TRANSFORMERS
The developments in the field of electrical engineering which have
rendered feasible the transmission of high pressure currents over long
distances, together with the reliability and efficiency of modern
generating units, have resulted in notable economies in the generation
and distribution of electric current.
This has been accomplished largely by the use of distant water power or
the centralization of the generating plants of a large territory in a
single power station.
The transformer is one of the essential factors in effecting the
economical distribution of electric energy, and may be defined as _an
apparatus used for changing the voltage and current of an alternating
circuit_. A transformer consists essentially of:
1. A primary winding;
2. A secondary winding;
3. An iron core.
~Basic Principles.~--If a current be passed through a coil of wire
encircling a bar of soft iron the iron will become a magnet; when the
current is discontinued the bar loses its magnetization.
_Conversely:_ If a bar of iron carrying a coil of wire be magnetized
in a direction at right angles to the plane of the coil a momentary
electric pressure will be induced in the wire; if the current be
reversed, another momentary pressure will be induced in the opposite
direction in the coil.
These actions are fully explained in chaps. X and XI, and as they are
perfectly familiar phenomena, a detailed explanation of the principles
upon which they depend is not necessary here.
From the first two statements given above it is evident that
if a bar of iron be provided with two coils of wire, one of
which is supplied from a source of alternating current, as
shown diagrammatically by fig. 1,916, at each impulse of the
exciting current a pressure will be induced in the secondary
coil, the direction of these impulses alternating like that of
the exciting current.
~Ques. What name is given to the coil through which current from the
source flows?~
Ans. _The primary winding._
[Illustration: Fig. 1,916.--Diagram of elementary transformer with
non-continuous core and connection with single phase alternator. The
three essential parts are: primary winding, secondary winding, and an
iron core.]
~Ques. What name is given to the coil in which a current is induced?~
Ans. _The secondary winding._
Similarly, the current from the source (alternator) is called
the _primary current_ and the induced current, the _secondary
current_.
~Ques. What is the objection to the elementary transformer shown in
fig. 1,916?~
Ans. The non-continuous core. With this type core, the flux emanating
from the north pole of the bar has to return to the south pole through
the surrounding air; and as the reluctance of air is much greater than
that of iron, the magnetism will be weak.
~Ques. How is this overcome?~
Ans. By the use of a continuous core as shown in fig. 1,917.
~Ques. Is this the best arrangement, and why?~
Ans. No. If the windings were put on as in fig. 1,917, the leakage of
magnetic lines of force would be excessive, as indicated by the dotted
lines. In such a case the lines which leak through air have no effect
upon the secondary winding, and are therefore wasted.
[Illustration: Fig. 1,917.--Diagram of elementary transformer with
continuous core and connections with alternator. The dotted lines show
the leakage of magnetic lines. To remedy this the arrangement shown in
fig. 1,918 is used.]
~Ques. How is the magnetic leakage reduced to a minimum in commercial
transformers?~
Ans. In these, and even in ordinary induction coils (the operating
principle of which is the same as that of transformers) the magnetic
leakage is reduced to the lowest possible amount by arranging the coils
one within the other, as shown in cross section in fig. 1,918.
~The Induced Voltage.~--The pressure induced in the secondary winding
will depend on the _ratio_ between the number of turns in the two
windings. For example, a transformer with 500 turns of wire in its
primary winding and 50 turns in its secondary winding would have a
transformation ratio of 10 to 1, and if it were supplied with primary
current at 1,000 volts, the secondary pressure at no load would be 100
volts.
[Illustration: Fig. 1,918.--Cross section showing commercial
arrangement of primary and secondary windings on core. One is
superposed on the other. This arrangement compels practically all of
the magnetic lines created by the primary winding to pass through the
secondary winding.]
EXAMPLE.--If ten amperes flow in the primary winding and the
transformation ratio be 10, then 10 × 10 = 100 amperes will
flow through the secondary winding.
Thus, a direct proportion exists between the pressures and
turns in the two windings and an inverse proportion between the
amperes and turns, that is:
_primary voltage: secondary voltage = primary turns: secondary turns_
_primary current: secondary current = secondary turns: primary turns_
From the above equations it is seen that the watts of the primary
circuit equal the watts of the secondary circuit.
~Ques. Are the above relations strictly true, and why?~
Ans. No, they are only approximate, because of transformer losses.
In the above example, the total wattage in the primary circuit
is 1,000 × 10 = 10 kw., and that in the secondary circuit
is 100 × 100 = 10 kw. Hence, while both volts and amperes
are widely different in the two circuits, the watts for each
are the same in the ideal case, that is, assuming perfect
transformer action or 100% efficiency. Now, the usual loss in
commercial transformers is about 10%, so that the actual watts
delivered in the secondary circuit is (100 × 100) × 90% = 9 kw.
[Illustration: FIG. 1,919.--Wagner transformer coil formed, ready for
taping. These are known as "pan cake" coils. They are wound with flat
cotton covered copper strip. In heavy coils, several strips in parallel
are used per turn in order to facilitate the winding and produce a more
compact coil.]
~The No Load Current.~--When the secondary winding of a transformer is
open or disconnected from the secondary circuit no current will flow in
the winding, but a very small current called the _no load current_ will
flow in the primary circuit.
The reason for this is as follows: The current flowing in
the primary winding causes repeated reversals of magnetic
flux through the iron core. These variations of flux induce
pressures in both coils; that induced in the primary called the
_reverse pressure_ is opposite in direction and very nearly
equal to the impressed pressure, that is, to the pressure
applied to the primary winding. Accordingly the only force
available to cause current to flow through the primary winding
is the difference between the impressed pressure and reverse
pressure, the _effective pressure_.
[Illustration: FIG. 1,920.--Wagner coils with insulation ready for core
assembly. The flat coils, sometimes called pancake coils are wound
of flat, cotton covered, copper strip with ample insulation between
layers. In heavy coils several flat strips in multiple are used per
turn in order to facilitate the winding and produce a more compact
coil. In many cases normal current flow per high tension coil is very
low and could be carried with a very small cross sectional area of
copper; however, flat strip is almost always used on account of the
increased mechanical stability thus obtained.]
~The Magnetizing Current.~--The magnetizing current of a transformer
is sometimes spoken of as that current which the primary winding takes
from the mains when working at normal pressure. The _true magnetizing
current_ is only that component of this total no load current which is
in quadrature with the supply pressure. The remaining component has
to overcome the various iron losses, and is therefore an "in phase"
component. The relation between these two components determines the
power factor of the so called "magnetizing current."
[Illustration: FIGS. 1,921 and 1,922.--Assembled coils of Westinghouse
10 and 15 kva. transformers; views showing ventilating ducts.]
The true magnetizing component is small if the transformer be well
designed, and be worked at low flux density.
~Action of Transformer with Load.~--If the secondary winding of a
transformer be connected to the secondary circuit by closing a switch
so that current flows through the secondary winding, the transformer is
said to be _loaded_.
The action of this secondary current is to oppose the
magnetizing action of the slight current already flowing in the
primary winding, thus decreasing the maximum value reached by
the alternating magnetic flux in the core, thereby decreasing
the induced pressure in each winding.
The amount of this decrease, however, is _very small_,
inasmuch as a very small decrease of the induced pressure in
the primary coil greatly increases the difference between the
pressure applied to the primary coil and the opposing pressure
induced in the primary coil, so that the primary current is
greatly increased. In fact, _the increase of primary current
due to the loading of the transformer is just great enough
(or very nearly) to exactly balance the magnetizing action of
the current in the secondary coil;_ that is, the flux in the
core must be maintained approximately constant by the primary
current whatever value the secondary current may have.
When the load on a transformer is increased, the primary of the
transformer automatically takes additional current and power
from the supply mains in direct proportion to the load on the
secondary.
When the load on the secondary is reduced, for example by
turning off lamps, the power taken from the supply mains by
the primary coil is automatically reduced in proportion to the
decrease in the load. This automatic action of the transformer
is due to the balanced magnetizing action of the primary and
secondary currents.
[Illustration: FIG. 1,923.--Rear view of Fort Wayne distributing
transformer, showing hanger irons for attaching to pole cross arm.]
~Classification of Transformers.~--As in the case of motors, the great
variety of transformer makes it necessary that a classification, to be
comprehensive, must be made from several points of view, as:
1. With respect to the transformation, as
_a._ Step up transformers;
_b._ Step down transformers.
2. With respect to the arrangement of the coils and magnetic circuit, as
_a._ Core transformers;
_b._ Shell transformers;
_c._ Combined core and shell transformers.
3. With respect to the kind of circuit they are to be used on, as
_a._ Single phase transformers;
_b._ Polyphase transformers.
4. With respect to the method employed in cooling, as
_a._ Dry transformers;
_b._ Air cooled transformers {natural draught;
{forced draught, or air blast;
_c._ Oil cooled transformers;
_d._ Water cooled transformers.
5. With respect to the nature of their output, as
_a._ Constant pressure transformers;
_b._ Constant current transformers;
_c._ Current transformers;
_d._ Auto-transformers.
6. With respect to the kind of service, as
_a._ Distributing;
_b._ Power.
7. With respect to the circuit connection that the transformer
is constructed for, as
_a._ Series transformers;
_b._ Shunt transformers.
~Step Up Transformers.~--This form of transformer is used to transform
a low voltage current into a high voltage current. Such transformers
are employed at the generating end of a transmission line to raise the
voltage of the alternators to such value as will enable the electric
power to be economically transmitted to a distant point.
[Illustration: FIG. 1,924.--Diagram of elementary _step up_
transformer. As shown the primary winding has two turns and secondary
10 turns, giving a ratio of voltage transformation of 10 ÷ 2 = 5.
Since only ⅕ as much current flows in the secondary winding as in the
primary, the latter requires heavier wire than the former.]
~Copper Economy with Step Up Transformers.~--To comprehend
fully the bearing of the matter, it must be remembered that the
energy supplied per second is the product of two factors, the
current and the pressure at which that current is supplied;
the magnitudes of the two factors may vary, but the value of
the power supplied depends only on the product of the two; for
example, the energy furnished per second by a current of 10
amperes supplied at a pressure of 2,000 volts is exactly the
same in amount as that furnished per second by a current of 400
amperes supplied at a pressure of 50 volts; in each case, the
product is 20,000 watts.
Now the loss of energy that occurs in transmission through a
well insulated wire depends also on two factors, the current
and the resistance of the wire, and in a given wire is
proportional to the square of the current. In the above example
the current of 400 amperes, if transmitted through the same
wire as the 10 amperes current, would, because it is forty
times as great, waste sixteen hundred times as much energy in
heating the wire. It follows that, for the same loss of energy,
the 10 ampere current at 2,000 volts may be carried by a wire
having only ¹/₁₆₀₀th of the sectional area of the wire used
for the 400 ampere current at 50 volts.
The cost of copper conductors for the distributing lines is
therefore very greatly economized by employing high pressures
for distribution of small currents.
[Illustration: FIG. 1,925.--Diagram of elementary step down
transformer. As shown the primary winding has 10 turns and the
secondary 2, giving a ratio of voltage transformation of 2 ÷ 10 = .2.
