Chapter LII: Transformers

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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._

~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.

~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.

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.

~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_.

~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."

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.

~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.

~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.

~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.

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.

~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.

~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.

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.

~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.

~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.

~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.

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).

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.

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.

~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.

~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.

~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.

~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.

~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.

~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.

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._

~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.

~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.

~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.

~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.

~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.

~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.

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.

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.

~Ques. How much circulating water is required?~

Ans. It depends upon the difference between the initial and discharge temperatures of the circulating water.

~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.

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.

~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.

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.

~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.

~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.

~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.

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.

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.

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.

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.

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.

~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.

~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?~

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.

~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.

~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.

~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.

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.

~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.

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.

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.

~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.

~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.

_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.

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.

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.

~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.

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.

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.

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.

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.

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.

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.

~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.

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 | | | |
|------------------+-----------+-----------+-----------+-----------+
| | Multiple | 1,100 | 10:1 | 5:1 |
|------------------+-----------+-----------+-----------+-----------+
| | Multiple | 1,100 | 9.05:1 | 4.52:1 |
|------------------+-----------+-----------+-----------+-----------+
| | Series | 2,200 | 20:1 | 10:1 |
|------------------+-----------+-----------+-----------+-----------+
| | Series | 2,200 | 19.05:1 | 9.5:1 |
|------------------+-----------+-----------+-----------+-----------+
| | 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. */

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Hawkins Electrical Guide v. 06 (of 10)Chapter LII: Transformers

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