3 Phase potentiometer

A bunch of old equipment I use at work (built between 1990 and around 2003) use what my coworkers call "Three Phase Potentiometers". They are rotary potentiometers with 5 connectors. Measuring their resistance didn't give me much sensible information, other than that it is not multiple regular potentiometers in one housing.

They are used to control the position of rotating equipment with a rotating lever, located up to 100m away. The equipment will rotate to the same position as the lever using hydraulics. When both potentiometers are aligned (and the equipment is no longer moving), the voltage between + and - on P2 is 0V.

The potentiometers are wired as followed:

simulate this circuit – Schematic created using CircuitLab

Unfortunately, none of the engineers that designed this equipment work with us anymore. We do still have some techs that know how to solve common issues, but we have no real understanding of the logic behind the design.

I have the following questions:

• Can anyone guide me to more information on how this type of potentiometer works?

• What are these potentiometers officially called?

• What are reasons to use a set up like this? Our newer designs use regular 3 pin potentiometers, and 4-20mA Hall effect transducers today.

• a picture of these "Three Phase Potentiometers" might help. Also, have you recorded the current, or relative voltages, on your three long wires? What is the "Control Circuit" in your figure? 1990-2003 mean that there's simply rotary encoders and a microcontroller in both ends, so in this current form, I guess all we can do about your question is guess. Jan 27, 2021 at 14:44
• Google on synchro/resolver. Jan 27, 2021 at 14:57
• These sound more like synchros, more like generator/motor than potentiometers, where the "slave" rotates synchronous to the "master" (the one with the knob on) They are available both as 3 phase devices, and 2 phase (I/Q).
– user16324
Jan 27, 2021 at 15:01

A three-phase potentiometer has 2 input wires (voltage) and three output wires, each with one phase voltage corresponding to the angle of the pot, 0 - 360 degrees continuous. They consist of one 360 degree winding in a ring with three taps taking off the three phases and two wipers connected at 180 degrees from one another on a rotating shaft. They have been superseded with rotary LVDTs or magnetic sensors these days.

• The 3 phase pots can rotate the 3 inputs to any phase output.

• The pots might be called 3 phase resolvers. Similar 2 phase Sin/Cos Pots exist for rotating phase in quadrature for rotating any signal by phase 0 to 360 deg.

• 3 phase pots continuously variable are expensive so other methods of phase-shifting can be done cheaper.

anecdote

In the 70's we used servo controlled sin/cos pots to shift the phase of calibration eddy current signals to produce 0 and 90 deg phase shifts for 1mm holes and 0.5mm change in wall thickness. A non-defect could then be displayed on an XY scope as a dot for impedance. The vector result of signals could then detect the type and size of an eddy current defect of the metal crystalline structure of a 1mm thick tube used to carry superheated heavy water. This was done by robotics during annual maintenance to detect strain of tubing to prevent a leak of expensive heavy water under many kilo-atmospheres of pressure in 0.2mm increments over every 20m tube. (thousands per boiler)

This method was equivalent to measuring the change of a coil vector impedance more than 10 ppm.