The current in the secondary being 5 times greater than in the primary
will require a proportionately heavier wire.]
~Step Down Transformers.~--When current is supplied to consumers
for lighting purposes, and for the operation of motors, etc.,
considerations of safety as well as those of suitability, require the
delivery of the current at comparatively low pressures ranging from 100
to 250 volts for lamps, and from 100 to 600 volts for motors.
This involves that the high pressure current in the transmission
lines must be transformed to low pressure current at the receiving
or distributing points by _step down transformers_, an elementary
transformer being shown in fig. 1,925.
[Illustration: FIGS. 1,926 and 1,927.--Core type transformer. It
consists of a central core of laminated iron, around which the coils
are wound. A usual form of core type transformer consists of a
rectangular core, around the two long limbs of which the primary and
secondary coils are wound, the low tension coil being placed next the
core.]
Transformers of this type have a large number of turns in the primary
winding and a small number in the secondary, in ratio depending on the
amount of pressure reduction required.
~Core Transformers.~--This type of transformer may be defined as one
having an iron core, upon which the wire is wound in such a manner that
the iron is enveloped within the coils, the outer surface of the coils
being exposed to the air as shown in figs. 1,926 and 1,927.
~Shell Transformers.~--In the shell type of transformer, as shown in
fig. 1,928, the core is in the form of a shell, being built around and
through the coils. A shell transformer has, as a rule, fewer turns and
a higher voltage per turn than the core type.
~Ques. What is the comparison between core and shell transformers?~
Ans. The relative advantages of the two types has been the subject of
considerable discussion among manufacturers; the companies who formerly
built only shell type transformers, now build core types, while with
other builders the opposite practice obtains.
[Illustration: FIG. 1,928.--Shell type transformer. In construction
the laminated core is built around and through the coils as shown. For
large ratings this type has some advantages with respect to insulation,
while for small ratings the core type is to be preferred in this
respect. The shell arrangement of the core gives better cooling; with
this arrangement minimum magnetic leakage is easily obtained.]
~Ques. Upon what does the choice between the two types chiefly depend?~
Ans. Upon manufacturing convenience rather than operating
characteristics.
The major insulation in a core type transformer consists of
several large pieces of great mechanical strength, while in
the shell type, there are required an extremely large number
of relatively small pieces of insulating material, which
necessitates careful workmanship to prevent defects in the
finished transformer, when thin or fragile material is used.
Both core and shell transformers are built for all ratings; for
small ratings the core type possesses certain advantages with
reference to insulation, while for large ratings, the shell
type possesses better cooling properties, and has less magnetic
leakage than the core type.
[Illustration: FIG. 1,929.--View illustrating the construction of cores
and coils of Maloney transformers.]
[Illustration: FIG. 1,930.--Maloney mica shield between primary and
secondary coils, showing lapping feature which prevents the wrinkling
and cracking of the mica.]
~Combined Core and Shell Transformers.~--An improved type of
transformer has been introduced which can be considered either as two
superposed shell transformers with coils in common, or as a single core
type transformer with divided magnetic circuit and having coils on only
one leg. It is best considered however, as a combined core and shell
transformer, and for small sizes it possesses most of the advantages
of both types. It can be constructed at less cost than can either a
core or a shell transformer having the same operating characteristics
and temperature limits.
[Illustration: FIGS. 1,931 and 1,932.--The Berry combined core and
shell transformer. It consists of a number of inner and outer vertical
and radial laminated iron blocks built up of the usual thin sheet iron,
with the coils between. The magnetic circuit is completed at the top
and bottom by other laminated blocks placed horizontally, and the whole
is held together between top and bottom cast iron frame plates by a
bolt passing right down the center. Fig. 1,931 gives a general view,
W being the winding, and B, B, B, etc., the outer laminated blocks.
The construction will be better understood from fig. 1,932, where it
may be supposed that the top cap and laminated cross pieces have been
removed. Here I, I, I and O, O, O are respectively the inner and outer
radial vertical blocks, P the primary, and S, S the secondary; the
latter being in two sections with the primary sandwiched between, as an
extra precaution against shock. It will be evident that this form of
transformer possesses excellent ventilation; and this is still further
enhanced by opening out the winding at intervals to leave ventilating
apertures, as at A, A, A. Fig. 1,932 shows only 6 sets of radial
blocks, but the usual plan is to provide 24 or 36, according to the
size of the transformer.]
Fig. 1,932 shows a cross section of the first transformer of this type
to be developed commercially, and known as an "iron clad" transformer;
this construction has been used in England for some time. Fig. 1,933
shows the American practice.
[Illustration: FIG. 1,933.--Plan of core of General Electric combined
core and shell transformer. The core used contains four magnetic
circuits of equal reluctance, in multiple; each circuit consisting of a
separate core. In this construction one leg of each circuit is built up
of two different widths of punchings forming such a cross section that
when the four circuits are assembled together they interlock to form
a central leg, upon which the winding is placed. The four remaining
legs consist of punchings of equal width. These occupy a position
surrounding the coil at equal distances from the center, on the four
sides; forming a channel between each leg and coil, thereby presenting
large surfaces to the oil and allowing its free access to all parts
of the winding. The punchings of each size transformer are all of the
same length, assembled alternately, and forming two lap joints equally
distributed in the four corners of the core, thereby giving a magnetic
circuit of low reluctance.]
~Ques. How is economy of construction obtained in designing combined
core and shell transformers?~
Ans. The cross section of iron in the central leg of the core is made
somewhat less than that external to the coils, in order to reduce the
amount of copper used in the coils.
~Single and Polyphase Transformers.~--A single phase transformer may be
defined as _one having only one set of primary and secondary terminals,
and in which the fluxes in the one or more magnetic circuits are all in
phase_, as distinguished from a polyphase transformer, or combination
in one unit of several one phase transformers with separate electric
circuits but having certain magnetic circuits in common. In polyphase
transformers there are two or more magnetic circuits through the core,
and the fluxes in the various circuits are displaced in phase.
~Ques. Is it necessary to use a polyphase transformer to transform a
polyphase current?~
Ans. No, a separate single phase transformer may be used for each phase.
[Illustration: FIGS. 1,934 and 1,935.--Top view showing core and
coils in place, and view of coils of Westinghouse distributing
transformer. The coils are wound from round wire in the smaller sizes
of transformers and from strap copper in the larger sizes. Strap wound
coils allow a greater current carrying conductor section than coils
wound from large round wire, as there is little waste space between the
different turns of the conductor. The coils are arranged concentrically
with the high tension winding between the two low tension coils, this
arrangement giving the fine regulation found in these transformers. The
low tension coils are wound in layers which extend across the whole
length of the coil opening in the iron, while the high tension coils
are wound in two parts and placed end to end. This construction reduces
the normal voltage strains to a value which will not give trouble
under any condition of service. The magnetic circuit is built up of
laminated, alloy steel punchings, each layer of laminæ being reversed
with reference to the preceding layer and all joints butted. This gives
a continuous magnetic circuit of low reluctance, low iron loss and low
exciting current. When assembled, the magnetic circuit consists of four
separate parallel circuits encircling the coils and protecting the
windings from mechanical injury. Separate high and low tension terminal
blocks of glazed porcelain are mounted upon extensions of the upper
end frames. All danger of confusing the leads or inadvertently making
an electrical connection between the high and low tension sides of the
transformer is thus averted. The high tension winding has four leads
brought to the studs in the terminal block. Adjustable brass connectors
or links between the studs provide for series or multiple connections
between two points of the high tension winding. The position of the
studs and the length of the links are so proportioned that wrong
connections on the block are impossible. Barriers on the porcelain
block separate the studs and prevent danger of arcing. Leads with means
of preventing creeping of oil by capillary action are attached to these
studs and brought out of the core through porcelain bushings.]
~Ques. Is there any choice between a polyphase transformer and separate
single phase transformers for transforming a polyphase current?~
Ans. Yes, the polyphase transformer is preferable, because less iron
is required than would be with the several single phase transformers.
The polyphase transformer therefore is somewhat lighter and also more
efficient.
[Illustration: FIGS. 1,936 and 1,937.--Core and shell types of three
phase transformer. In the core type, fig. 1,936, there are three cores
A, B, and C, joined by the yokes D and D'. This forms a three phase
magnetic circuit, since the instantaneous sum of the fluxes is zero.
Each core is wound with a primary coil P, and a secondary coil S. As
shown, the primary winding of each phase is divided into three coils
to ensure better insulation. The primaries and secondaries may be
connected _star_ or _mesh_. The core B has a shorter return path than
A and C, which causes the magnetizing current in that phase to be less
than in the A and C phases. This has sometimes been obviated by placing
the three cores at the corners of an equilateral triangle (as in figs.
1,939 and 1,940), but the extra trouble involved is not justified, as
the unbalancing is a no load condition, and practically disappears
when the transformer is loaded. The shell type, fig. 1,937, consists
practically of three separate transformers in one unit. The flux paths
are here separate, each pair of coils being threaded by its own flux,
which does not, as in the core type, return through the other coils.
This gives the shell type an advantage over the core type, for should
one phase burn out, the other two may still be used, especially if the
faulty coils be short circuited. The effect of such short circuiting is
to prevent all but a very small flux from threading the faulty coil.]
~Ques. Name two varieties of polyphase transformer?~
Ans. The core, and the shell types as shown in figs. 1,936 and 1,937.
~Ques. How should a three phase transformer be operated with one phase
damaged?~
Ans. The damaged windings should be separated electrically from the
other coils.
The pressure winding of the damaged phase should be short
circuited upon itself and the corresponding low pressure
winding should also be short circuited upon itself. The winding
thus short circuited will choke down the flux passing through
the portion of the core surrounded by them without producing
in any portion of the winding a current greater than a small
fraction of the current which would normally exist in such
portion at full load.
~Transformer Losses.~--As previously mentioned, the ratio between
the applied primary voltage and the secondary terminal voltage of a
transformer is not always equal to the ratio of primary to secondary
turns of wire around the core.
The commercial transformer is not a perfect converter of energy, that
is, the ~input~, or watts applied to the primary circuit is always more
than the ~output~ or watts delivered from the secondary winding.
[Illustration: FIG. 1,938.--Interior of General Electric oil cooled 500
kva. 33,000 volt outdoor transformer showing lifting arrangement.]
This is due to the various losses which take place, and the difference
between the input and the output is equal to the sum of these losses.
They are divided into two classes:
1. The ~iron~ or ~core~ losses;
2. The ~copper~ losses.
The iron or core losses are due to
1. Hysteresis;
2. Eddy currents;
3. Magnetic leakage (negligibly small).
[Illustration: FIGS. 1,939 and 1,940.--Triangular arrangements of cores
of three phase transformer. Fig. 1,939, form with three cornered yokes
at bottom and top of cores; fig. 1,940, form with circular yokes. While
these designs give perfect symmetry for the three phases, there is some
trouble in the mechanical arrangement of the yokes. If these be stamped
out triangularly and inserted horizontally between the three cores, it
is necessary to interpose a layer of insulation, otherwise there would
be objectionable eddy currents formed in the stampings.]
