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 rot r windings to limit starting current. If these windings are connected to a common starting resistance, the two rotors will remain synchronized during starting. (Figure ) This is usefull 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 appplication 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 ) 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 .
Transmitter – receiver
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 ) 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 brough 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 ) 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.
Differential transmitter – receiver
A synchro differential transmitter (TDX) has both a three phase rotor and stator. (Figure ) 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 isplays 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 postion 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 ) 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
Addition vs subtraction
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.
The torque inputs in Figure < are TX and TDX. The torque output angular difference is TR.
The torque inputs in Figure are TX and TDX. The torque output angular sum is TR.
The torque inputs in Figure are TX1 and TX2. The torque output angular difference is TDR.
The torque inputs in Figure are TX1 and TX2. The torque output angular sum is TDR.
A variation of the synchro transmitter is the control transformer. It has three equally spaced stator windings like a TX. Its rotor is wound with more turns than a transmitter or receiver to make it more sensitive at detecting a null as it is rotated, typically, by a servo system. The CT (Control Transformer) rotor output is zero when it is oriented at a angle right angle to the stator magnetic field vector. Unlike a TX or RX, the CT neither transmits nor receives torque. It is simply a sensitive angular position detector.
Control transformer (CT) detects servo null.
In Figure , the shaft of the TX is set to the desired position of the radar antenna. The servo system will cause the servo motor to drive the antenna to the commanded position. The CT compares the commanded to actual position and signals the servo amplifier to drive the motor until that commanded angle is achieved.