases, the matter of first cost is important
and deserves careful consideration. It should be remembered, however,
that high grade electric machinery cannot be built out of low grade
materials and with poor workmen; therefore, when necessity compels
the selection of a cheap machine, it should not be expected that its
service will be as satisfactory as that of a first class machine.
=Number and Size of Units.=--The best number and size of units
for an electrical plant is usually governed by the requirements of
the driving engines. As a rule, dynamos and motors are not much less
efficient at quarter load than at full load, and the smaller dynamos
are fully equal to the larger machines in this respect, therefore, a
generating plant can be subdivided, and if so desired, without any
detrimental results except those to a multiplicity of units.
=Ques. What is the important consideration with respect to
efficiency?=
Ans. Efficiency at maximum load is not so important as efficiency at
average load.
For instance, in the diagram, fig. 439, the rated efficiency of
one dynamo as shown by the curve A, is 95 per cent., and that
of another, as shown by curve B, is 91 per cent., but it will
be observed that the average efficiency of B is much higher,
being 75 per cent. at quarter-load, 89 per cent. at half-load,
and 91 per cent. at three-quarter load, to 55, 77 and 89 per
cent. of A, at the corresponding loads. In this case, A is
higher than B only at full load, and as full load is a limit
which should not be reached except in special cases, and then
only for short intervals of time, the service rendered by B
would be much more satisfactory in the long run. In order to
avoid the difficulties possible under these conditions, a
guarantee to carry 25 per cent. overload for two hours without
injury should be required, and either this or the rated load be
taken, as the full load, so as to give a factor of safety of 25
per cent.
[Illustration: FIG 439.--Efficiency curves for 100 K. W.
dynamos. The efficiency of a dynamo at maximum load is not so important
as at average load. For instance, if in the figure the curve O B C
represent the efficiency of a 100 K. W. dynamo and O A D, that of
another machine, it would be in accordance with common practice to
compare them at rated load, at which the efficiency of the first is
only 91%, while the other is 93%. The first machine, however, is far
better than the second, since its average efficiency is much higher,
being nearly 91% between half load and 25% overload. It should be noted
that full load is a limit which should be but occasionally reached, and
then only for short periods of time.]
=Ques. Upon what does the choice of field winding of a dynamo
depend?=
Ans. The different classes of field winding have already been
discussed, but in general the conditions governing selection are as
follows: The series dynamo is used where a constant current at variable
voltage is desired, as in series arc lamp circuits. A shunt dynamo is
used on constant voltage circuits, where the distance from the machine
to the load is not great, that is, where there is small line loss. With
a compound dynamo there is compensation for line loss, that is, it can
be constructed so that the voltage at its terminals, or at the load can
be maintained constant or allowed to increase or decrease with a change
in load. It can thus operate lamps at constant voltage though they be
located at some distance, or the voltage at the end of the line can be
made to increase with an increase of load, as is frequently the case in
railway work.
[Illustration: FIG. 440.--Holzer-Cabot performance curves of
standard 20 H. P. motor, showing efficiency, speed regulation, and
amperes input.]
=Ques. For what conditions of service are series motors adapted?=
Ans. They are used on constant current circuits, and also on constant
voltage circuits as in railway work and similar purposes where an
attendant is always at hand to regulate the speed.
=Ques. Name some advantages and disadvantages of series motors.=
Ans. They are easily started even under heavy loads, the winding is
cheaper than the other types and the speed is nearer constant than
shunt motors when operated on constant current circuits. When used
on constant pressure circuits, such as is employed for incandescent
lighting, the speed will depend on the load.
=Ques. What kind of circuit is suitable for shunt motors?=
Ans. They are used on constant voltage circuits.
=Ques. What are the advantages of shunt motors?=
Ans. The speed remains nearly constant for variable load.
=Ques. State the disadvantages.=
Ans. They start less easily under a heavy load than do series motors,
and the speed cannot be varied through any wide range without
considerable loss. The shunt motor requires more attention than the
series type and is more liable to be burnt out.
=Location.=--The place chosen for the dynamo or motor should be
dry, free from dust, and preferably where a cool current of air can be
had. It should allow sufficient room for a belt of proper length when a
belt drive is used.
=Foundations.=--It is most important to secure a good foundation
for every dynamo, and great care should be taken to have them entirely
separate from those of the walls of the building in which the machine
is installed, and if the dynamo be directly driven, but not on the same
bed plate as the engine, a foundation large enough for both together
should be laid down. Stone or concrete may be used, or brick built with
cement, having a large thick stone bedded at the top.
For small machines the holding down bolts may be set with lead or
sulphur in holes in the stone top, but for large machines the bolts
should be long enough to pass down to the bottom, where they should be
anchored with iron plates.
