Synchro control transformer

A synchro control transformer is used in conjunction with a synchro transmitter to act as error sensor of mechanical components. Basically, the construction of a synchro control transformer is very similar to that of the synchro transmitter, except that the rotor is cylindrically shaped so that the air gap flux is uniformly distributed around the rotor.

This is essential to a control transformer since its rotor terminals are usually connected to an amplifier. The cylindrical shape of the rotor of the synchro control transformer helps to keep the change of impedance in the rotor coil circuit with the change in angular position. The symbol “CT” is often used to designate a synchro control transformer.


For the application of detection of the error, the synchros transformer should be connected to the synchros detector. The stator leads of the synchro transmitter are connected to the stator leads of a synchro control transformer.

The CX rotor is connected to an AC source and the rotor coils of the synchronization control transformer are connected to the input of an error amplifier. For small deviations, the voltage at the rotor terminals of the control transformer is proportional to the deviation between the two positions of the rotor.

Because of the similarity in the magnetic construction, the flux patterns produced in the two synchros will be the same if all losses are neglected. For example, if the rotor coil of the transmitter is held parallel to the stator coil S2, the direction of the flux in the air gap in the synchro control transformer will be such that the induced voltage in its rotor coil will be maximum when it is parallel to its stator coil S2 and is minimum when the rotor coil makes an angle of 90° with the S2 coil.

With proper design, the minimum voltage can be made practically zero and this position of the rotor of the synchro control transformer, with respect to that of the control transformer is referred to as electrical zero position.

The rotor voltage of the control transformer is approximately proportional to the difference between the positions of the rotors of the transmitter and the control transformer. For small discrepancies of the controlled and reference shaft positions, the transfer function of the synchro error detector can be written as:


E = error voltage θr = reference shaft position in degrees θc = controlled shaft position in degrees θe = θr – θc = error in angular positions between the reference shaft and the controlled shaft in degrees Ks = sensitivity of the error detector in volts per degree.