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¶ U.S. GOVERNMENT PRINTING OFFICE; 1945 - 618779
Many electrical devices and machines operate on the principle of
TRANSFORMER ACTION. THEY ARE NOT TRANSFORMERS - but the theory of their
operation is best explained by considering them AS IF THEY WERE TRANSFORMERS.
This term "transformer action" comes from the current and voltage relationships
in a transformer. Bound up in the term are a number of separate meanings.
Each one is important and each one finds application in special electrical
If you thoroughly understand "transformer action"
you can understand more than half of all the electrical machinery built.
Let's take each item in the general term "transformer action"
and analyze it.
Power transfer means that the device has TWO circuits. The power
is fed into one circuit and is transferred to the other by magnetic
induction. This means that you are always dealing with a magnetic circuit.
And you'll run in to lots of iron to preserve this magnetism. Whenever
you see TWO, circuits in a machine, one of them having no connection
to the power source, you can be pretty sure that transformer action
makes the machine run.
Voltage changes usually take place whenever you have transformer
action. Check your primary and secondary circuits for differences in
the number of conductors. If the number of conductors goes up - you
can count on the voltage going up. Then too - check for movement between
your primary and secondary. If one moves in relation to the other, more
flux is going to be cut-you can count on the secondary voltage being
higher than if there was no relative movement.
Remember that you do not get something for nothing. If the secondary
voltage goes up, there will be an increase in secondary current. Since
secondary current controls primary input, you can be sure that primary
current goes up too.
The powers of the primary and secondary
will always be equal (neglecting losses). Whatever is TAKEN OUT in increased
voltage and current must be paid for by increased current INPUT.
THEY'RE ALWAYS OPPOSITE. The two currents - primary and secondary
- are always flowing in opposite directions. This means that the magnetic
poles produced in the two circuits are always opposite to each other.
In transformer action you'll always find three voltages to consider.
First there is the applied voltage from the source. This is the primary
voltage or Ep
. Second, there is the voltage of self-induction
on the primary. It's produced by the primary's own field - Esi
Third, there is the induced voltage in the secondary - the Es
is produced by the primary field) Esi
are alike in direction but both are opposite to the
If there is no relative movement between primary and secondary,
frequencies are equal. But, if there is MOVEMENT, the NUMBER of cuttings
received by the secondary depends on both PRIMARY FREQUENCY and the
RELATIVE SPEED OF MOVEMENT. With relative motion the frequency of the
secondary either goes up or down.
The AMOUNT of current, EVERYWHERE, in transformer action, is controlled
by conditions in the secondary. Watch what happens to the current in
the secondary - that's the key to what goes on in the primary. Don't
forget that Esi
has the primary current almost choked off.
It's up to the secondary field to reduce the Esi
current to flow in the primary.
Induction motors certainly don't look like transformers. But let's
CONSIDER a three-phase squirrel cage motor as a transformer. The one
in figure 226 is a good example. It is shown in a cross-sectional view.
You must consider the stator as the primary - it's connected to the
source. And the rotor as the secondary - its power comes from the field
of the primary.
First, how about the two voltages? Well, is
it a step-down or step-up? Step-down - because the secondary (rotor)
has fewer turns than the primary (stator). What do you think would happen
if this was reversed? Imagine a rotor with more turns than the stator
- a step-up job. The rotor, being short circuited by its end rings,
would have a tremendous current. So would the stator - it's controlled
by rotor current. This job would cook in short order! And that is exactly
why squirrel cage rotors have fewer turns than their stators. The designer
not only understood transformer action, but he made use of it.
Figure 226. - lnduction motor as a transformer.
Why does the rotor turn? Because it's a secondary, and its poles
are opposite to those of the primary. Attraction between primary and
secondary produces torque, and torque produces rotation.
does a large induction motor have to be protected against high starting
current? Because, at the start, the rotating field (primary) is cutting
the rotor (secondary) at a furious rate. Extremely high voltages are
induced in the secondary - high current flows. The primary, subject
to secondary control, likewise carries a high current. If this high
primary current wasn't cut down by a starter, the primary would go up
in smoke. All right - then why isn't a starting resistance needed when
the motor is running? Because when the secondary is revolving WITH the
primary field there is less relative motion between them. Less flux
is cut - secondary voltage and current is less - and likewise the primary
current is reduced to a safe value.