Those which are classed as copper losses are due to
1. Heating the conductors (the I²R loss);
2. Eddy currents in conductors.
~Hysteresis.~--In the operation of a transformer the alternating
current causes the core to undergo rapid reversals of magnetism. This
requires an expenditure of energy which is converted into heat.
[Illustration: FIG. 1,941.--View showing mechanical construction of
coil and core of Moloney pole type ½ to 50 kw. transformer. Moloney
standard transformers of these sizes are regularly wound for 1,100 to
2,200 primary volts. For 1,100 volts the primary coils are connected
in parallel by means of connecting links; for 2,200 volts, they are
connected in series. The porcelain primary terminal board is provided
with two connecting links so that connections can be made for either
1,100 or 2,200 volts.]
This loss of energy as before explained is due to the work required
to change the position of the molecules of the iron, in reversing the
magnetization. Extra power then must be taken from the line to make up
for this loss, thus reducing the efficiency of the transformer.
~Ques. Upon what does the hysteresis loss depend?~
Ans. Upon the quality of the iron in the core, the magnetic density at
which it is worked and the frequency.
~Ques. With a given quality of iron how does the hysteresis loss vary?~
Ans. It varies as the 1.6 power of the voltage with constant frequency.
~Ques. In construction, what is done to obtain minimum hysteresis loss?~
Ans. The softest iron obtainable is used for the core, and a low degree
of magnetization is employed.
[Illustration: FIG. 1,942.--Fort Wayne transformer coils and core
complete.]
[Illustration: FIG. 1,943.--Top view of Fort Wayne (type A) transformer
cover removed, showing assembly of coils and core and disposition of
leads.]
~Eddy Currents.~--The iron core of a transformer acts as a closed
conductor in which small pressures of different values are induced in
different parts by the alternating field, giving rise to eddy currents.
Energy is thus consumed by these currents which is wasted in heating
the iron, thus reducing the efficiency of the transformer.
~Ques. How is the loss reduced to a minimum?~
Ans. By the usual method of laminating the core.
The iron core is built up of very thin sheet iron or steel
stampings, and these are insulated from each other by varnish
and are laid face to face at right angles to the path that the
eddy currents tend to follow, so that the currents would have
to pass from sheet to sheet, through the insulation.
~Ques. In practice, upon what does the thickness of the laminæ or
stampings depend?~
Ans. Upon the frequency.
The laminæ vary in thickness from about .014 to .025 inch,
according as the frequency is respectively high or low.
[Illustration: FIG. 1,944.--General Electric 10 kva., (type H)
transformer removed from tank. That part of the steel core composing
the magnetic circuit outside of the winding is divided into four
equal sections. Each section is located a sufficient distance from
the winding so that all portions of the winding and core are equally
exposed to the cooling action of the oil. On all except the very
smallest sizes the winding is divided by channels and ducts through
which a continual circulation of oil is maintained. The result is
uniform temperature throughout the transformer, thus eliminating the
detrimental effects of unequal expansion in the coils with consequent
rubbing and abrasion of the insulation.]
~Ques. Does a transformer take any current when the secondary circuit
is open?~
Ans. Yes, a "no load" current passes through the primary.
~Ques. Why?~
Ans. The energy thus supplied balances the core losses.
[Illustration: FIG. 1,945.--Cover construction of Wagner 350 kva., oil
filled 1,100-2,200 volt transformer. In transformers with corrugated
cases, the base and top ring are cast to the corrugated iron sheets.]
~Ques. Are the iron or copper losses the more important, and why?~
Ans. The iron losses, because these are going on as long as the primary
pressure is maintained, and the copper losses take place only while
energy is being delivered from the secondary.
Strictly speaking, on _no load_ (that is when the secondary
circuit is open) a slight copper loss takes place in the
primary coil but because of its smallness is not mentioned.
It is, to be exact, included in the expression "iron losses,"
as the precise meaning of this term signifies _not only the
hysteresis and eddy current losses but the copper loss in the
primary coil when the secondary is open_.
The importance of the iron losses is apparent in noting that in
electric lighting the lights are in use only a small fraction
of the 24 hours, but the iron losses continue all the time,
thus the greater part of each day energy must be supplied to
each transformer by the power company to meet the losses,
during which time no money is received from the customers.
Some companies make a minimum charge per month whether any
current is used or not to offset the no load transformer losses
and rent of meter.
[Illustration: FIGS. 1,946 to 1,948.--Methods of connecting the low
tension sides of Westinghouse transformers using the connectors
illustrated in figs. 1,949 to 1,953.]
~Ques. How may the iron losses be reduced to a minimum?~
Ans. By having short magnetic paths of large area and using iron or
steel of high permeability. The design and construction must keep the
eddy currents as low as possible.
As before stated the iron losses take place continually, and
since most transformers are loaded only a small fraction of a
day it is very important that the iron losses should be reduced
to a minimum.
With a large number of transformers on a line, the magnetizing
current that is wasted, is considerable.
During May, 1910, the U. S. Bureau of Standards issued a
circular showing that each watt saving in core losses was a
saving of 88 cents, which is evident economy in the use of high
grade transformers.
~Copper Losses.~--Since the primary and secondary windings of a
transformer have resistance, some of the energy supplied will be lost
by heating the copper. The amount of this loss is proportional to
square of the current, and is usually spoken of as the I²R loss.
[Illustration: FIGS. 1,949 to 1,953.--Westinghouse low tension
transformer connectors for connecting the low tension leads to the
feeder wires. The transformers of the smaller capacities have knuckle
joint connectors and those of the larger sizes have interleaved
connectors. These connectors form a mechanically strong joint of high
current carrying capacity. Since the high tension leads are connected
directly to the cut out or fuse blocks, connectors are not required on
these leads. The use of these connectors allows a transformer to be
removed and another of the same or a different capacity substituted
usually without soldering or unsoldering a joint. The connectors also
facilitate changes in the low tension connections.]
~Ques. Define the copper losses.~
Ans. The copper losses are the sum of the I²R losses of both the
primary and secondary windings, and the eddy current loss in the
conductors.
~Ques. Is the eddy current loss in the conductors large?~
Ans. No, it is very small and may be disregarded, so that the sum of
the I²R losses of primary and secondary can be taken as the total
copper loss for practical purposes.
~Ques. What effect has the power factor on the copper losses?~
Ans. Since the copper loss depends upon the current in the primary and
secondary windings, it requires a larger current when the power factor
is low than when high, hence the copper losses increase with a lowering
of the power factor.
[Illustration: FIG. 1,954.--Method of bringing out the secondary leads
in Wagner central station transformers. Each primary lead is brought
into the case through a similar bushing. Observe the elimination of all
possibility of grounding the cable on the case or core.]
~Ques. What effect other than heating has resistance in the windings?~
Ans. It causes poor regulation.
This is objectionable, especially when incandescent lights are
in use, because the voltage fluctuates inversely with load
changes, that is, it drops as lamps are turned on and rises
as they are turned off, producing disagreeable changes in the
brilliancy of the lamps.
~Cooling of Transformers.~--Owing to the fact that a transformer is a
stationary piece of apparatus, not receiving ventilation from moving
parts, its efficient cooling becomes a very strong feature of the
design, especially in the case of large high pressure transformers. The
effective cooling is rendered more difficult because transformers are
invariably enclosed in more or less air tight cases, except in very dry
situations, where a perforated metal covering may be permitted.
[Illustration: FIGS. 1,955 and 1,956.--Westinghouse transformer
terminal blocks for high and low tension conductors.]
The final degree to which the temperature rises after continuous
working for some hours, depends on the total losses in iron and copper,
on the total radiating surface, and on the facilities afforded for
cooling.
There are various methods of cooling transformers, the cooling mediums
employed being
1. Air;
2. Oil;
3. Water.
The means adopted for getting rid of the heat which is inevitably
developed in a transformer by the waste energy is one of the important
considerations with respect to its design.
~Ques. What is the behaviour of a transformer with respect to heating
when operated continuously at full load?~
Ans. The temperature gradually rises until at the end of some hours it
becomes constant.
The difference between the constant temperature and that of the
secondary atmosphere is called the temperature rise at full
load. _Its amount constitutes a most important feature in the
commercial value of the transformer._
[Illustration: FIGS. 1,957 to 1,960.--Porcelain bushing for
Westinghouse transformers.]
~Ques. Why is a high rise of temperature objectionable?~
Ans. It causes rapid deterioration of the insulation, increased
hysteresis losses, and greater fire risk.
~Dry Transformers.~--This classification is used to distinguish
transformers using air as a cooling medium from those which employ a
liquid such as water or oil to effect the cooling.
~Air Cooled Transformers.~--This name is given to all transformers
which are cooled by currents of air without regard to the manner in
which the air is circulated. There are two methods of circulating the
air, as by
1. Natural draught;
2. Forced draught, or blast.
~Ques. Describe a natural draught air cooled transformer.~
Ans. In this type, the case containing the windings is open at the top
and bottom. The column of air in the case expands as its temperature
rises, becoming lighter than the cold air on the outside and is
consequently displaced by the latter, resulting in a circulation of air
through the case. The process is identical with furnace draught.
[Illustration: FIGS. 1,961 to 1,963.--Fuse blocks for Westinghouse
transformers. The fuses furnished with the transformers are mounted in
a weather proof porcelain fuse box of special design. The stationary
contacts are deeply recessed in the porcelain and are well separated
from each other. The contacts are so constructed that the plug is held
securely in place by giving it a partial turn after inserting it. When
the plug is in position, the fuse is in sight and its condition can be
noted which eliminates all danger of pulling the fuse while same is
still intact and the transformer is under load.]
~Ques. Describe a forced draught or air blast transformer.~
Ans. The case is closed at the bottom and open at the top. A current of
air is forced through from bottom to top as shown in fig. 1,964 by a
fan.
~Ques. How are the coils best adapted to air cooling?~
Ans. They are built up high and thin, and assembled with spaces between
them, for the circulation of the air.
~Ques. What are the requirements with respect to the air supply in
forced draught transformers?~
Ans. Air blast transformers require a large volume of air at a
comparatively low pressure. This varies from one-half to one ounce
per square inch. The larger transformers require greater pressure to
overcome the resistance of longer air ducts.
[Illustration: FIG. 1,964.--Forced draught or "air blast" transformer.
As is indicated by the classification, this type of transformer is
cooled by forcing a current of air through ducts, provided between the
coils and between sectionalized portions of the core. The cold air
is forced through the interior of the core containing the coils by a
blower, the air passing vertically through the coils and out through
the top. Part of the air is sometimes diverted horizontally through
the ventilating ducts provided in the core, passing off at one side of
the transformer. The amount of air going through the coils, or through
the core, may be controlled independently by providing dampers in the
passages.]
~Ques. How much air is used ordinarily for cooling per kw. of load?~
Ans. About 150 cu. ft. of air per minute.
