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Table of Contents.
¶ U.S. GOVERNMENT PRINTING OFFICE; 1945  618779
CHAPTER 20
TRANSFORMERS
WHAT FOR
Transformers get their name from the kind of work they dothey TRANSFORM ELECTRICAL POWER. Suppose you had to
supply a galley stove at 220 volts, the ship's lighting system at 110 volts, and a 6volt signalbell system.
Three different generators for three different voltages? Absolutely not! Use transformers, and then one generator
can supply all three loadseach at its proper voltage. Your generator would have a standard output, say 110 volts.
One transformer would STEP this UP to 220 volts. Another would STEP it DOWN to 6 volts.
This sounds like
something for nothingbut it isn't. TRANSFORMERS DO NOT INCREASE OR DECREASE POWER. Remember that power is
voltage multiplied by current. And when a transformer changes voltage it also changes current. If you INCREASE the
voltage, the current is DECREASED. One goes up and the other goes down power INPUT. The VOLTAGE and CURRENT of a
circuit ARE CHANGED BY TRANSFORMERS. But the TOTAL POWER remains the SAME.
HOW IT WORKS
The transformer is a mutual induction circuit. And that should tell you what to expect 
1. There is an iron core.
2. There are TWO electrical circuits.
3. Energy is transformed from one circuit to the other by a field of flux.
Look at figure 218. This is a SIMPLE transformer. Modern transformers are not built this way, but every
important transformer principle can be illustrated by this diagram.
Figure 218.  Simple transformer.
FIRST, notice the core  it forms a closed iron path for flux. This is the path of energy transfer between the
two circuits. The efficiency of a transformer depends on this core and it is always designed to carry the maximum
flux.
SECOND, notice the two circuits  they are formed in coils around the iron of the core. The coil, or
winding, that receives energy from the source is called the PRIMARY. In figure 218, the winding on the left is
connected to the line, or source  it is the primary. The other winding, the one connected to the load, is called
the SECONDARY. In the figure, the winding on the right supplies the load  it is the secondary. Never let the
relative number of turns on the two windings confuse you. The source winding is always the primary and the load
winding is always the secondary. If the load and source connections of figure 218 were exchangedthe names of the
windings would also exchange.
Figure 219.  Transformer flux.
THIRD, notice the flux set up by the transformer primary. Figure 219 shows this better than figure 218. You
know that this is a mutual induction circuit. And you know something else about ityou know that flux lines must
be cut for transfer of energy. Neither the primary nor the secondary coils can move. So the flux field must move.
And that means just one thing  ONLY A.C. and PULSATING D.C. can be used in transformers  they produce moving
fields.
Imagine that 60 cycle a.c. is being fed into the primary of figure 219. A field blossoms out around EVERY
PRIMARY TURN. The iron preserves this field and carries it to the secondary. At the secondary EVERY TURN is cut by
the field. Voltage is induced in the secondary and power has 'been transferred from primary to secondary.
In 60 cycle a.c., you know that current changes direction 120 times each second (two alternations to the cycle).
Which means that the secondary is cut 120 times a second. Therefore, the frequency of the secondary is EXACTLY the
same as the frequency of the primary.
HOW MUCH?
The question, "How much?" always comes up. That's because you are using transformers to change voltage. And
what's the sense of using a device unless you know what you're going to get out of it?
Figure 220 is a
typical transformer setup. Notice the voltmeter readings. They tell the story of transformer voltage. The voltage
of the primary (E
_{p}) is 110 volts. The voltage of the secondary (E
_{s}) is 220 volts. You'll
notice that the secondary is unloaded  an open circuit. Therefore, the secondary current is zero. But, how much
current is there in the primary? Looking at the circuit, you see that the primary is connected directly across a
110 volt line. You'd expect a heavy current. But you're wrong  the voltage of selfinduction (E
_{si}) in
the primary is very high. The tight windings and the iron core cause every turn of the winding to be cut by almost
the total of the coil's flux. The result is that E
_{si}
is almost equal to E
_{p}. Not only equal  but OPPOSITE. And this nearly equal and opposite E
_{si}
chokes the current down to an extremely small value. The small current that does flow is called the MAGNETIZING
CURRENT of a transformer.
Let's make figure 220 an example. The resistance of the coil is 10 ohms. And the
voltage of selfinduction is 109 volts. This gives you these values 
E
_{p} = 1;1.0 v.
E
_{si}
= 109 v.
R = 10 Ω
I
_{p} = (110  109)/10 = 1/10 amp. (by Ohm slaw)
Figure 220.  Stepup transformer.
The magnetizing current is only onetenth of an ampere.
Notice that E
_{si}
was subtracted from Ep. Remember that Ep and E
_{si} are OPPOSITE  they cancel each other, so one must be
subtracted from the other to get THE NET VOLTAGE ACTING ON THE CURRENT. It's like the counteremf in a dc motor.
