How Synchros are used as Position Transducers

Let’s consider the transformer in Figure 1.0 whose secondary winding can be rotated with the respect to the primary winding.

At an angle ϴ, the output voltage will be given by:

V_o = KV_icosϴ     where K is a constant

The output amplitude is dependent on the angle, and the signal can be in phase or out of phase. This principle is the basis of synchros and resolvers.

A synchro link in its simplest form comprises of a transmitter and receiver connected as shown below:

Even though the above figure may look superficially like a three-phase circuit, it is fed from a single-phase supply. The voltage applied to the transmitter induces in-phase or out-phase voltages in the windings and causes currents to flow through the stator windings at the receiver. These currents produce a magnetic field at the receiver which aligns with the angle of the transmitter rotor.

The receiver rotor also produces a magnetic field. If the two receiver fields do not align, torque will be produced on the receiver shaft which causes the rotor to rotate until the rotor and stator fields align, that is, the receiver rotor is at the same position as the transmitter rotor.

Such a link is called a torque link and can be used to remotely position an indicator. An example of circuit illustrated in Figure 1.3 can be used to give an electrical signal which represents the difference in angle of the two shafts. The receiving device is referred to as a control transformer.

In Figure 1.3 above, the output voltage is zero when the rotor on the control transformer is at 90° to the rotor on the transmitter. Note, there are two zero positions 180° apart.  The transmitter operates in similar way as I have described above and induces a magnetic field in the control transformer. An ac voltage will be induced in the control transformer’s rotor if it is not perpendicular to the field. The magnitude of this voltage is related to the angle error and the phase to the direction.

The ac output signal must be converted to dc by a phase sensitive rectifier. Cowan rectifier is commonly applied here. 

Synchros are primarily used in angle measurements and are commonly employed in control engineering as parts of servomechanism, machine tools, antennas, etc. You may find synchros in a variety of devices, including: control transformers, resolvers, Scott T transformers, phase-sensitive demodulators, analog to digital converters.

Also read:

• What is a Microsyn Transducer?
• How cycloconverter is used as a direct frequency converter
• The General Principle of using a Servo Motor for Position Control
• Supervisory Control and Data Acquisition in Power Systems