In forced draught transformers, the air pressure maintained
by the blower varies from ½ to 1½ oz. per square inch. Forced
draught or air blast transformers are seldom built in small
sizes or for voltages higher than about 35,000 volts.
~Oil Cooled Transformers.~--In this type of transformer the coils and
core are immersed in oil and provided with ducts to allow the oil to
circulate by convection and thus serve as a medium to transmit the heat
to the case, from which it passes by radiation.
[Illustration: FIG. 1965.--Looking down into a Wagner central station
transformer, showing the connection board, which provides facility for
varying the ratio of transformation and also for interchanging the
primaries.]
~Ques. Explain in detail the circulation of the oil.~
Ans. The oil, heated by contact with the exposed surfaces of the core
and coils, rises to the surface, flows outward and descends along the
sides of the transformer case, from the outer surface of which the heat
is radiated into the air.
~Ques. How may the efficiency of this method of cooling be increased?~
Ans. By providing the case with external ribs or fins, or by "fluting"
so as to increase the external cooling surface.
[Illustration: FIG. 1966.--Section through Westinghouse ½ kilovolt
ampere type S transformer. FIG. 1967.--Section through Westinghouse 50
kilovolt ampere type S transformer showing large oil ducts.]
~Ques. In what types of transformer is this mode of oil cooling used?~
Ans. Lighting transformers.
In such transformers, the large volume of oil absorbs
considerable heat, so that the rise of temperature is retarded.
Hence, for moderate periods of operation, say 3 or 4 hours,
the average lighting period, the maximum temperature would not
be reached.
~Ques. In what other capacities except that of cooling agent, does the
oil act?~
Ans. It is a good insulator, preserves the insulation from oxidation,
increasing the breakdown resistance of the insulation, and generally
restores the insulation in case of puncture.
[Illustration: FIG. 1,968.--Wagner 300 kva, 4,400 volt three phase oil
cooled transformer. In this type of transformer the case is filled with
oil and fluted so as to increase the cooling surface, an oil drain
valve is screwed to a wrought iron nipple cast into the base, the duct
to which is in such a position as to make it possible not only to drain
all of the oil from the transformer, but when desirable, to draw off a
small quantity from the bottom. Should any moisture be in the oil it is
therefore drawn off first.]
~Ques. What is the special objection to oil?~
Ans. Danger of fire.
~Ques. What kind of oil is used in transformers?~
Ans. Mineral oil.
~Ques. What are the requirements of a good grade of transformer oil?~
Ans. It should show very little evaporation at 212° Fahr., and should
not give off gases at such a rate as to produce an explosive mixture
with the air at a temperate below 356°. It should not contain moisture,
acid, alkali or sulphur compounds.
[Illustration: FIG. 1,969.--Section through Fort Wayne (type A)
transformer showing interior of case, core conductors, and insulation,
also division of laminæ.]
The presence of moisture can be detected by thrusting a red
hot nail in the oil; if the oil "crackle," water is present.
Moisture may be removed by raising the temperature slightly
above the boiling point, 212° Fahr., but the time consumed
(several days) is excessive.
~Water Cooled Transformers.~--A water cooled transformer is one in
which water is the cooling agent, and, in most cases, oil is the
medium by which heat is transferred from the coils to the water. In
construction, pipes or a jacketed casing is provided through which the
cooling water is passed by forced circulation, as shown in figs. 1,970
and 1,971.
[Illustration: FIG. 1,970.--Water cooled transformer with internal
cooling coil, that is, with cooling coil within the transformer case.
In this type, the cooling coil, through which the circulating water
passes, is placed in the top of the case or tank, the latter is
filled with oil so that the coil is submerged. The oil acts simply
as a medium to transfer the heat generated by the transformer to the
water circulating through the cooling coil. In operation a continual
circulation of the oil takes place, as indicated by the arrows, due
to the alternate heating and cooling it receives as it flows past the
transformer coils and cooling coil respectively.]
In some cases tubular conductors are provided for the circulation of
the water.
Water cooled transformers may be divided into two classes, as those
having:
1. Internal cooling coils;
2. External cooling coils.
~Ques. Describe the first named type.~
Ans. Inside the transformer case near the top is placed a coil of
wrought iron pipe, through which the cooling water is pumped. The case
is filled with oil, which by _thermo-circulation_ flows upward through
the coils, transferring the heat absorbed from the coils to the water;
on cooling it becomes more dense (heavier) and descends along the
inside surface of the casing.
[Illustration: FIG. 1,971.--Water cooled transformer with external
cooling coil. In this arrangement the cooling coil is placed in a
separate tank as shown. Here forced circulation is employed for both
the heat transfer medium (oil) and the cooling agent (water), two
pumps being necessary. The cool oil enters the transformer case at
the lowest point and absorbing heat from the transformer coils it
passes off through the top connection leading to the cooling coil and
expansion tank. Since the transformer tank is closed, an expansion tank
is provided to allow for expansion of the oil due to heating. The water
circulation is arranged as illustrated.]
~Ques. How much circulating water is required?~
Ans. It depends upon the difference between the initial and discharge
temperatures of the circulating water.
[Illustration: FIG. 1,972.--Interior of General Electric water cooled
140,000 volt transformer showing cooling coil.]
~Ques. In water cooled transformers how much cooling surface is
required for an internal cooling coil?~
Ans. The surface of the cooling coil should be from .5 to 1.3 sq. in.
per watt of total transformer loss, depending upon the amount of heat
which the external surface of the transformer case will dissipate.
For a water temperature rise of 43° Fahr., 1.32 lbs. of water
per minute is required per kw. of load.
~Transformer Insulation.~--This subject has not, until the last few
years, been given the same special attention that many other electrical
problems have received, although the development of the transformer
from its original form, consisting of an iron core enclosed by coils of
wire, to its present degree of refinement and economy of material, has
been comparatively rapid.
In transformer construction it is obviously very important that
the insulation be of the best quality to prevent burn outs and
interruptions of service.
~Ques. What is the "major" insulation?~
Ans. The insulation placed between the core and secondary (low
pressure) coils, and between the primary and secondary coils.
[Illustration: FIG. 1,973.--Assembled coils of General Electric water
cooled 500 kva., 66,000 volt transformer.]
It consists usually of mica tubes, sometimes applied as sheets
held in place by the windings, when no ventilating ducts are
provided, or moulded to correct form and held between sheets of
tough insulating material where ducts are provided for air or
oil circulation.
~Ques. Describe the "minor" insulation.~
Ans. It is the insulation placed between adjacent turns of the coils.
Since the difference of pressure is small between the adjacent
turns the insulation need not be very thick. It usually
consists of a double thickness of cotton wrapped around each
conductor. For round conductors, the ordinary double covered
magnet wire is satisfactory.
~Ques. What is the most efficient insulating material for transformers?~
Ans. Mica.
It has a high dielective strength, is fireproof, and is the
most desirable insulator where there are no sharp corners.
[Illustration: FIG. 1,974.--Three Westinghouse 20 kva, outdoor
transformers, for irrigation service. These are mounted on a drag
so that they may be readily transported from place to place. 33,000
volts high tension; 2,200 and 440 volts low tension, 50 cycles. These
outdoor transformers are of the oil immersed, self-cooling type and
have been developed to meet the requirements for transformers of
capacities greater or of voltages higher than are usually found in
distribution work. They are in reality distributing transformers for
high voltage, outdoor installations, single or three phase service,
for voltages up to 110,000. Where the magnitude of the load does not
warrant an expensive installation, transformers of the outdoor type are
particularly applicable. The cost of a building and outlet bushings
which is often the item of greatest expense is eliminated where outdoor
type transformers are installed.]
~Oil Insulated Transformers.~--High voltage transformers are insulated
with oil, as it is very important to maintain careful insulation not
only between the coils, but also between the coils and the core. In the
case of high voltage transformers, any accidental static discharge,
such as that due to lightning, which might destroy one of the air
insulated type, might be successfully withstood by one insulated with
oil, for if the oil insulation be damaged it will mend itself at once.
By providing good circulation for the oil, the transformer can get rid
of the heat produced in it readily and operate at a low temperature,
which not only increases its life but cuts down the electric resistance
of the copper conductors and therefore the I²R loss.
~Efficiency of Transformers.~--The efficiency of transformers is _the
ratio of the electric power delivered at the secondary terminals to the
electric power absorbed at the primary terminals_.
Accordingly, the output must equal the input minus the losses. If the
iron and copper losses at a given load be known, their values and
consequently the efficiency at other loads may be readily calculated.
EXAMPLE.--If a 10 kilowatt constant pressure transformer at full load
and temperature have a copper loss of .16 kilowatt, or 1.6 per cent.,
and the iron loss be the same, then its
output 10
efficiency = ------ = -------------- × 100 = 96.9 per cent.
input 10 + .16 + .16
At three-quarters load the output will be 7.5 kilowatts; and as the
iron loss is practically constant at all loads and the copper loss is
proportional to the square of the load, the
output 7.5
efficiency = ------ = --------------- × 100 = 96.8 per cent.
input 7.5 + .16 + .09
The matter of efficiency is important, especially in the case of large
transformers, as a low efficiency not only means a large waste of power
in the form of heat, but also a great increase in the difficulties
encountered in keeping the apparatus cool. The efficiency curve shown
in fig. 1,975, serves to indicate, however, how slight a margin
actually remains for improvement in this particular in the design and
construction of large transformers.
[Illustration: FIG. 1,975.--Efficiency curve of Westinghouse 375 kw.,
transformer. Pressure 500 to 15,000 volts; frequency 60. Efficiencies
at different loads: full load efficiency, 98%; ¾ full load efficiency,
98%; ½ full load efficiency, 97.6%; ¼ full load efficiency, 96.1%;
regulation non-inductive load, 1.4%; load having .9 power factor, 3.3%.]
The efficiency of transformers is, in general, higher than that
of other electrical machines; even in quite small sizes it
reaches over 90 per cent., and in the largest, is frequently as
high as 98.5 per cent.
To measure the efficiency of a transformer directly, by
measuring input and output, does not constitute a satisfactory
method when the efficiency is so high. A very accurate
result can be obtained, however, by measuring separately, by
wattmeter, the core and copper losses.
The core loss is measured by placing a wattmeter in circuit
when the transformer is on circuit at no load and normal
frequency.
The copper loss is measured by placing a wattmeter in circuit
with the primary when the secondary is short circuited, and
when enough pressure is applied to cause full load current to
flow.
If it be desired to separate the load losses from the true
I²R loss, the resistances can be measured, and the I²R loss
calculated and subtracted from the wattmeter reading. The
losses being known, the efficiency at any load is readily found
by taking the core loss as constant and the copper loss as
varying proportionally to the square of the load. Thus,
output
efficiency = --------------- × 100
output + losses
~All Day Efficiency of Transformers.~--This denotes the ratio of the
total watt hour output of a transformer to the total watt hour input
taken over a working day. To compute this efficiency it is necessary to
know the load curve of the transformer over a day. Suppose that this is
equivalent to 5 hours at full load, and 19 hours at no load. Then, if
W₁ be the core loss in watts, W₂ the copper loss at rated load, and W
the rated output,
output = 5 × W,
losses = 5 (W₁ + W₂) + 19 W₁,
input = 5 (W + W₁ + W₂) + 19 W₁,
and the all day efficiency is equal to
5W × 100
--------------------- per cent.
