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Synchro Phasing???


Jun 27, 2013
Jun 27, 2013
The synchro is a control transformer (CT) that is wired as though it is a control transmitter (CX -- with the input excitation going to the rotor instead of the stator). The input is 23 vac, A-phase to R2 and 5.2 vac B-phase to R1. That is effectively 26 vac as A and B phases are 120 degress out of phase with each other. Between the 5.2 vac input and R1, there is a 147 Ohm resistor (that actually measured 147.6 -- called a "phasing resistor" in the technical order) so the synchro rotor circuit is essentially a series RL.

My question is: How does applying 23 volts to one of the rotor leads and 5.2 volts (that is 120 degrees out of phase with the 23 vac) to the other rotor lead differ from simply applying 26 volts to one side and ground to the other? Is the phase angle affected? And how does the "phasing resistor" affect the output of the synchro?

Here is what I got when I applied 26 vac, 400Hz to rotor lead R1:

1. When 26.13 vac, 400Hz was applied to rotor lead R1 (bypassing the resistor), the current measured 37.4mA. This means the inductive reactance (XL) of the rotor coil must be 699.198 Ohms.

2. If XL=699.198, then L must be 278.2mH.

3. Total impedance then must be 714.6 Ohms.

4. Therefore, total current must be 36.6mA, but I got 34.4mA when measuring total circuit amps (including the resistor). I'm not sure how significant the 2 mA discrepancy is...

5. Phase angle should be 78.08 degrees.

6. Voltage drop across the resistor should be 5.4 volts, but I measured 5.08 vac.

7. Voltage drop across the rotor coil should be 25.59 volts, but I measured 24.07 vac.

8. When 26 vac is applied to the rotor coil (bypassing the resistor), the stator (S1 & S3) output at 90 degrees is 11.8 vac which confirms the 0.454 transformation ratio.

9. When 26 vac is applied with the resistor included, the stator output dropped to 10.8vac which is consistent with the voltage drop I measured.

Will all this change when I apply 23 vac A-phase to one side of the rotor and 5.2 vac B-phase to the other? If so, how do I calculate the phase shift and stator outputs?

More info: This is for an aircraft spoiler position transmitter for a Bendix PB-20 autopilot system


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