It's interesting to imagine
what the conditions would be IF the rotor ever caught up to the stator
field. This would mean that each rotor conductor would ride right along
with the flux of the rotating field. No relative motion! No flux cut.
. No current in the motor. No torque! Which tells you
that an induction rotor can NEVER rotate as fast as the stator's magnetic
How about the rotor frequency? When the rotor is standing
still (at startup), it is cut by every pulse of the stator's a-c field.
Rotor and stator frequencies are the same. But as the rotor picks up
speed, it rides WITH the field. There is less and less relative motion
and the rotor frequency becomes lower and lower.
These are the
most important facts about a squirrel cage motor. The engineer designs
the motor on the oasis of these facts. And all of them can be understood
by simply considering the motor as a transformer.
induction motor can be considered the same as the squirrel cage. Both
motors operate on the same principle.
INDUCTION REGULATORS are transformer devices used to regulate the
voltage in a-c lines. Figure 227 shows a schematic of a regulator and
its connection in a line. The primary winding is mounted on a movable
cylindrical iron core and connected ACROSS the line. The secondary is
wound in slots on a stationary core surrounding the primary. It is connected
IN the line. The secondary is connected so that its voltage will add
to the line to offset line drop. The machine looks like a motor but
it is used as a transformer.
The primary has a line current
and establishes a field around the movable core. When the primary is
in position A, all its flux cuts the secondary. This induces voltage
in the secondary which adds to the line voltage (line and secondary
are in series). But when the primary is in position B, its flux does
not cut the secondary. No voltage is added to the line. Changing the
primary position alters the amount of flux which cuts the secondary.
More or less voltage is induced in the secondary depending on the primary
position. In this way, the line can have any voltage added to it that
Figure 227. - lnduction regulator.
This is a much better method of voltage regulation than the use
of a rheostat. Rheostats consume power in their resistance. The power
is wasted as heat. But the induction regulator delivers back just about
as much power as it consumes. When it is in the zero position, B, the
reduces current in the primary almost to zero. Therefore,
there is only a slight loss whether the regulator is boosting the voltage
or is turned completely off. These devices are built for either automatic
or hand operation.
Usually a.c. is generated at a frequency of 60 cycles. But a higher
frequency is required for some high speed motors, radio circuits, and
heating devices. One of the easiest ways of producing this higher frequency
is by means of a FREQUENCY CONVERTER.
Frequency converters are
built like wound-rotor motors. The primary is the stator and the secondary
is the rotor. The secondary voltage is taken off the rotor by slip rings.
With the secondary standing still the two frequencies are equal. But
if the secondary is connected to a motor and driven BACKWARDS, it meets
the rotating field of the primary. Thus it is cut by the primary field
more rapidly than if it just stood still. Suppose the secondary was
turned AGAINST the rotating field at exactly the same speed that the
primary field is rotating. The secondary would be cut just twice' as
many times and the frequency would be double.
By adjusting the
rpm of the secondary, any frequency can be taken from the co er. In
addition, if a voltage step-up or step-down is required, the number
of turns on the primary and secondary can be adjusted to fit the requirements.
Here is a problem. The gyrocompass is located deep in a ship. But
the reading of the gyro is needed on the bridge. It MIGHT BE POSSIBLE
to transmit the reading by a system of gears and shafts. But it wouldn't
be practical. Gears and shafts running the 300 or 400 feet between the
gyro and bridge would never stay lined up. A flexible cable would not
work for any such length. Any mechanical device would fail sooner or
later because of the length, the twists, and the bends.
But an electrical
transmission line carries current just as well around corners as it
does in a straight line. For electrical transmission, you would need
some electrical device to pick up the gyro's reading. And another device
to duplicate this reading at the bridge end of the line. The SYNCHRO
fills the bill. Thus synchros are electrical devices built to transmit
readings from one place to another.
Figure 228. - Synchro windings.
Figure 228 is the schematic of a synchro. Notice that the stator
winding is just like the winding on a three-phase motor or alternator
stator - a three-phase job. The rotor has a single phase winding and
is connected to a 60 cycle, 115 v., a-c line. The rotor shaft is coupled
to the gyro and turns every time the gyro turns. This synchro is called
At the bridge end of the line, a second synchro
repeats the gyro reading. Its rotor is coupled to a repeater compass.
