I am measuring the electric power (P = VI) in a gear-motor. I measure it of two ways. One way is measure with a multimeter the resistance of the motor, then with V=RI, I measure the current, then I multiply that by the voltage and a obtain something near 2000 watts. In the other hand I have a multimeter hook that measures high values of current, but with it I obtain a different amperage and that leads to an electric power near of 600 watts. Why is this? I think is a big difference, which is the real one? Since I am controlling it with AC current, Is AC resistance or amperage Different than DC? Thanks in advance

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    \$\begingroup\$ V=IR doesn't account for back EMF. So it is only accurate when the motor is stalled. When it's running unloaded, it generates enough voltage to cancel most (possibly 90-95%) of the source voltage and takes 5-10% of the stall current. If it's taking 30% of the stall current it's quite heavily loaded, running about 70% of its nominal speed (maybe 75% of its unloaded speed) \$\endgroup\$
    – user16324
    May 9, 2016 at 18:22

1 Answer 1


AC circuit behavior is much different from DC. AC circuits have impedance rather than ordinary resistance. Impedance includes both resistance and inductive or capacitive reactance. An ohmmeter measures only the resistance.

In addition, power in an AC circuit is voltage X current x power factor. Power factor is a number between 0 and 1 that accounts for the effect of the phase relationship between voltage and current.

In addition, motor current is mostly determined by the load. In an AC motor, the load is like a variable resistance that is infinite when the motor is not running. When the motor is running the load resistance appears to be connected in parallel with the fixed internal resistance and reactance of the motor.

In a DC motor, the load is like a generator inside the motor that has a variable voltage that opposes the supply voltage. The opposing voltage is usually called "back EMF" (reverse polarity electromotive force).

The above is a simplification that attempts to present the general idea. You will need to study the subject in detail to really understand it.


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