5(W + W₁ + W₂) + 19W₁
Commercial or all day efficiency is a most important point in a good
transformer. The principal factor in securing a high all day efficiency
is to keep the core loss as low as possible. The core loss is
constant--it continues while current is supplied to the primary, while
copper loss takes place only when the secondary is delivering energy.
In general, if a transformer is to be operated at light loads the
greater part of the day, it is much more economical to use one designed
for a small iron loss than for a small full load copper loss.
[Illustration: FIGS. 1,976 and 1,977.--Westinghouse double pole fuse
box; views showing box open with tubes in place, and with tubes
removed.]
~Transformer Fuse Blocks.~--These may be of either the single pole or
double pole type. Fig. 1,976 shows a double pole fuse box opened, and
fig. 1,977, the fuse box opened and the tubes removed. Of the four
wires, W, W, W, W, entering the box from beneath, two are from the
primary mains, and two lead to the primary coil of the transformer.
These wires terminate in metallic receptacles R, R, R, R, in the
porcelain plate P, fig. 1,977, which are bridged over in pairs by fuse
wires placed inside porcelain tubes T, T, as shown in fig. 1,976. These
tubes are air tight except for a small outlet O in each, which fit into
the receptacles B, B, in the porcelain plate and open out at the back
of the block, as shown in fig. 1,977.
The fuse wires are connected between metallic spring tubes S, S, S, S,
which fit into the receptacles R, R, R, R.
If a sudden load or a short circuit occur in the transformer, the
intense heat, accompanying the melting or blowing of the fuse,
causes a rapid expansion of the air inside the tube, so that a strong
blast of air rushes through the outlet O of the tube and immediately
extinguishes the arc.
By this arrangement, sustained arcing is prevented, as the
action of the tube causes the arc to extinguish itself
automatically when the current is interrupted.
The porcelain tubes are held in position by the spring K, and
the primary of the transformer becomes entirely disconnected
from the circuit when the tubes are lifted out.
This form of construction enables the lineman to detach
the tubes from the fuse box, and insert the fuse at his
convenience. Furthermore, when inserting a fuse in a short
circuited line, he does not run the risk of being hurt, as
the heated vapor of the exploding fuse can escape through
the outlet provided for that purpose, and in a predetermined
direction.
The method of attaching the lid not only permits of quick
access to the interior of the box, but enables the lineman to
tighten the joints by means of the thumb screws L, L, so as to
keep the box waterproof.
[Illustration: FIG. 1,978.--Diagram illustrating connections and
principles of auto-transformers as explained in the accompanying text.]
~Auto-transformers.~--In this class of transformer, there is only one
winding which serves for both primary and secondary. On account of its
simplicity it is made cheaply.
Auto-transformers are used where the ratio of transformation
is small, as a considerable saving in copper and iron can be
effected, and the whole transformer reduced in size as compared
with one having separate windings.
Fig. 1,978 illustrates the electrical connections and the
relations between the volts and number of turns.
By using the end wire and tapping in on turn No. 20 a current
at 20 volts pressure is readily obtained which may be used for
starting up motors requiring a large starting current and yet
not draw heavily on the line.
Since the primary is connected directly to the secondary it
would be dangerous to use an auto-transformer on high pressure
circuits. This type of transformer has only a limited use,
usually as compensator for motor starting boxes.
[Illustration: FIGS. 1,979 and 1,980.--Two winding transformer and
single winding or auto-transformer. Fig. 1,979 shows a 200:100 volt
transformer having a 10 amp. primary and a 20 amp. secondary, the
currents being in opposite directions. If these currents be superposed
by using one winding only, the auto-transformer shown in fig. 1,980
is obtained where the winding carries 10 amp. only and requires only
one-half the copper (assuming the same mean length of turn). If R be
the ratio of an auto-transformer, the relative size of it compared
with a transformer of the same ratio and output is ((R - 1) / R):1.
For example, a 10 kw. transformer of 400 volts primary and 300 volts
secondary could be replaced by an auto-transformer of
10 × (1.33 - 1) / 1.33 = 2.5 kw.; or, in other words, the amount
of material used in a 2½ kw. transformer could be used to wind an
auto-transformer of 400:300 ratio and 10 kw. output.]
~Constant Current Transformers for Series Arc Lighting.~--The
principle of the constant current transformer as used for series arc
lighting is readily understood by reference to the elementary diagram
shown in fig. 1,981. A constant alternating current is supplied to
the stationary primary coil which induces a current in the movable
secondary coil. The pressure induced in the coil will depend on the
number of lines of flux which pass through it and by changing its
position in the magnetic field over the primary a variable e.m.f. can
be produced and a constant current maintained in the lighting circuit
when the lamps are turned on or off, or if the resistance of the
circuit be lowered by the consumption of the carbons.
[Illustration: FIG. 1,981.--Elementary diagram illustrating the
principles of constant current transformer as used for series arc
lighting.]
Since the induced currents in the secondary are repelled by the primary
there is a tendency for the secondary coil to jump out of the primary
field, and in case of a very large current due to a short circuit in
the lamp circuit, the secondary current is quickly reduced to normal by
the rapid movement of the coil upward.
By adjusting the counterweight for a given number of amperes required
by the arc, the current will be maintained constant by the movement of
the secondary coil.
The magnetic field produced by the primary must be kept the same by a
constant current from the alternator, therefore, when the lamp load is
increased the primary voltage increases similar to that of an ordinary
series wound direct current dynamo. In other words the alternator and
regulating transformer supply a constant current and variable voltage.
[Illustration: FIG. 1,982.--Mechanism of General Electric air cooled
constant current transformer. It operates on the principle explained in
the accompanying text and is built to supply 25 to 100 arc lamps at 6.6
to 7.5 amperes. The transformers are interchangeable and will operate
on 60 or 125 cycles. The relative positions of the two coils may be
changed in order to regulate the strength of the current more closely,
by shifting the position of the arc carrying the counterbalance by
means of the adjusting screw on it. A dash pot filled with special oil
prevents sudden movements of the secondary coil and keeps the current
through the lamps nearly constant, when they are being cut in or out
of the circuit. In starting up a constant current transformer, it is
necessary to separate the two coils as far as possible and then close
the primary circuit switch and allow the two coils to come together.
If the primary circuit be thrown directly on the generator the heavy
rush of current which will follow due to the two coils being too close
together might injure the lamps.]
Constant current incandescent lighting systems for use in small towns
also use this method for automatically regulating the current.
~Regulation.~--This term applies to the means adopted either to obtain
constancy of pressure or current. In the transformer, regulation is
_inherent_, that is, the apparatus automatically effects its own
regulation. The regulation of a transformer means, _the change of
voltage due to change of load on the secondary_; it may be defined more
precisely as: _the percentage increase in the secondary voltage as the
load is decreased from its normal value to zero_. Thus, observation
should be made of the secondary voltage, at full load and at no load,
the primary pressure being held constant at the normal value.
[Illustration: FIG. 1,983.--General Electric air cooled constant
current transformer. View showing external appearance with case on.]
The regulation is said to be "good" or "close," when this
change is small. In the design of a transformer, good
regulation and low iron losses are in opposition to one another
when the best results are desired in both. A well designed
transformer, however, should give good results, both as to
regulation and iron losses, the relative value depending upon
the class of work it has to do, and size.
~Transformer Connections.~--The alternating current has the advantage
over direct current, in the ease with which the pressure and current
can be changed by different connections of transformers.
On single phase circuits the transformer connections can be varied to
change current and pressure, and in addition on polyphase circuits the
phases can also be changed to almost any form.
~Single Phase Connections.~--The method of connecting ordinary
distributing transformers to constant pressure mains is shown by the
elementary diagram, fig. 1,984, where a transformer of 10 to 1 ratio is
indicated with its primary winding connected to a 1,000 volt main, and
a secondary winding to deliver 100 volts.
[Illustration: FIG. 1,984.--Single phase transformer connection with
constant pressure main.]
[Illustration: FIG. 1,985.--Usual method of single phase transformer
connections for residence lighting with three wire secondaries. A
balancing transformer is connected to the three wire circuit near the
center of distribution as shown.]
Fig. 1,986 shows a transformer with each winding divided into two
sections. Each primary section is wound for 1,000 volts, and each
secondary section for 50 volts. By connecting the entire primary
winding in series, the transformer may be supplied from a 2,000 volt
main, as indicated, and if the secondary winding be also connected
all in series, as shown, the no load voltage will be 100 between the
secondary terminals.
[Illustration: FIG. 1,986.--Diagram of single phase transformer having
primary and secondary windings in two sections, showing voltages per
section with series connections.]
The sections of the primary winding may be connected in parallel to
a 1,000 volt main, and 100 volts obtained from the secondary, or the
primary and secondary windings may be connected each with its two
sections in parallel, and transformations made from 1,000 to 50 volts
as represented in fig. 1,987.
This is a very common method of construction for small
transformers, which are provided with convenient terminal
blocks for combining the sections of each winding to suit the
requirements of the case. When the two sections of either
winding are connected in parallel as shown in fig. 1,987, _care
must be taken to connect_ ~corresponding ends~ _of the two
sections together_.
~Combining Transformers.~--Two or more transformers built to operate
at the same pressure and frequency may be connected together in a
variety of ways; in fact, the primary and secondary terminals may each
be considered exactly as the terminals of direct current dynamos, with
certain restrictions.
[Illustration: FIG. 1,987.--Diagram of single phase transformer with
primary and secondary windings of two sections each, showing voltages
per section with parallel connection.]
~Ques. What are the two principal precautions which must be observed in
combining transformer terminals?~
Ans. The terminals must have the same polarity at a given instant, and
the transformers should have practically identical characteristics.
The latter condition is not absolutely essential, but it
is emphatically preferable. For example, if a transformer,
which has 2 per cent. regulation, be connected in parallel,
as indicated in fig. 1,988, with one which has 3 per cent.
regulation, at no load the transformers will give exactly the
same voltage at the secondary terminals, but at full load one
will have a secondary pressure of, say, 98 volts, while the
other has 97 volts. The result is that the transformer giving
only 97 volts will be subject to a reverse pressure of one
volt from its mate. This will not cause excessive current to
flow backward through the secondary winding of the low voltage
transformer, but it will disturb the phase relations and lower
the power factor and efficiency of the combination. In such a
case it is much better to work the secondary circuits of the
two transformers separately.
In case the transformers have practically the same
characteristics it is necessary, as stated above, to make sure
that the secondary terminals connected together have the same
polarity at a given instant; it is not necessary to find out
definitely what the polarity is, merely that it is the same for
both terminals. This can be easily done as shown in fig. 1,989.
[Illustration: FIG. 1,988.--Diagram showing unlike single phase
transformers in parallel.]