This synchro is called the MOTOR. Although these two synchros have different
names, they are EXACTLY ALIKE ELECTRICALLY. Both have three-phase windings
on their stators and single phase windings on their rotors.
Notice in figure 229 that BOTH rotors are energized from the same single
phase a-c line. Also, the two stator windings are connected together
in series. It is important that the X winding of the generator be connected
to the X winding of the motor. And likewise that the two Y and the two
Z windings are each connected together. In synchros the leads from the
X, Y, and Z windings are usually marked S1
, And the rotor leads are marked R1
Figure 229. - Synchro generator and motor.
Now, consider each unit as a transformer - the rotor is the primary
and the stator is the secondary. The two rotors are in the same position-say
opposite the S1
windings. Both primary fields are cutting
the secondary windings and inducing a voltage. But the VOLTAGES are
UNABLE to move any CURRENT because they are EQUAL and OPPOSITE. EQUAL
because the voltages are induced by duplicate fields and OPPOSITE because
they both try to force current OUT on the line connecting the two S1
windings. Figure 229 shows the two voltages meeting head on.
The other S2
windings are acting just like
winding. Notice that their voltages likewise cancel.
Generator and motor voltages balance - zero current flow. The total
effect of this is ZERO. Nothing happens - the two rotors remain in their
positions opposite the S1
Now the ship changes
course. The rotor of the generator synchro is turned by the gyro - say
half way to the S3
winding (30°). The two synchro rotors
are no longer in the same relative position. And the voltages induced
in the two stator windings no longer balance. The generator's S1
voltage is weaker and its S3
voltage is stronger. Current
flows from the generator's S3
to the motor's S3
and from the motor's S1
to the generator's S1
Figure 230 shows the two synchros with their rotors in the new position.
This time the arrows indicate CURRENT direction.
The new conditions
are this - both S1
and S3 windings have fields which are
trying to pull their rotors back into identical relative positions.
The transmitter CANNOT move - the gyro holds its rotor in the new position.
But the motor rotor can move. It does just that AND CARRIES THE REPEATER
COMPASS WITH IT. The motor rotor moves to the point where all the S1
, and S3
voltages balance again and the stator
currents become zero. This point is where the transmitter and motor
rotors are in identical positions again. The total effect is a rotation
of the motor's rotor exactly following the rotation of the transmitter's
rotor. You might look at it this ,way-each rotor is a primary inducing
a voltage in three secondaries. As long as the three secondary voltages
balance against each other, no current flows. But move the transmitter
primary and, thereby increase or decrease the voltage on any of its
secondaries and current flows. This current flows in both transmitter
and motor - their secondaries are connected together. A field
is set up by this current and it acts on the motor primary. Actually
this is MOTOR ACTION. The motor primary is forced to move by the torque
of the motor action. The primary stops moving only' when the torque
is again zero. And zero torque is produced only when. no current flows
in the secondaries. You know when that happens - at the point where
the two primaries are in identical positions. The secondary voltages
will then balance against each other.
Figure 230. - Torque produced in a synchro
Thus every move of the transmitter's rotor produces torque on the
motor's rotor. The motor's rotor, answering this torque, follows every
move of the transmitter's rotor. Thus, for every shift of 337 the gyrocompass,
a corresponding shift occurs in the bridge repeater.
run into synchros of a different type. They have a single phase winding,
with a.c. impressed, on the stator. And the rotor has a winding just
like the three phase wound rotor. With this type of synchro, you'll
have to consider the stator winding as the primary and the rotor winding
as the secondary. The leads are numbered differently too - R1
, and R3
come from the rotor slip rings and
are the stator leads.
rotors of both generator and motor are in the same position, voltages
on the R1
, and R3
- no current and no torque. But if the generator rotor is turned, this
balance is upset - current flows in the rotor windings and torque is
produced. Motor action forces the motor rotor to a balancing position
- duplicating the generator rotor's position.
electrically new in this type of synchro - it just has the rotor and
stator windings reversed. Study it as a transformer and you'll "get
These have been only a few examples of machines and transformer
action. When you run into something new, examine the new device for
transformer action. It's the easiest way to get the low-down on new
Chapter 21 Quiz