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Table of Contents. • U.S. Government Printing Office; 1945 - 618779
Chapter 21: Some Electrical
Machines - Transformer Action
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 equipment.
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
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
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 (this Es is produced by the primary field) Esi and
Es are alike in direction but both are opposite to the Ep.
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 and permit
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
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
Why 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 Es.
No current in the motor. No torque! Which tells you that an induction rotor can NEVER
rotate as fast as the stator's magnetic field.
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.
The wound-rotor 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 is necessary.
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 Esi 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 the generator.
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, S2, and S3,
And the rotor leads are marked R1 and R2.
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 and S3 windings are acting just like the S1
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 windings.
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
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, S2, 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 motor.
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
You'll 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, R2, and R3 come from the rotor slip rings
and S1 and S2 are the stator leads.
When the rotors of both generator and motor are in the same position, voltages on
the R1, R2, and R3 leads balance - 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.
There's nothing 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 it."
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 equipment.
Chapter 21 Quiz