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¶ U.S. GOVERNMENT PRINTING OFFICE; 1945 - 618779
SOME ELECTRICAL MACHINES
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
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
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 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
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.
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
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
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
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
, and S3
, 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
The other S2
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
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
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 -
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
, 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 repeater.
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
, and R3
come from the rotor slip rings and S1
are the stator leads.
When the rotors
of both generator and motor are in the same position, voltages on the R1
, 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