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Normally, the rotor windings of a wound rotor induction motor are
shorted out after starting. During starting, resistance may be placed in
series with the rotor windings to limit starting current. If these
windings are connected to a common starting resistance, the two rotors
will remain synchronized during starting. (Figure below) This is useful
for printing presses and draw bridges, where two motors need to be
synchronized during starting. Once started, and the rotors are shorted,
the synchronizing torque is absent. The higher the resistance during
starting, the higher the synchronizing torque for a pair of motors. If
the starting resistors are removed, but the rotors still paralleled,
there is no starting torque. However there is a substantial
synchronizing torque. This is a selsyn, which is an abbreviation
for “self synchronous”.
Starting wound rotor induction motors from common
resistors.
The rotors may be stationary. If one rotor is moved through an angle
θ, the other selsyn shaft will move through an angle θ. If drag is
applied to one selsyn, this will be felt when attempting to rotate the
other shaft. While multi-horsepower (multi-kilowatt) selsyns exist, the
main application is small units of a few watts for instrumentation
applications-- remote position indication.
Selsyns without starting resistance.
Instrumentation selsyns have no use for starting resistors. (Figure
above) They are not intended to be self rotating. Since the rotors are
not shorted out nor resistor loaded, no starting torque is developed.
However, manual rotation of one shaft will produce an unbalance in the
rotor currents until the parallel unit's shaft follows. Note that a
common source of three phase power is applied to both stators.
Though we show three phase rotors above, a
single phase powered rotor is sufficient as shown in Figure below.
Small instrumentation selsyns, also known as sychros, use
single phase paralleled, AC energized rotors, retaining the 3-phase
paralleled stators, which are not externally energized. (Figure below)
Synchros function as rotary transformers. If the rotors of both the
torque transmitter (TX) and torque receiver (RX) are at the
same angle, the phases of the induced stator voltages will be identical
for both, and no current will flow. Should one rotor be displaced from
the other, the stator phase voltages will differ between transmitter and
receiver. Stator current will flow developing torque. The receiver shaft
is electrically slaved to the transmitter shaft. Either the transmitter
or receiver shaft may be rotated to turn the opposite unit.
Synchros have single phase powered rotors.
Synchro stators are wound with 3-phase windings brought out to
external terminals. The single rotor winding of a torque transmitter or
receiver is brought out by brushed slip rings. Synchro transmitters and
receivers are electrically identical. However, a synchro receiver has
inertial damping built in. A synchro torque transmitter may be
substituted for a torque receiver.
Remote position sensing is the main synchro application. (Figure
below) For example, a synchro transmitter coupled to a radar antenna
indicates antenna position on an indicator in a control room. A synchro
transmitter coupled to a weather vane indicates wind direction at a
remote console. Synchros are available for use with 240 Vac 50 Hz, 115
Vac 60 Hz, 115 Vac 400 Hz, and 26 Vac 400 Hz power.
Synchro application: remote position indication.
A synchro differential transmitter (TDX) has both a three
phase rotor and stator. (Figure below) A synchro differential
transmitter adds a shaft angle input to an electrical angle input on the
rotor inputs, outputting the sum on the stator outputs. This stator
electrical angle may be displayed by sending it to an RX. For example, a
synchro receiver displays the position of a radar antenna relative to a
ship's bow. The addition of a ship's compass heading by a synchro
differential transmitter, displays antenna position on an RX relative to
true north, regardless of ship's heading. Reversing the S1-S3 pair of
stator leads between a TX and TDX subtracts angular positions.
Torque differential transmitter (TDX).
A shipboard radar antenna coupled to a synchro transmitter encodes
the antenna angle with respect to ship's bow. (Figure below) It is
desired to display the antenna position with respect to true north. We
need to add the ships heading from a gyrocompass to the bow-relative
antenna position to display antenna angle with respect to true north.
antenna + gyro
Torque differential transmitter application:
angular addition.
antenna-N = antenna + gyro
rx = tx + gy
For example, ship's heading is 30o, antenna position
relative to ship's bow is 0o, antenna-N is:
rx = tx + gy
30o = 30o + 0o
Example, ship's heading is 30o, antenna position relative
to ship's bow is 15o, antenna-N is:
45o = 30o + 15o
For reference we show the wiring diagrams for subtraction and
addition of shaft angles using both TDX's (Torque Differential
transmitter) and TDR's (Torque Differential Receiver). The TDX has a
torque angle input on the shaft, an electrical angle input on the three
stator connections, and an electrical angle output on the three rotor
connections. The TDR has electrical angle inputs on both the stator and
rotor. The angle output is a torque on the TDR shaft. The difference
between a TDX and a TDR is that the TDX is a torque transmitter and the
TDR a torque receiver.
TDX subtraction.
The torque inputs in Figure above are TX and TDX. The torque output
angular difference is TR.
TDX Addition.
The torque inputs in Figure above are TX and TDX. The torque output
angular sum is TR.
TDR subtraction.
The torque inputs in Figure above are TX1 and TX2.
The torque output angular difference is TDR.
TDR addition.
The torque inputs in Figure above are TX1 and TX2.
The torque output angular sum is TDR.
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