I have a permanent magnet synchronous machine (PMSM) that I intend to run as a generator. I'd like to test its load characteristics under resistive load. The PMSM is Y-connected. Assuming Y-connected load, the circuit will look like this.


simulate this circuit – Schematic created using CircuitLab

My objective is to plot the load characteristics of the generator (load voltage vs. load current), for 10-12 different values of load resistance, starting from a maximum resistance to a minimum resistance. I want to be able change the load resistance while keeping the load balanced. However, I only have access to potentiometers and simple resistors. If I manually change the POT resistances by hand at each step, there is bound to be imprecision and thus, load will become unbalanced.

My questions: (1) Is there any way I can achieve this using potentiometers? (2) Is there an alternative way to do it, keeping the load balanced? Thanks a lot!


2 Answers 2


Use a triple bank of switches that simultaneously switch the three loads to the three phases. Using pots is not a good idea because as the wiper gets close to one end of the pot, the power in that small fraction of resistance is going to sky rocket and burn the pot.

Alternatively make a 3 phase rectifier (6 diodes) and have one adjustable load on the rectified output. It's not ideal because there will be pulses of current taken from the windings but it may be a useful experiment.

  • \$\begingroup\$ On a slightly different note, for the Y connection, should I ground the load side neutral or not? \$\endgroup\$
    – Adeel
    May 8, 2015 at 13:05
  • \$\begingroup\$ It's not needed functionally and what if it were a delta load? You couldn't ground anything! What output voltage will it produce? \$\endgroup\$
    – Andy aka
    May 8, 2015 at 13:33
  • \$\begingroup\$ Leaving it unconnected lets you measure its voltage to check for phase balance, or connect it via an AC ammeter. Any significant current indicates imbalance which may show a fault in one of the phases. \$\endgroup\$
    – user16324
    May 8, 2015 at 13:39

If you also have access to mechanical parts, you could gear multiple pots together, to a fourth shaft which controls all three. Use either gears, or chain drive to sprockets, or toothed belt according to what is available.

Carbon track pots tend to be inaccurate, but high power pots should be wirewound pots which track each other reasonably closely (similar resistance/angle). Ten turn pots tend to track fairly accurately.

Tracking accuracy should be specified in the datasheet and you can easily measure resistance/angle before connecting to the generator.


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