Synchros are a.c. electromagnetic devices that find wide application in servomechanism1. However they can also be used as indicating instruments for registering mechanical displacements remotely.

Note - 1A servomechanism is an error-actuated feedback control system in which controlled variable is mechanical.

There are four basic types of synchros—control transmitter (CX), control transformer (CT), control differential (CD), and control receiver (CR).

Further, synchros are classified according to whether they have torque or control capabilities. The difference of course, depends on whether they have the ability to position mechanical loads or not. There is a letter designation that identifies the function of synchros as follows:

Constructional Features

Control Transmitter (CX)

The contruction of synchro is akin to miniature version of three-phase synchronous generator. However, there is difference in the way these two machines are used. The synchronous generator is excited with d.c. source and is driven at constant speed and produces three-phase a.c. output at stator terminals. In contrast, synchros are excited with a.c. source and is often merely displaced by finite amounts from electrical zero and induces single-phase voltages in stator windings.

Control Receiver (CR)

The basic structure of control receiver is same as that of control transmitter—it has balanced three-phase stator winding and a salient-pole rotor. One detail in which it differs is inclusionof a mechanical viscious damper on its shaft. In normal use, both stator and rotor windings are fed with single-phase a.c. supply, the fields produced by these windings interact to produce torque and rotation. The purpose of damper is prevent the rotor from overshooting its mark. If overshoot is large, average torque may be produced causing receiver to run as a single-phase motor.

Control Transformer (CT)

There is some difference in construction of control transformer from contro transmitter—one important difference is that air-gap is uniform owing to cylindrical or umbrella-like construction of rotor. This keeps magnetizing current drawn to a minimum. Other difference is that impedance of stator windings is higher than those of transmitter to pemit feeding of several control transformers from a single transmitter.

Control Differential (CD)

The differential synchro has a balanced three-phase distributed winding in both stator and rotor. Although three-phase windings are involved, these units deal solely with single-phase voltages.

Electrically, there is no difference between a differencial transmitter and a differential receiver. If we apply elecrical input to both the differential stator and rotor winding, the rotor will turn if it is not mechanically locked. In those differentials that are designed to function as a receiver, there is a damping plate connected to the rotor to cut down oscillations.

Determination of Electrical Zero

For control transmitter (CX) and control receiver (CR)

Electrical zero occurs when the potential difference between S₁ and S₃ is zero and S₂ is in phase with R₁.

(a) Jumper Method:

Here we have insured zero voltage between S₁ and S₃ by connecting them together and referenced to R₂. By connectiong R₁ to S₂, these two will be forced into an equal phase. As the rotor is free to run, it will automatically rotate to above shown position.

(b) Voltmeter Method:

We change the position of rotor till V₁ is zero. But, there will be two positions 180° apart, at which V₁ will be zero. The correct position occurs when V₂ is less than the line voltage.

For control transformer (CT)

First we set the transmitter at electrical zero. Now the output of transformer will be zero only when it is perpendicular to S2 (which is its electrical zero). But since there are two such positions possible at 180° part, to get the correct position, we rotate the transformer shaft clockwise and observe the phase of the output voltage. If it is in phase with the transmitter, then it is the correct zero position. It it is 180° out of phase, then the other zero output position is the correct electrical zero position.

For control differential (CD)

In actual practice, the differential would be connected through a mechanical linkage in such a fashion that its rotor would not be free to move once it was positioned in a particular setting.

We know that for a system employing only a TX and TR, it we revert two leads, the TR rotor turns exactly opposite to that of TX. So in a differential also, if we revert the same leads, we can get addition of angles instead of subtraction; e.g. we will get –
αR = αX + αD instead of αR = αX - αD.


Chapter 10 "Synchros" from book by Vincent Del Toro


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