~Ques. What may be said with respect to operating transformer
secondaries in parallel?~
Ans. It is seldom advantageous. Occasionally it may be necessary as a
temporary expedient, but where the load is such as to require a greater
capacity than that of a transformer already installed, it is much
better to replace it by a large transformer than to supplement it by an
additional transformer of its own size.
~Ques. How are the secondaries arranged in modern transformers and why?~
Ans. The secondary windings are divided into at least two sections so
that they may be connected either in series or parallel.
~Ques. Explain how secondary connections are made for different
voltages.~
Ans. If, for instance, the secondary pressure of a transformer having
two sections be 100 volts with the terminals in parallel, as in fig.
1,990, then connecting them in series will give 200 volts at the free
secondary terminals, as indicated in fig. 1,991.
~Ques. What precaution should be taken in connecting secondary sections
in parallel in core type if the two sections be wound on different
limbs of the cores?~
[Illustration: FIG. 1,989.--Method of comparing instantaneous polarity.
Two of the terminals are connected as shown by a small strip of fuse
wire, and then touching the other two terminals together. If the fuse
blows, then the connections must be reversed; if it does not, then they
may be made permanent.]
Ans. It will be advisable to make the connections ample and permanent,
so that there will not be any liability to a difference between the
current flowing in one secondary winding and that flowing through the
other.
~Two Phase Connections.~--In the case of two phase distribution each
circuit may be treated as entirely independent of the other so far as
the transformers are concerned. Two transformers are used, one being
connected to one primary phase and supplying one secondary phase, the
other being connected to the other primary phase and supplying the
other secondary phase as indicated in fig. 1,996, exactly as though
each primary and secondary phase were an ordinary single phase system,
independent of the other phase.
[Illustration: FIGS. 1,990 and 1,991.--Methods of altering the
secondary connections of a transformer having two sections in the
secondary to obtain a different voltage. Fig. 1,990 shows the two
sections in parallel giving say 100 volts; fig. 1,991 shows the two
sections in series giving 200 volts.]
~Ques. Is the above method usually employed?~
Ans. No, the method shown in fig. 1,997 is generally used.
~Three Phase Connections.~--There is not so much freedom in making
three phase transformer connections, as with single or two phase,
because the three phases are inseparably interlinked. However, the
system gives rise to several methods of transformer connection, which
are known as:
1. Star;
2. Delta;
3. Star-delta;
4. Delta-star.
[Illustration: FIGS. 1,992 to 1,995.--Three phase transformer
connections. Fig. 1,992 ~delta~ connection; fig. 1,993 ~star~
connection; fig. 1,994 ~delta star~ connection; fig. 1,995 ~star-delta~
connection.] /#
~Delta Connection.~--In the delta connection both primaries and
secondaries are connected in delta grouping, as in fig. 1,992.
~Star Connection.~--This method consists in connecting both the
primaries and secondaries in star grouping, as in fig. 1,993.
~Delta-star Connection.~--In this method the primaries are connected in
delta grouping and the secondaries in star grouping, as in fig. 1,994.
~Star-delta Connection.~--This consists in connecting the primaries in
star grouping, and the secondaries in delta grouping, as in fig. 1,995.
[Illustration: FIG. 1,996.--Two phase transformer connections. Two
single phase transformers are used and connections made just as though
each phase were an ordinary single phase system.]
~Ques. What advantage has the star connection over the delta
connection?~
Ans. Each star transformer is wound for only 58% of the line voltage.
In high voltage transmission, this admits of much smaller transformers
being built for high pressure than possible with the delta connection.
~Ques. What advantages are obtained with the delta connection?~
Ans. When three transformers are delta connected, one may be removed
without interrupting the performance of the circuit, the two remaining
transformers in a manner acting in series to carry the load of the
missing transformer.
[Illustration: FIG. 1,997.--Two phase transformer connections, with
secondaries arranged for three wire distribution, the primaries
being independently connected to the two phases. In the three wire
circuit, the middle or neutral wire is made about one-half larger than
each of the two outer wires. In fig. 1,996 it makes no difference
which secondary terminal of a transformer is connected to a given
secondary wire, so long as no transformers are used in parallel. For
example, referring to the diagram, the left hand secondary terminal
of transformer, A, could just as well be connected to the lower wire
of the secondary phase, A, and its right hand terminal connected to
the upper wire, the only requirement being that the two pairs of mains
shall not be "mixed;" that is, transformer, A, must not be connected
with one secondary terminal to phase, A and the other to phase, B. In
the case shown by fig. 1,997, there is not quite so much freedom in
making connections. One secondary terminal of each transformer must be
connected to one of the outer wires and the other two terminals must be
both connected to the larger middle wire of the secondary system. It
makes no difference, however, which two secondary terminals are joined
and connected to the middle wire so long as the other terminal of each
transformer is connected to an outer wire of the secondary system.]
The desire to guard against a shut down due to the disabling
of one transformer has led to the extensive use of the delta
connection, especially for the secondaries or low pressure
side.
It should be noted that if one transformer be disabled, the
efficiency of the other two will be greatly reduced. To operate
a damaged three phase transformer, the damaged windings must
be separated electrically from the other coils, the damaged
primary and secondary being respectively short circuited upon
themselves.
~Ques. What kinds of transformers are used for three phase current?~
Ans. Either a three phase transformer, or a separate single phase
transformer for each phase.
[Illustration: FIG. 1,998.--Three wire connections for transformer
having two secondary sections on different legs of the core. If the
secondary terminals be connected up to a three wire distribution, as
here shown diagrammatically, it is advisable to make the fuse, 2,
in the middle wire, considerably smaller than necessary to pass the
normal load in either side of the circuit, because, should the fuse,
1, be blown, the secondary circuit through the section, Sa, will be
open, and the corresponding half of the primary winding, Pa, will
have a much higher impedance than the half of the primary winding,
Pb, the inductance of which is so nearly neutralized by the load on
the secondary winding, Sb. The result will be that the voltage of
the primary section, Pa, will be very much greater than that of the
section, Pb, and as the sections are in series the current must be the
same through both halves of the winding; the drop or difference of
pressure, therefore, between the terminals of Pa will be much higher
than that between the terminals of Pb, consequently, the secondary
voltage of Sb will be greatly lowered and the service impaired. As the
primary winding, Pa, is designed to take only one-half of the total
voltage, the unbalancing referred to will subject it to a considerably
higher pressure than the normal value; consequently, the magnetic
density in that leg of the transformer core will be much higher than
normal, and the transformer will heat disastrously. If the fuse, 2,
in the middle wire be made, say, one-half the capacity of each of the
other fuses, this condition will be relieved by the blowing of this
fuse, and as the lamps in the live circuit would not be anywhere near
candle power if the circuit remained intact, the blowing of the middle
fuse will not be any disadvantage to the user of the lamps. Some makers
avoid the contingency just described by dividing each secondary coil
into two sections and connecting a section on one leg in series with a
section on the other leg of the core, so that current applied to either
pair of the secondary terminals will circulate about both legs of the
core.]
[Illustration: FIGS. 1,999 to 2,002.--Three phase delta, and star
connections using three transformers. There are two ways of connecting
up the primaries and secondaries, one known as the "delta" connection,
and illustrated diagrammatically by fig. 1,999, and the other known
as the "star" connection, and illustrated by fig. 2,001. In both
diagrams the line wires are lettered, A, B and C. Fig. 2,000 shows the
primaries and secondaries connected up delta fashion, corresponding
to fig. 1,999, and fig. 2,002 shows them connected up star fashion,
corresponding to fig. 2,001. In both of the latter sketches the
secondary wires are lettered to correspond with the respective primary
wires. When the primaries are connected up delta fashion, the voltage
between the terminals of each primary winding is the same as the
voltage between the corresponding two wires of the primary circuit,
and the same is true of the secondary transformer terminals and
circuit wires. The current, however, flowing through the transformer
winding is less than the current in the line wire, for the reason
that the current from any one line wire divides between the windings
of two transformers. For example, in figs. 1,999 and 2,000, part of
the current from the line wire, A, will flow from A to B through the
left hand transformer, and part from A to C through the right hand
transformer; if the current in the line wire, A, be 100 amperes, the
current in each transformer winding will be 57.735 amperes. When
transformers are connected up star fashion, as in figs. 2,001 and
2,002, the current in each transformer winding is the same as that
in the line wire to which it is connected, but the voltage between
the terminals of each transformer winding is 57.735 per cent. of the
voltage from wire to wire on the circuit. For example, if the primary
voltage from A to B is 1,000 volts, the voltage at the terminals of
the left hand transformer (from A to J) will be only 577.35 volts,
and the same is true of each of the other transformers if the system
is balanced. These statements apply, of course, to both primary and
secondary windings, from which it will become evident that if the
three transformers of a three phase circuit be connected up star
fashion at the primaries, and delta fashion at the secondaries, the
secondary voltage will be lower than if both sides are connected up
star fashion. For example, if the transformers be wound for a ratio
of 10 to 1, and are connected up with both primaries and secondaries
alike, no matter whether it be delta fashion or star fashion, the
secondary voltage will be one-tenth of the primary voltage; but if the
primaries be connected up star fashion on a 1,000 volt circuit, and
the secondaries be connected up delta fashion, the secondary voltage
will be only 57.735 volts, instead of 100 volts. The explanation of the
difference between the voltage per coil in a delta system and that in
a star system is that in the former each winding is connected directly
across from wire to wire; whereas in the star system, two windings are
in series between each pair of line wires. The voltage of each winding
is not reduced to one-half, however, because the pressures are out of
phase with each other, being 120°, or one-third of a cycle, apart;
consequently, instead of having 500 volts at the terminals of each coil
in fig. 2,001 the voltage is 577.35. The same explanation applies to
the current values in a delta system. The current phase between A and
B, in fig. 1,999, is 120° removed from that in the winding between A
and C; consequently the sum of the two currents, in the wire, A, is
1.732 times the current in each wire; or, to state it the opposite way,
the current in each winding is 57.735% of the current in the wire, A.
It will be well for the reader to remember that in all cases pressures
differing in phase when connected in series, combine according to the
well-known law of the parallelogram of forces; currents differing in
phase, and connected in parallel, combine according to the same law.]
~Ques. What points are to be considered in choosing between three
phase and single phase transformers for the three phase current
transformation?~
Ans. No specific rule can be given regarding the selection of single
phase or three phase transformers since both designs are equally
reliable; local conditions will generally determine which type is
preferable.
The following general remarks may, however, be helpful:
Single phase transformers are preferable where only one
transformer group is installed and where the expense of a spare
transformer would not be warranted. In such installations
the burn out of one phase of a three phase unit would cause
considerable inconvenience for the reason that the whole
transformer would have to be disconnected from the circuit
before repairs could be made.
If single phase transformers be used and connected in delta
on both primary and secondary, the damaged transformer can
be cut out with a minimum amount of trouble and the other two
transformers can be operated at normal temperature open delta
at 58 per cent. of the normal capacity of the group of three
transformers, until the third unit can be replaced.