Now to get back to the why and wherefore of that 220 volts on the secondary. It's like this  the magnetizing
current's flux produced 109 volts of selfinduction on the 11 turns of the primary. That's just slightly under 10
volts per turn. Call it an even 10 volts. Is there any reason why this flux is not producing a like voltage  10
volts per turn  on the secondary winding? No reason in the world  both coils are on the same core, so whatever
flux cuts  one, cuts the other. BUT  and it's a big "but"  the secondary has 22 turns. And at 10 volts per turn
 that's 220 volts.
This tells you that the ratio of voltages is the same as the ratio of turns.
Mathematically it says 
E
_{p}/E
_{s}
= T
_{p}/T
_{s} in which
E
_{p} = voltage in primary;
E
_{s}
= voltage in secondary;
T
_{p} = turns in primary coil;
T
_{s} = turns in secondary coil.
Suppose a
transformer has 600 volts and 2,400 turns on the primary. What will be the voltage of a 400 turn secondary?
E
_{p}/E
_{s}
= T
_{p}/T
_{s} 600/E
_{s} = 2,400/400 = 100 volts.
Here's another way
to work this problem. How many volts per turn in the primary? 600 volts divided by 2,400 turns  1/4 volt per
turn. The volts per turn of the primary and secondary are just about equal, so 400 turns multiplied by 1/4 volt is
100 volts on the secondary. Notice that this transformer is a stepdown job. The voltage goes from 600 volts to
100 volts.
Transformers are highly efficient. As high as 98 percent. That's why you can ignore the difference between E
_{p}
and E
_{si} in calculating volts per turn.
Now put a 44ohm load on the transformer of figure 220.
You have the circuit of figure 221. The current in the secondary is 
I
_{s} = E
_{s}/R
_{s}
= 220/44 = 5 amps.
Right here is a good place for you to get this fact straight. THE SECONDARY CURRENT IS CONTROLLED BY THE
LOAD. If the load had been 22 ohms instead of 44 ohms, the current would have been 10 amperes instead of 5
amperes.
The 5 ampere secondary current is OPPOSITE in direction to the primary current. You can prove
this by the coil hand rule. Or  reason it out this way  the current in the secondary is produced by the voltage
induced by the primary field. You know that E
_{si} is opposite to E
_{p} and you know that both E
_{s}
and E
_{si}
are produced by the same primary field. Therefore, Es must be opposite to E
_{p}. This is the same general
rule you got for mutual induction circuits.
Figure 221.  Loaded transformer.
Now, what is the EFFECT of opposite primary and secondary currents? Simply thisTHEIR FIELDS TEND TO CANCEL
EACH OTHER. The 5 ampere secondary sets up a field which destroys some of the primary flux. Result  less E
_{si}
and more current in the primary. The primary current will increase until the primary field strength balances the
secondary's field strength. Field strengths are determined by ampereturns (IT). When the two fields are equal,
their ampereturns are equal 
I
_{p}T
_{p} = I
_{s}T
_{s}
or I
_{p}/Is = Ts/T
_{p}Which says that, the currents in the two windings are
INVERSELY proportional to their number of turns.
In the example 
I
_{p}/I
_{s}
= T
_{s}/T
_{p}
I
_{p}/5 = 22/11
and I
_{p} = 10 amps.
Which means that the
primary current increases to 10 amperes for a secondary load of 5 amperes.
Suppose another transformer has a
primary of 70 turns, a secondary of 350 turns, and a 30 ampere load. What is the primary current?
I
_{p}/I
_{s}
= T
_{s}/T
_{p} I
_{p}/30 = 350/70
and Ip = 150 amps.
You
can prove this is correct by using the fact that primary and secondary fields are of equal strength.
I
_{p}T
_{p} = I
_{s}T
_{s}
150 x 70 = 350 x 30
10,500 = 10,500
This brings you to an important fact about
transformers. The load current in the SECONDARY controls the current in the PRIMARY. This control is centered in
the action of the secondary flux. Every change of secondary flux changes the E
_{si} on the primary and
consequently the primary current. It is an automatic control. No load on the secondary reduces primary current
almost to zero (only magnetizing current flows). As the secondary is loaded, its current. increases and the
primary current increases with it.
INPUTOUTPUT
Since transformers are so highly efficient, their efficiency is usually considered to be 100 percent. This
assumption introduces a small error but for all practical purposes it is close enough. Considered 100 percent
efficient, the transformer's input power and output power must be equal. Therefore 
P
_{p} = P
I
_{p}E
_{p}
= l
_{s}E
_{s}Let's check this against an example. Figure 222 is a good practical problem.
And these are the things you can solve for  Es, Is, Iv, Ps, and Pv.
Figure 222.  Power in a transformer.
For E
_{s} 
E
_{p}/E
_{s} = T
_{s}/T
_{p}
50/E
_{s} = 250/1000
E
_{s} = 200 v.
For I
_{s} 
I
_{s} = E
_{s}/R
_{s} = 200/8 = 2.5 amps.
For I
_{p}

I
_{p}/I
_{s} = T
_{s}/T
_{p}
I
_{p}/2.5 = 1000/250
I
_{p} = 10 amps.