=Setting up of Dynamos and Motors.=--In unpacking the machines
care should be taken to avoid injury to any part, and in putting the
parts together, each part should be carefully cleaned, and all the
parts put together in exactly the right way. The shafts, bearings,
magnetic joints, and electrical connection should receive especial
attention and be thoroughly cleaned of every particle of dirt, grit,
dust, metal clippings, etc.
=Ques. Who should preferably assemble the machines?=
Ans. Whenever possible, they should be assembled by someone thoroughly
familiar with the construction; but if the services of such a person
cannot be had, no one should attempt to put a machine together unless
he has a drawing or photograph of the same for a general guide.
=Ques. What precaution should be taken with the armature?=
Ans. It should be handled carefully to avoid any injury to the wires of
the winding and their insulation.
If it become necessary to lay the armature on the ground it
should be laid on clean paper or cloth, but it is better to
support it by the shaft on two wooden horses or other supports,
and thus avoid any strain on the armature body or commutator.
[Illustration: FIG. 441.--Foundation. It may be made either
of concrete, stone or brick. The machinery is held firmly in place
on the foundation by anchor bolts built into it; the proper position
for the bolts are determined by a wooden template suspended above the
foundation as shown. The bolts are surrounded by iron pipe that fixes
them vertically but permits a little side play to allow for any slight
errors in locating the centers on the template.]
=Connecting Up Dynamos.=--The manner in which the connections
of the field magnet coils, brushes, and terminals, are connected to
one another depends entirely upon the type of machine. The field
magnet shunt coils of shunt and compound wound dynamos, are invariably
arranged in series with one another, and then connected as a shunt to
the brushes or terminals of the machine. The series coils of series and
compound wound machines are arranged either in series or in parallel
with one another, according to conditions of operation, and then
connected in series to the armature and external circuit.
=Coupling Up Field Magnet Coils.=--In coupling up the coils
of either salient or consequent pole field magnets, assume each of
the pole pieces to have a certain polarity (in bipolar dynamos two
poles only, a north and south pole respectively, are required; in
multipolar dynamos the poles must be arranged in alternate order
around the armature, the number of N and S poles being equal), then
apply Flemming's rule as given under fig. 132, to each of the coils,
and ascertain the direction in which the magnetizing current must
flow in each in order to produce the assumed polarity in each of the
pole pieces. Having marked these directions on the coils, they can
be coupled up in either series or parallel connection according to
requirements, so that the current flows in the proper direction in each.
[Illustration: FIG. 442.--Comparison of space occupied by
direct and belt connected dynamos. In office buildings space is of
value and the room required by belt connected dynamos can always be
put to profitable use. For this reason the direct connected unit has
become generally adopted in the best type of office buildings. In large
factories the direct connected unit is generally adopted also to save
space. Where these conditions do not obtain, belted type of dynamo can
be used to advantage as a given output can be obtained with a smaller
size machine than where it is direct connected to the engine. This is
due to the limited rotative speeds at which engines can be run. The
illustration shows the relative space required by the two types.]
=The Drive.=--Various means are employed to connect the engine or
other prime mover with the dynamo, or the motor with the machinery to
driver. Among these may be mentioned the following:
1. Direct drive;
2. Belt drive;
3. Rope drive;
4. Gear drive;
5. Friction drive.
[Illustration: FIG. 443.--General Electric type M P, marine
generating set with tandem compound engine. The requirements of such
units are compactness, light weight, simplicity, freedom from vibration
and noise at high speed, perfect regulation and durability. By adopting
a short stroke for the engines and a special armature winding for the
dynamos, the height and length of the sets have been reduced. The
bed is carried out to the full width of the dynamo frame, making an
ample base surface for foundation without increasing the floor space
required. While the construction gives a massive appearance, the bed
has been cored out and the various parts so designed that the complete
sets have an approximate capacity of 3½ watts per pound. All of
the moving parts are enclosed by the engine column, excluding dust
and reducing wear and attention to a minimum. The bearing are oiled
automatically under pressure. These sets are made in sizes from 25 K.
W. to 75 K. W., the cylinder dimensions for the smallest size being
6½ and 10½ by 5, and for the largest size 10½ and 18 by 8.
Single cylinder sets are made in sizes ranging from 2½ K. W. to 50
K. W., the cylinder dimensions ranging from 3½ × 3 to 12 × 11. See
fig. 730.]
=Ques. What is a direct drive?=
Ans. One in which the driving member is connected direct to the driven
member, without any interposed gearing.
Fig. 443 shows a direct connected unit, which is an example of
direct drive.