With a three phase shell type transformer, if both the primary
and secondary be delta connected, trouble in one phase will not
prevent the use of the other two phases in open delta. By short
circuiting both primary and secondary of the defective phase,
and cutting it out of circuit the magnetic flux in that section
is entirely neutralized. This cannot be done, however, with any
but delta connected shell type transformers.
[Illustration: FIG. 2,003.--Diagram showing three wire secondary
connections General Electric (type H) transformer. As will be seen,
the method adopted consists of distributing equally, on each side of
the primary coil, both halves of the secondary winding, so that each
secondary throughout its length is closely adjacent to the entire
primary winding. In order to insure the exact equality of resistance
and reactance in the two secondary windings necessary to obtain perfect
regulation of the two halves, the inside portion of the secondary
winding on one side of the primary coil is connected in series with the
outside portion of that on the other side. As a result, the drop of
voltage in either side of the secondary under any ordinary conditions
of unbalanced load, does not exceed the listed regulation drop. This
particular arrangement is used because it is the simplest and best
method for this construction.]
Where a large number of three phase transformers can be used,
it is generally advisable to install three phase units, the
following advantages being in their favor as compared with
single phase units:
1. Require less floor space than three single phase units;
2. Weigh less than the single phase units;
3. Simpler connections, as only three primary and three
secondary leads are generally brought out;
4. Transformer presents a symmetrical and compact appearance.
~Ques. What is the character of the construction of three phase
transformers?~
Ans. The three phase transformer is practically similar to that of
the single phase, except that somewhat heavier and larger parts are
required for the core structure.
~COMPARISON OF AIR BLAST, WATER COOLED, AND OIL COOLED
TRANSFORMERS~
======================+=======================+=======================
~Air blast type~ | ~Water cooled type~ | ~Oil cooled type~
----------------------+-----------------------+-----------------------
~1. COST~
----------------------+-----------------------+-----------------------
~A. First cost~ | |
Necessarily more |Least expensive of all |Necessarily more
expensive than the |types. |expensive than the air
water cooled type of | |blast and water cooled
similar rating. | |type of similar rating.
| |
~B.~ | |
The installation is |Being heavier than the |Being heavier than the
extremely simple. |air blast type, these |air blast and water
|transformers, as a |cooled type, these
Moisture that may |rule, require heavier |transformers require
have collected on the |apparatus for |heavier apparatus for
surfaces during |installing. Both |installing. Both
transportation |transformer and tank |transformer and tank
or storage should be |should be thoroughly |should be thoroughly
thoroughly dried out. |dried out before being |dried out before being
|filled with oil. |filled with oil.
| |
|The oil is usually |
|supplied in 50 gal. |
|hermetically sealed |
|steel barrels to |
|minimize possibility of|
|moisture during |
|transportation. |
----------------------+-----------------------+-----------------------
~C. Auxiliary | |
apparatus~| |
A duct, or chamber, |In most cases, cooling |Do not require cooling
of considerable size |water may be obtained |water or blower.
is required under the |with sufficient natural|
transformers in order |head. However, there |
to conduct the cooling|are frequent cases in |
air to them. |which it can be |
|obtained only by the |
A blower outfit for |use of pumps. |
supplying air is | |
required. |A system of piping for |
|the cooling water and |
|oil drainage is |
|required, the cost of |
|which depends, of |
|course, on the station |
|layout. |
~D. Maintenance~ | |
An occasional |A water pumping outfit |No air or water
cleaning, for which a |would possibly require |circulation to demand
supply of compressed |a trifle more attention|attention.
air at about 20 lb. |than a blower outfit in|
pressure is |which there are no |
recommended. |valves or piping. |
| |
The blower outfit | |
requires no more care | |
than any other similar| |
apparatus. | |
======================+=======================+=======================
~2. FLOOR SPACE~
----------------------+-----------------------+-----------------------
Always requires space |Extra space only |Only require space for
for cooling apparatus.|required when auxiliary|the transformer as no
|pumping apparatus is |extra apparatus is
|necessary. |necessary
======================+=======================+=======================
~3. LOCATION~
----------------------+-----------------------+-----------------------
As the transformers |Transformers are |Transformers are
are open at the top |completely enclosed but|completely enclosed but
they should not be |location should be such|location should be such
located where there is|that no water will fall|that no water will fall
much dust or dirt nor |on leads or bushings. |on leads or bushings.
where water from any | |
source is liable to | |The building should be
fall on them. | |well ventilated.
| |
| |There is no auxiliary
| |apparatus.
The blower should be |Location of auxiliary |
so situated as to |apparatus will depend |
obtain clean dry air |on the station layout. |
of a temperature not | |
greater than 77° Fahr.| |
======================+=======================+=======================
~4. GENERAL APPEARANCE~
----------------------+-----------------------+-----------------------
Terminal leads may |Leads are brought out |Leads are brought out
be located in the |of the top of the |of the top of the
base and the air |transformers. |transformers.
chamber may be used | |
for conducting and |Water cooling pipes |
distributing the |are connected at the |
connecting wiring. |top in most cases. |
| |
The absence of | |
overhead wiring aids | |
in simplifying the | |
appearance of the | |
station. | |
======================+=======================+=======================
~5. OPERATION~
----------------------------------------------------------------------
Equal reliability in all three types.
While full load efficiencies are practically equal in the three
designs, it is necessary to change the proportion of iron and copper
losses somewhat as the copper loss of the air blast transformer is a
smaller part of its total loss than of the water cooled and oil cooled
types. As a result, the regulation of the air blast transformer is a
trifle better.
======================================================================
~6. GENERAL~
----------------------------------------------------------------------
The above information regarding selection of type is not applicable
to air blast transformers for circuits materially in excess of 33,000
volts.
On account of the great thickness of the solid insulation needed and
the consequent difficulty in radiating heat from the copper, it is
impracticable to design the air blast type for more than this voltage.
The oil immersed designs are therefore recommended for transformers
above 33,000 volts.
Both oil cooled and water cooled types are available for all
voltages, being restricted in this respect only by the limitations of
transmission facilities.[8]
======================================================================
[8] NOTE.--No special foundations are necessary for any type of
transformer other than a good, even floor, having sufficient strength
to support the weight.
[Illustration: FIGS. 2,004 to 2,011.--Connections of standard
transformers. All stock transformers are wound for some standard
transformation ratio, such as 10 to 1, but various leads are brought
out by means of which ratios of 5, 10 and 20 to 1 may be obtained for
one transformer. The figures show the voltage combinations possible
with a standard transformer.]
~Ques. How are transformers connected for four wire three phase
distribution?~
Ans. When the secondaries of three transformers are star connected, a
fourth wire may be run from the neutral point, thus obtaining the four
wire system.
[Illustration: FIG. 2,012.--Method of determining core loss. Connect
voltmeter and wattmeter as shown in the illustration to the low tension
side of the transformer. By means of a variable voltage transformer
bring the applied voltage to the point for which the transformer
is designed. The wattmeter indicates directly the core loss, which
includes a very small loss due to the current in the copper.
~Cautions.~--1. Make sure of the voltage and frequency. The
manufacturers' tabulated statements refer to a definite voltage and
frequency and these have a decided influence upon the core loss. 2. The
high tension circuit must remain open during the test.]
The voltage between any main wire and the neutral will be
57 per cent. of the voltage between any two main wires. For
general distribution this system is desirable, requiring less
copper and greater flexibility than other systems.
Three phase 200 volt motors may be supplied from the main wires
and 115 volt lamps connected between each of the three main
wires and the neutral; if the lamp load be very nearly balanced
the current flowing in the neutral wire will be very small, as
in the case of the ordinary three wire direct current system.
~How to Test Transformers.--~The troubles incident to gas or water
service have their parallels in electric power distribution.
[Illustration: FIG. 2,013.--Method of determining copper loss. Connect
ammeter and wattmeter to high tension side of transformer short circuit
secondary leads, as shown in the illustration, and by means of a
variable voltage, adjust current to the full load value for which the
transformer is intended. The wattmeter reading shows the copper loss at
full load. The full load primary current of any transformer is found
from the following equation.
_full load current = full load watts ÷ primary volts_
EXAMPLE: To find proper full load current on a five kw. 2,200 volt
transformer, divide 5,000 watts by 2,200 volts, the full load current
will then be 2.27 amperes. A slight variation in primary current
greatly increases or decreases the copper loss.
~Remarks.~--Copper loss increases with temperature because the
resistance of the metal rises. Do not overload the current coil of the
wattmeter. For greater accuracy the I²R drop of potential method should
be used.]
Companies engaged in the former, credit a large percentage of their
losses to leaky valves and defective mains. The remedy may involve
heavy expense and the loss is often tolerated as the lesser of two
evils.
In electric power distribution the transformer takes in part the place
of the valve and pipe system. An inferior or defective transformer
usually treats both the central station and its customers badly, being
in this respect more impartial than the gas or water pipe which may
annoy but one of the interested parties at a time.
Like a neglected or defective gas fixture a transformer can
menace life, failing, however, to give the warning the former
gives, and with a more hidden threat on account of its location.
[Illustration: FIG. 2,014.--Diagram of connections for regulation
test. Connect transformer under test to high tension supply circuit.
A second transformer with same or other known change ratio is also
to be connected up, as illustrated. By means of a double pole double
throw switch, the voltmeter can be made to read the pressure on the
secondary of either transformer. Supposing the same change ratio it
is evident that if both remain unloaded the voltmeter will indicate
the same pressure. A gradually increasing lamp load up to the limit
of the transformer capacity, will be attended by a drop in pressure
at the terminals. This drop can be read as the difference of the
voltmeter indications, and when expressed in per cent. of secondary
voltage stands for "regulation." ~Remarks:~ The auxiliary transformer
is necessary in order to make sure of the high tension line voltage. A
large transformer under test may cause primary drop in taking power.
This must be set down against it in testing regulation. The second
transformer gives notice of such drop, whatever be the cause. Figs.
2,012 to 2,014 used by courtesy of the Moloney Electric Co.]
Apart from this, corresponding to an exasperated customer who
complains at home and to his friends of dim lamps, blackened
lamps, you will find in the power station the manager,
who, also worried and in no better humor, contemplates the
difference in meter readings at the end of the line.
His business does not increase and would not increase even if
he could lower the rates, which he cannot do because of these
meter readings.
He may be confident of his engines and generators, and that
his line is up and all right, but he very seldom knows what
the transformers are doing on top of the poles. Perhaps he
feels that this waste is so slight that it makes no material
difference. This can be readily ascertained by means of a set
of testing instruments.
Perhaps the transformers were purchased because of their attractive
prices and never tested.
Water, plumbing, gas and steam fittings are subjected to test. Why not
transformers? Even more so because transformers take constant toll from
the company installing them, while gas and water fittings, once passed,
are off the contractor's hands.
The busy manager has little time for complicated treatises and
monographs on electrical measurements and even handbooks confront
him with forbidding formulæ. Accordingly the methods of transformer
testing, which are very simple, are illustrated in the accompanying
cuts. Managers of electric power and lighting companies should study
them carefully.
