# Phase Angle Problem in a Synchro Rotor

Discussion in 'Electronics Homework Help' started by gacman, Jul 3, 2013.

1. ### gacman

6
0
Jun 27, 2013
Will the phase angle of the voltage across a series RL circuit be affected if two separate voltages that are out of phase with each other are applied to it?

Refer to the attached schematic:

Circuit A has 5.2 volts, B-phase applied to the rotor coil lead R1 and 23 volts, A-phase applied to R2. Phases A and B are apart by 120 degrees in phase effectively making the sum of the two voltages equivalent to 26 volts AC being applied to the synchro rotor. Will the 120-degree phase split have an effect on the phase angle of the series rotor coil/resistor in the circuit? And if so, then what should the stator outputs be?

Circuit B shows the same circuit but with a single phase 26 volts applied to the rotor coil through the phasing resistor. This is the same configuration that they teach in school for series RL circuits. I can calculate all the circuit values using the standard equations for series RL circuits. What I need to know is: Do the standard equations still apply if the input voltage is applied in the same configuration as circuit A?

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2. ### woodchips

43
0
Feb 8, 2013
As you say, both circuit A and B have 26V applied to the rotor, so they are identical. What is different is the phase angle between the stator voltages and the A, B or 26V applied to the rotor because of the vector sum of A and B. Easiest solution is to draw the vector voltages. Still can't see what you are trying to do, just power the synchro rotor from 26V without the series resistor which doesn't do anything.

3. ### gacman

6
0
Jun 27, 2013

I agree it would be easier to do it the way you suggested. The reality though, is that the aircraft is actually wired like circuit A with the 5.2 VAC B-Phase coming in on rotor lead R1 and 23 VAC, A-phase coming in on rotor lead R2. There is what they refer to as a "phasing resistor" in series with the rotor coil (on the R1 side). Other than reducing the voltages on the stators, I can see no reason for it to be there.

Here is my situation: I have about 20 of these synchro transmitters that were found in a locker. I checked them using a synchro tester and a 26 volt, single phase power source. I got fairly consistent readings on most of them. When I compare those readings to what I see when I check them on the aircraft, I get different readings. The only difference is the way the power comes in. Checking them on the aircraft is like circuit A and testing them in my shop is like circuit B. My technical data tells me only to null them below 50mV.

I am still trying to get additional technical data from the manufacturer...who seems to be dragging his feet. But in the meantime, I am trying to mathematically figure out what the readings "should" be so that I can at least confirm the ones that are truly bad. I will wait for the official technical data before I certify any of them to be good, of course...but this whole deal has sparked my curiosity about the engineering behind this particular system. Why design it like that? There must be a reason, right?

4. ### woodchips

43
0
Feb 8, 2013
You don't say where you are but in the UK aircraft 3 phase systems always have the B phase earthed. Assuming this is similar to your C phase earthed. Also the synchro must be a control type rather than torque? In any case using the two phases introduces a phase shift between, say, the A phase and the synchro output. It appears that the resistor simply acts as a phase trimming device, as you say. If the transformers had more taps then don't see that it is necessary.

When tested they should give exactly similar readings, a synchro is a pretty precise angle transducer. What, numerically, is the difference between the test bench and aircraft readings? Presumably the tester will resolve the stator voltages to an angle. What instrument does the synchro drive?

Use pencil and paper to simply draw the vector diagram, might explain what is happening when you can superimpose what you measure on to it.

5. ### gacman

6
0
Jun 27, 2013
Autopilot system

These synchros are actually control transformers (CT) that are wired up as though they were control transmitters (CX). They are used as spoiler position transmitters for a Bendix PB-20 autopilot system on Saudi E-3A AWACS aircraft. In fact, there are two of them that are differentially connected to give a resultant zero output as long as the spoilers are extended equally (regardless of how much they are extended). They provide a phase proportional signal to the amplifier/computer when either of the spoilers is extended more than the other. Only the S1 and S3 leads are connected to the computer.

Numerically, on the bench, I get about 6.2 volts between stator leads S1 and S3 with the rotor at 90 degrees. On the aircraft, I get about 9.5 volts.

In the shop, when I bypass the resistor, I get 11.8 volts. This is exactly what I would expect given the transformer ratio is 0.454.

6. ### woodchips

43
0
Feb 8, 2013
Ah, a CT is not the same as a CX. The CX has an H form rotor to induce the signal into the stator windings, a CT has a wound rotor like a squirrel cage motor. The end result is that the stator induced voltages will, can, vary. The impedance of each type of rotor must also be considerable different.

It looks like just using the two stator windings makes the device essentially single phase. Connecting the two synchos in series opposition will then do as you say, if equal position then equal voltages then zero volts across the two synchros. If the synchros are not at the same position then the stator voltages will not completely cancel giving both a voltage and phase shift, relative to rotor, across two synchros.

That seems a reasonable explanation.

Presumably you have measured the voltage AB on the aircraft and it is 26V? Have you looked at the voltage with a scope though? Might have 26V but is it mean or RMS, could be significant waveform distortion. If you are using phase as the output from the synchros then you really need as little distortion as possible, less than 1% say?

The aircraft is big, lots of wire inside it, the bench set up is exactly the opposite.

I would get a scope and have a look.