We have a 3-phase permanent magnet motor and want to test its efficiency in generator mode.

We can give a mechanical input power with an external motor (torque meter and tachometer installed) and we also have an oscilloscope, a 3-phase balanced fixed resistance and a 3-phase variac (auto-transformer), so I was wondering if I can measure the efficiency of the motor by loading its output with variac&resistance.

Since the output of the motor will not be pure resistive because of the variac, do I have to consider the phase difference shift between the output voltage and current of the motor?

Or is this setup not proper for efficiency testing at all? Is pure resistive load necessary instead of an inductive load?

My oscilloscope can measure the phase-to-phase voltage and current, and output the active power as Watt (and I have to multiply it by square root of 3 for 3-phase power I think).

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    \$\begingroup\$ By "3-phase balanced fixed resistance" do you mean equal fixed resistances on each phase? \$\endgroup\$ Commented Oct 22, 2019 at 8:31
  • \$\begingroup\$ Yes that's true. \$\endgroup\$ Commented Oct 22, 2019 at 8:33

1 Answer 1


The inductive effect of the Variac is not important if the power measurement system is adequate. The power measurement system must be able to take into account any phase difference between the current and voltage. You could attempt to eliminate inductance from the load, but that can never be entirely successful. It is best to have a high quality three-phase wattmeter that is fully capable and accurate. There are oscilloscopes that can calculate power properly, but you need a good current transducer.


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