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I am trying to measure the power factor of a single phase, 208V compressor motor that is connected across phases B & C of a three-phase service and drawing 8-10A. My first attempt using a Fluke 345 set to measure single phase is reading 0.997 power factor. If I set my meter to measure as three-phase, I see 0.07 power factor.

Neither of these can be correct and I'm not sure how to measure the true power factor for this setup. Is this even a reasonable measurement to ask for from this configuration?

(If it matters, there is another single phase motor connected across other phases (A & B) that is active at the same time and if I attempt to read it I get 0.47 or 0.88 power factor as single phase or three-phase measurement.)

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  • \$\begingroup\$ Forgive me, but is there a reason you are concerned about the power factor of this motor? \$\endgroup\$
    – Tyler
    Dec 19, 2016 at 0:18
  • \$\begingroup\$ I need to get the actual wattage of the motor to monitor the efficiency of the overall system. \$\endgroup\$
    – GrizzlyGuru
    Dec 19, 2016 at 5:06

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Connecting a single-phase load to two phases of a three-phase service is the same as connecting the load to a single phase service. Your meter should be set to single-phase and there should be no connection to the third phase. If the motor has a capacitor that remains connected while it is running, the power factor could be close to 1. A universal motor could also have a high power factor.

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