For P
_{s} 
P
_{s} = E
_{s}I
_{s}
P
_{s} = 200 x 2.5 = 500 w.
For P
_{p} 
P
_{p} = E
_{p}I
_{p}
P
_{p} = 50 x 10 = 500 w.
Note that P
_{p} = P
_{s}.
SUMMARY OF THEORY
A summation of the preceding information gives you three important equations and two important facts.
EQUATIONS 
1. E
_{p}/E
_{s} = T
_{s}/T
_{p} 2. I
_{p}/I
_{s}
= T
_{s}/T
_{p} 3. I
_{p}E
_{p} = I
_{s}E
_{s}FACTS 
1. The voltage of the secondary is always opposite in direction to the voltage of
the primary.
2. The current drawn by the primary is controlled by the secondary load current.
MODERN CONSTRUCTION
Perhaps you are wondering why transformers are so efficient. The reason is their lack of moving parts. When
moving parts are eliminated from a device, mechanical friction, the biggest source of loss, is gone. Losses become
very small.
Modern transformers are designed with just one idea  cut down the losses to as small a value
as possible. In general, the losses that are unavoidable are divided into two classes  IRON losses and COPPER
losses.
Iron losses occur in the core. Part of these losses is due to the resistance of the molecular
magnets. They must turn over every time the a.c. reverses. In a 60cycle transformer, the molecules must shift
around 120 times each second. Molecules resist this shifting; and their desire to stand still is called
HYSTERESIS. You might say that hysteresis is really FRICTION. And you know that friction produces heat.
The iron cores themselves act as wires. They are cut by the flux of their winding's fields, and carry small
induced currents. These induced currents are called EDDY CURRENTS because they flow entirely within the iron core.
Eddy currents produce heat  they are really small short circuits within the core.
Thus, IRON LOSSES are
made up of two factors  HYSTERESIS and EDDY CURRENTS. Both produce heat , and both represent losses which must be
subtracted from the output power.
COPPER LOSSES occur within the windings. They are due to just one thing
 the heat generated by the current in the winding conductors. Now here is an important fact  copper (and most
other conductors) increases its resistance as it gets hotter. This means that if the heat resulting from iron and
copper losses is allowed to accumulate, the windings will get hotter. And the hotter they get, the higher is their
resistance. Which increases the power loss due to resistance. It's a vicious circle. Losses produce heat and heat
produces even higher losses and so on.
IRON LOSSESDOWN
Hysteresis losses are reduced by using either a soft iron or a special transformer steel containing silicon.
These metals allow their molecules to shift easily  with a minimum of friction. Eddy currents are broken up by
constructing the cores out of thin plates of iron instead of a solid piece. This LAMINATED structure breaks up the
eddy currents by insulating each plate from its neighbors. Eddy currents still exist  but they are so small that
the loss they cause can be neglected.
Figure 223.  Transformer cores.
Figure 223 shows a number of transformer cores. Notice that they are all complete magnetic circuits, all are
laminated, and all use iron generously.
COPPER LOSSES  DOWN
Reducing copper losses is a pipe. The windings are designed to be as short and as heavy as possible. Both
short turns and large wire tend to decrease resistance and reduce heat.
Finally, the overall loss in the windings, due to accumulated heat, is cut down by special cooling devices. Oil
baths, radiators, and air blowers are all used to keep transformers cool.
CORE SHAPES
There are three general types of core design. Each has its own special advantage. But all put both primary and
secondary windings on the same leg of iron.
Figure 224 shows all three types. A  the CORE TYPE, is best
for high voltage use. Its winding turns are short and IR drops are at a minimum. B  the SHELL type has longer
turns because the middle leg is twice the size of the outside legs. The IR drops are larger but the flux path is
shorter. Therefore, this type is best for heavy current loads. C  the MODIFIED SHELL type is a combination of
both core and shell. The modified type has some of the advantages of both the simple types.
Figure 224A.  Core type.
Figure 224B.  Shell type.
Figure 224C.  Modified shell type.
WINDING DESIGNS
There are two winding designs which can be used on ANY of the three types of cores. In figure 225, you'll see
crosssections of both types.
Figure 225.  Types of winding design.
A  the CYLINDRICAL design consists of a primary cylinder and a secondary cylinder. One is fitted over the top
of the other. And then the whole winding assembly is slipped over the iron core. B  the PANCAKE design separates
each winding into sections or pancakes. Then alternate pancakes of primary and secondary are slipped over the
core. In the final assembly, all pancakes of the primary are connected together in series, and all pancakes of the
secondary are. connected together in series.
The cylindrical winding is a little cheaper to construct, but
it is harder to repair. The pancake winding costs a little more but is easier to repair because it is broken up
into sections. You'll run into both designs of winding and probably all three types of core. The best advice is to
understand each, but leave them as is.
SUMMARY
Transformers are built to do just one job  change voltage and current values. They are designed to do it as
efficiently as possible. Cores are laminated, winding turns are shortened, and artificial cooling is used. All
these make the transformer the most troublefree and efficient device used by electricians.
Chapter 20 Quiz
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