Efficiency Test of Permanent Magnet (PM) Motor With Variac&Fixed Resistance
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.