[Illustration: FIG. 2,015.--Wagner central station core type
transformer repair unit consisting of one half set of primary and
secondary windings together with the section of the iron core upon
which the coils are wound.]
An ammeter, voltmeter and wattmeter are required to make the tests.
Losses are small in ~good~ transformers and hence the instruments
~should be accurate~. For the same reason instruments should be chosen
of the proper capacity to give their best readings. If there be any
doubt about the testing instruments being correct, they should be
calibrated before being used. The testing circuits should be properly
fused for the protection of the instruments. It is hardly logical, but
a very common practice is to mistrust meters and to watch them closely,
while the transformers are guilty of theft unchallenged, and keep
busily at it on a large scale.
[Illustration: FIG. 2,016.--Moloney tubular air draft oil filled
transformer. The case is made of cast iron, with large steel tubes
passing from the bottom through the top. In operation the air in the
tubes becomes hot and expands; a draft is thus produced which carries
away considerable heat.]
~Transformer Operation with Grounded Secondary.~--The operation of a
transformer with a grounded secondary has been approved by the American
Institute of Electrical Engineers, and by the National Board of Fire
Underwriters.
This method of operation effectually prevents a high voltage occurring
upon the low tension wires in case of a breakdown or other electrical
connections occurring between the primary and secondary windings.
[Illustration: FIG. 2,017.--Moloney pressure transformer adapted for
switchboard work in connection with voltmeters, wattmeters the, etc.,
in sizes from 25 to 500 watts.]
In case of a breakdown without the secondary grounded, any one touching
a part of the low tension system, such as a lamp socket, might receive
the full high pressure voltage. With the low tension grounded, the fuse
in the high tension circuit will blow and the fault be discovered upon
replacing it.
~Transformer Capacity for Motors.~--The voltage regulation of a well
designed transformer is within 3 per cent. of its rated voltage on a
non-inductive load such as incandescent lamps, but when motors are
connected to the circuit their self-induction causes a loss of 5 per
cent. or more, and if the load be fluctuating, it is better to use
independent transformers for the motor, which will prevent considerable
fluctuations in the incandescent lamps. Arc lamps do not show slight
voltage changes as much as incandescent lamps. The proper rating of
transformers for two phase and three phase induction motors is given in
table on the next page.
[Illustration: FIG. 2,018.--Moloney current transformer switchboard or
indoor type. It is used ordinarily for insulating an ammeter, a current
relay, the current coil of a watt meter or watt hour meter from a high
tension circuit, for reducing the line current to a value suitable for
these instruments.]
A three phase induction motor may be operated from three single phase
transformers or one three phase transformer. While the one three phase
transformer greatly reduces the space and simplifies the wiring, the
use of three single phase transformers is more flexible and, in case
one transformer burns out, the connection can be readily changed so
that two transformers will operate the motor at reduced load until the
burned out transformer is replaced or repaired.
[Illustration: FIG. 2,019.--Diagram showing a method of operating a
three phase motor on a two phase circuit, using a transformer having
a tap made in the middle of the secondary winding, so as to get the
necessary additional phase. While this does not give a true balanced
three phase secondary, it is close enough for motor work. In the above
arrangement, the main transformer supplies 54 per cent. of the current
and the other with the split winding 46 per cent.]
It is well to allow one kilowatt per horse power of the motor in
selecting the size for the transformers, excepting in the small sizes
when a little larger kilowatt rating is found to be the most desirable.
~Transformers for Two and Three Phase Motors~
-----------+-------------------------------------------------
| Single phase transformer voltages
Delivered +------------------------+------------------------
voltage of | 110 volt motor | 220 volt motor
circuit +-----------+------------+----------+-------------
| Primary | Secondary | Primary | Secondary
-----------+-----------+------------+----------+-------------
1,100 | 1,100 | 122 | 1,100 | 244
2,200 | 2,200 | 122 | 2,200 | 244
-----------+-----------+------------+----------+-------------
Very small transformers should not be used, even when the motor is
large compared to the work it has to do, as the heavy starting current
may burn them out.
The following tables give the proper sizes of transformer for three
types of induction motor and the approximate current taken by three
phase induction motors at 220 volts.
-----------------------------------------------------------
~Capacities of Transformers for Induction Motors~
---------------+-------------------------------------------
| Kilowatts per transformer
Size of motor +--------------+--------------+-------------
horse power | Two single | Three single | One three
| phase | phase | phase
| transformers | transformers | transformer
---------------+--------------+--------------+-------------
1 | 0.6 | 0.6 |
2 | 1.5 | 1.0 | 2.0
3 | 2.0 | 1.5 | 3.0
5 | 3.0 | 2.0 | 5.0
7 | 4.0 | 3.0 | 7.5
10 | 5.0 | 4.0 | 10.0
15 | 7.5 | 5.0 | 15.0
20 | 10.0 | 7.5 | 20.0
30 | 15.0 | 10.0 | 30.0
50 | 25.0 | 15.0 | 50.0
75 | 40.0 | 25.0 | 75.0
100 | 50.0 | 30.0 | 100.0
---------------+--------------+--------------+-------------
~Current taken by Three Phase Induction Motors at 220 Volts~
-------------+-----------------+------------+----------------
Horse power |Approximate full |Horse power |Approximate full
of motor | load current | of motor |load current
-------------+-----------------+------------+----------------
1 | 3.2 | 20 | 50.
2 | 6.0 | 30 | 75.
3 | 9.0 | 50 | 125.
5 | 14.0 | 75 | 185.
10 | 27.0 | 100 | 250.
15 | 40.0 | 150 | 370.
-------------+-----------------+------------+----------------
~Transformer Connections for Motors.~--Fig. 2,020 shows the connection
of a three phase so called delta connected transformer with the three
primaries connected to the lines leading from the alternator and the
three secondaries leading to the motor.
The connections for a three phase motor using two transformers is shown
in fig. 2,021 and is identical with the previous arrangement, except
that one transformer is left out and the other two made correspondingly
larger.
The copper required in any three wire three phase circuit for a given
power and loss is 75 per cent. that necessary with the two wire single
phase or four wire two phase system having the same voltage between
lines.
[Illustration: FIG. 2,020.--Three phase motor transformer connections;
the so-called Delta connected transformers.]
The connections of three transformers for a low tension system of
distribution by the four wire three phase system are shown in fig.
2,022. The three transformers have their primaries joined in delta
connection and the secondaries in "~Y~" connection. The three upper
lines of the secondary are the three main three phase lines, and the
lowest line is the common neutral.
[Illustration: FIG. 2,021.--Three phase motor connections using two
transformers.]
The voltage across the main conductors is 200 volts, while
that between either of them and the neutral is 115 volts; 200
volt motors should be joined to the mains while 115 volt lamps
are connected between the mains and neutral. The arrangement
is similar to the Edison three wire system and the neutral
carries current only when the lamp load is unbalanced.
The voltage between the mains should be used in calculating the
size of wires, and the size of the neutral wire should be made
in proportion to each of the main conductors that the lighting
load is to the total load.
[Illustration: FIG. 2,022.--Delta-star connection of three transformers
for low pressure, three phase, four wire system.]
When lights only are used the neutral should be the same as the
main conductors. The copper required in such a system for a
given power and loss is about 33.3 per cent. as compared with
a two wire single phase system or a four wire two phase system
using the same voltage.
[Illustration: FIG. 2,023.--Diagram of transformer connections for
motors on the monocyclic system.]
~Monocyclic Motor System.~--Motors on the monocyclic system are
operated from two transformers connected as shown in fig. 2,023. In
the monocyclic system the single phase current is used to supply the
lighting load and two wires only are necessary, but if a self-starting
induction motor be required, a third or _teaser wire_ is brought to the
motor and two transformers used.
The teaser wire supplies the quarter phase current required to start
the motor, which afterwards runs as a single phase synchronous motor
and little or no current flows through the teaser circuit as long as
the motor keeps in synchronism; in case it fall behind, the teaser
current tends to bring it up to speed instead of the motor stopping, as
would be the case of a single phase motor.
[Illustration: FIG. 2,024.--Moloney flaming auto type arc lamp
transformer for 110 volts primary to 55 volts secondary. A hook in
bottom of case provides means for suspension of lamp. The transformer
may be operated on circuits from 100 to 120 volts primary, 50 to 60
volts secondary. The secondary capacity is 8 to 12 amperes.]
The voltage of the transformers should be tested by means of a
voltmeter or two incandescent lamps joined in series, before starting
up the motor, to see if the proper transformer connections have been
made and prevent an excessive flow of current.
If one of the transformers be reversed the voltage will be
almost doubled; in fact, it is a good plan to check up all the
transformer connections with the voltmeter or lamps which will
often save a burn out.
+------------------+-----------+-----------+-----------------------+
| | | |Ratio of transformation|
| | | | at no load |
| | | +-----------+-----------+
| Arrangement of | Primary | For | with | with |
| links on the | coils | circuit | secondary | secondary |
| connecting board | will be | voltage | coils in | coils in |
| | connected | normal at | multiple | series. |
| | in | | | |
|------------------+-----------+-----------+-----------+-----------+
| [ILLUSTRATION] | Multiple | 1,100 | 10:1 | 5:1 |
|------------------+-----------+-----------+-----------+-----------+
| [ILLUSTRATION] | Multiple | 1,100 | 9.05:1 | 4.52:1 |
|------------------+-----------+-----------+-----------+-----------+
| [ILLUSTRATION] | Series | 2,200 | 20:1 | 10:1 |
|------------------+-----------+-----------+-----------+-----------+
| [ILLUSTRATION] | Series | 2,200 | 19.05:1 | 9.5:1 |
|------------------+-----------+-----------+-----------+-----------+
| [ILLUSTRATION] | Series | 2,200 | 18.1:1 | 9.05:1 |
|------------------+-----------+-----------+-----------+-----------+
FIGS. 2,025 to 2,029.--Diagrams of Wagner transformer connection
board, and table showing various arrangements of the terminal links,
corresponding transformation ratios, and suitable primary voltages. */
[Illustration: FIG. 2,030.--Installation of a transformer on pole;
view showing method of attachment and disposition of the primary and
secondary leads, cutouts, etc.]
[Illustration: FIG. 2,031.--Diagram of static booster or regulating
transformer. It is used for regulating the pressure on feeders. In
the figure, B are the station bus bars, R the regulable transformer,
F the two wire feeders, and T a distant transformer feeding into the
low pressure three wire distributing network N. The two ends of the
primary, and one end of the secondary of R, are connected to the bus
bars as shown. The other end of the secondary, as well as a number of
intermediate points, are joined up to a multiple way switch S, to which
one of the feeder conductors is attached, the other feeder main being
connected to the opposite bus bar. As will be evident from the figure,
by manipulating S extra volts may be added to the bus bar pressure at
will, and the drop along F compensated for. R is a step transformer,
the total secondary difference of pressure being comparatively small.
The above device possesses rather serious drawbacks, in that the switch
S has to carry the main current, and that the supply would be stopped
if the switch got out of order. Kapp improved on the arrangement by
putting the switch in the primary circuit.]