=Ques. What may be said with respect to direct drive?=
Ans. It is the simplest method and the space required is less than with
belt drive. With direct drive the engine and dynamo must run at the
same speed; this is a disadvantage because the desirable speeds of the
two machines may not agree.
Since the usual engine speeds are slower than dynamo speeds,
direct drive involves the use of a larger dynamo for a given
output than would be necessary with belt connection, and
involves a corresponding increase in cost and greater friction
loss due to the rotation of larger and heavier parts.
[Illustration: FIG. 444.--Belt clamp for stretching belt and
holding the ends while making joint. It consists of a _stretching
frame_, the two ends of which are coupled by screwed bars; used for
pulling the ends of a belt together with the proper tension, when
lacing or joining the ends.]
=Ques. Mention some of the features of belt drive.=
Ans. Greater flexibility in the original design of a plant is possible
and new arrangements of old apparatus can be made at any time. It gives
conveniently any desired speed ratio and permits the use of high speed
dynamos and motors.
=Ques. State some of the disadvantages of belt drive.=
Ans. Considerable space is required and the action is not positive.
Belts exert a side pull on the bearings which results in wear, also
loss of power by friction.
[Illustration: FIGS. 445 and 446.--Two methods of lacing a
belt. In fig. 445 two rows of oval holes should be made with a punch,
as indicated. The nearest hole should be ¾ inch from the side, and
the first row 7/8 inch from the end, and the second row 1¾ inches
from the end of the belt. In large belts these distances should be
a little greater. A regular belt lacing (a strong, pliable strip
of leather) should be used, beginning at hole No. 1, and passing
consecutively through all the holes as numbered. In fig. 446 the holes
are all made in a row. This method has the advantage of making the
lacers lie parallel with the motion on the pulley side. The lacing is
doubled to find its middle, and the two ends are passed through the
two holes marked "1" and "1_a_" precisely as in lacing a shoe. The two
ends are then passed successively through the two series of holes in
the order in which they are numbered, 2, 3, 4, etc., and 2_a_, 3_a_,
4_a_, etc., finishing at 13 and 13_a_, which are additional holes for
fastening the ends of the lacer.]
=Ques. Give a rule for determining the proper size of belt.=
Ans. _A single belt travelling 1,000 feet per minute will transmit one
horse power per inch of width; a double belt will transmit twice this
amount._
EXAMPLE.--What size of double belt is required to transmit 50
horse power at 4,000 ft. speed, and what diameter pulley must
be used for 954 revolutions per minute at 4,000 ft. speed of
belt?
[Illustration: FIG. 447.--Wrong way to run a belt. The pull
should _not_ come on the top side, because, with slack at bottom there
is a tendency to slip.]
The horse power transmitted per inch is
4,000
----- × 2 = 8
1,000
accordingly, the width of belt required to transmit 50 horse
power is
50 ÷ 8 = 6.25, say 6".
For 4,000 ft. per minute belt speed, the distance _in inches_
travelled by the belt _per revolution_ of the pulley.
4,000 × 12
---------- = 50.31 inches
954
This is equal to the circumference of the pulley, and the
corresponding diameter is
50.31
---------- = 16.1, say 16 inches.
π
=Ques. What is the proper speed for a belt?=
Ans. From 3,000 to 5,000 feet per minute, depending on conditions.
Points Relating to Belts.
1. The amount of power that a belt of given size can transmit
is not a very definite quantity. The rule just given is
conservative and will give an amply large belt for ordinary
conditions.
2. A belt should make a straight run through the air and over
the pulleys without wabbling; it should maintain an even and
perfect contact with that part of the pulley with which it
comes in contact. In order to do this it should be kept soft,
pliable, and have no abrasions or rough places.
[Illustration: FIG. 448.--Right way to run a belt. The pull should come
on the lower side bringing the slack on top.]
3. When belt fasteners give way there is too much strain upon
belt. The greatest amount of slack in a belt is found where
it leaves the driving pulley, hence the tightener should be
near the driving pulley, as it takes up the slack, prevents
vibration and diminishes strain on belts and bearings. More
than 100 degrees of heat is injurious to belts.
4. Double belts should always run with the splices, and not
against them. Quarter turn belts should be made of two ply
leather, so as to diminish the side strain.
5. Friction is greatest when the pulleys are covered with
leather. Friction depends upon pressure, but adhesion depends
upon surface contact; the more a belt adheres to pulley surface
without straining, through too much tightening, the better the
driving power. Slipping occurs on wet days because the leather
absorbs dampness.
6. A leather covered pulley will produce more resistance than
polished or rough iron ones. A good belt dressing makes a
smooth, resisting surface, and as it contains no oils which
create a slippery surface to belts, it increases belt adhesion.
The friction of leather upon leather is five times greater than
leather upon iron.
