Secrets of Synchros

Synchros (aka Selsyns) seem to fall in a gray area of electronics. Neither my electronic reference nor electrical reference books cover them with any clarity. I have found that there is quite a bit of misinformation in print and on the net regarding their application.

The Synchro is a specially wound "motor" comprised of two windings.

The Rotor is a single winding with slip rings & brushes.

The Stator is a 3 section winding, Y connected.

Many synchros have rear mounted screw terminals marked with Rx & Sx designations. Others have flying leads. The Standard Color Code for units with flying leads is:

R1 Red (or White/Red)
R2 Black (or White/Black)

S1 Blue
S2 Black
S3 Yellow

With a sinusoidal excitation voltage (VR) applied to the rotor, the stator voltages are nominally:

V(S1-S3) = K * VR * Sin(θ)
V(S2-S3) = K * VR * Sin(θ+120)
V(S2-S1) = K * VR * Sin(θ-120)

Where θ (theta) is the shaft angle referenced to electromechanical zero and K is a constant defined by the winding ratios.

Synchros are (were?) commonly manufactured with these (rotor) voltage ratings:

115V at 400Hz       115V at 60Hz
90V at 400Hz           90V at 60Hz
26V at 400Hz.

Normal system operation connects the corresponding rotor (Rx) and stator (Sx) wires of a synchro transmitter to a synchro receiver with a five wire cable. The position of the synchro transmitter's input shaft is then repeated on the synchro receiver's output shaft.

The Four-Wire connection.

The US military, and also the commercial aviation sector, employed many (26V / 400Hz) synchros in this manner for Radio Compass applications. One of the rotor leads, R2 (Black), is commoned with one of the stator connections, S2 (Black), to save a pin on the connectors and a wire in the interconnecting cable -- a valid consideration when building several hundred thousand units. Note: I have seen military drawings which incorrectly identify the phases even though the connections are correct.

An AN3102-14S-2P connector (Note: The cable mate is an AN3106-14S-2S) is commonly used on the later version units but, the pinout is NOT consistent.

I have seen:

On the ARN-6 series

R2-S2       A
S3             B
S1             C
R1            D

On a Lear unit

R2-S2       A
R1             B
S1             C
S3             D

Electromechanical zeroing

A four wire system may be directly calibrated. In a five wire system, temporarily connect a jumper lead between R2 and S2.

1. Connect an AC voltmeter, set to a range at least 2X the excitation voltage, between S1 & S3.

2. Apply power to the synchro(s). Rotate the synchro to be calibrated until the voltmeter reads zero. For finest adjustment the meter may be switched to a lower range as the reading approaches zero..

3. Now measure the voltage between R1 and either S1 or S3. If the indicated voltage is greater than the excitation voltage, the synchro is misoriented by 180 degrees. Turn the shaft or body approx. 180 degrees and repeat steps 1 & 2.

Amplifying or repeating the synchro signal

A 26 volt 4-wire connected system may be extended to multiple readout devices, or alternatively employing a small transmitter to drive a larger receiver, by buffering just the S1 & S3 signals with a pair of power Op-Amps (Typ: Texas Instrument OPA-544T) connected as followers operating on +/- 24 volt rails referenced to S2 -- system "ground" in this instance.

My "Telephone Wire" 4 conductor connection color code:

R1 Red
R2-S2 Black
S1 Green
S3 Yellow

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