# Calculating kW of rotational power required by an engine to generate 3 phase power when phases are unbalanced

With respect to phase balance -- or lack thereof, if one phase is much higher than the others (for example 75, 100, 75) the total power according to my generator's display is 46 kW, but would the diesel engine be loaded more? Maybe as if they were all 100 A, trying to spin and make the force required for largest magnetic field? (I'm not sure if my terminology is correct), or is it an average of them? Or root? Or something else?

The goal is to quantify the energy (fuel) being wasted (or not) by unbalanced loads on my generator. I'm not necessarily looking for an all encompassing equation to calculate the exact kW of power the engine output needs to produce 46 kW of electrical power given the unbalanced phases condition, but I would like to understand the major variables' proportional effect on the power required to make more power. ;-)

• @Transistor You beat me to it. Mar 4 at 22:13
• @winny. You've got to be quick around here to stamp out kelvin-watts (KW). Mar 4 at 22:20

Consider the case of a single-phase, 2-pole generator; the load would vary with the angle reaching a peak every 180° and zero in between those positions.

With three phases the loading on the engine will be smoother even with some inbalance.

Add in a flywheel and whatever other inertia is there including the engine and alternator rotor and I'd say you can average it out for modest imbalance.

You might find my answer to https://electronics.stackexchange.com/a/390400/73158 of some interest.

In unbalanced conditions generator current consists of positive, negative and possibly zero sequence. Negative sequence current produces magnetic field rotating backwards, thus rotor rotates with doubled synchronous speed relative to it. Zero sequence current produces non-rotating magnetic field, thus rotor rotates with synchronous speed relative to it.

These magnetic fields induce currents in the rotor steel and winding. Negative sequence magnetic field induce current with double frequency. This all cause additional heating and harmonics in the generator voltage.

Heating means more power dissipation. Measured in kW it might be not that much, about 2-3 times of that in normal balanced state. But chances are that you may burn your generator. You might want to refer to the manufacturer on the allowed load unbalance.