7. Moisture and water distend the fibres, change the properties
of the tanner's grease and softening compounds. Repeated
saturation and drying will soon destroy leather. Leather well
filled with tanner's grease or animal oil, if allowed to hang
in a warm room for several months without handling, will dry
out, become harsh, and will readily crack.
8. A running belt is stretched and relaxed at different times
and unless there be perfect elasticity in all its parts there
will not be uniform distension.
9. There should be 25 per cent. margin allowed for adhesion
before a belt begins to slip.
[Illustration: FIGS. 449 to 451.--Method of aligning engine
and dynamo. In fig. 449, a line is stretched from A to E and the dynamo
shifted until the line contacts with points A, D, I, and E. In a small
dynamo, the pulley may be loosened and set back on the shaft as in fig.
450, while lining up the faces, and then moved back to its original
position as in fig. 451. When the pulley is not easily shifted the
distances at A and D (fig. 449) may be measured.]
=Rules for Calculating Speed and Sizes of Pulley.=--When two
pulleys are working together connected by a belt, the one which
communicates the motion is called the _driver_ and the other which
receives it, the _driven pulley_.
=To Find the Size of the Driving Pulley:= Multiply the
diameter of the driven pulley by its required number of
revolutions, and divide the product by the revolutions of the
driver. The quotient will be the diameter of the driver.
=To Find the Number of Revolutions of the Driven Pulley:=
Multiply the diameter of the driver by its number of
revolutions, and divide by diameter of driven. The quotient
will be the number of revolutions of the driven.
=To Find the Diameter of the Driven that shall Make a Given
Number of Revolutions, the Diameter and Revolutions of the
Driver Being Given:= Multiply the diameter of the driver by
its number of revolutions, and divide the product by the number
of revolutions of the driven pulley. The quotient will be the
diameter of the driven pulley.
=Rope Drive.=--In this method of power transmission, rope is run
in V-shaped grooves in the rims of the pulleys; this form of drive, in
some cases, is more desirable than others.
[Illustration: FIG. 452.--General Electric C Q back geared
motor driving Hamilton sensitive drill. When slowly moving machines
are to be driven, or where, for any reason, very moderate belt speeds
are required, the back geared motor is desirable. Two ratios of gear
reduction have been adopted as standard; they are:--4 to 1 and 8 to 1.]
=Ques. What are some of the advantages of rope drive?=
Ans. More power can be transmitted with a given diameter and width of
pulley, on account of the increased grip in the grooves. Rope drive can
be employed for long or short distances by reason of its lightness and
the action of the grooves.
=Gear Drive.=--This method is used where a positive drive is
desired, as for elevator or railway motors. It admits of any degree of
speed reduction without attending difficulties as would be encountered
with belt drive.
Thus, with the worm type of gear as used on elevator motors a
great reduction in velocity can be made without incurring the
expense of countershaft as with a belt.
[Illustration: FIG. 453.--Watson vertical motor designed
to operate a vertical shaft, either through belt connection, or by
direct drive. Hess-Bright ball bearings are used, taking the downward
thrust due to the weight of the armature. For mounting on the floor or
ceiling, a tripod base (as shown) is furnished, the standard sliding
base being used on a side wall. The armature shaft may be extended for
pulley or coupling either above or below the motor.]
=Friction Drive.=--This is a very simple mode of transmitting
power and has the advantages of simplicity and compactness. In
operation, the driving wheel is pressed against the wheel to be driven,
transmitting motion to the latter by the frictional grip. The drive is
thrown out of gear by slightly moving the machine on its sliding base.
In construction, the friction may be increased by making one wheel of
the pair of wood, compressed paper, or leather.
=Electrical Connections.=--Circuits for dynamos and motors should
be carefully planned so as to secure the simplest arrangement, and to
avoid unnecessary expense and delay, the wiring should be installed in
accordance with the requirements of the National Electrical Code.
[Illustration: FIG. 454.--Sling for handling armatures. In
raising an armature it should be supported by the shaft to avoid any
strain on the armature body or commutator.]
=Ques. What may be said with respect to exposed and concealed
wiring?=
Ans. Exposed wiring is cheap and accessible; a short circuit or ground
is easily located and repaired. Concealed wiring, especially when
placed under the floor, has the advantage of being out of the way, and
thus protected from injury.
=Ques. In wiring a dynamo what are the considerations with respect to
size of wire?=
Ans. All conductors, including those connecting the machine with the
switchboard, as well as the bus bars on the latter, should be of ample
size to be free from overheating and excessive loss of voltage. The
drop between the generator and switchboard should not exceed one-half
per cent. at full load, because it interferes with proper regulation
and adds to the less easily avoided drop on the distribution system.