Single Phasing

Well, part of it is that the unit will try to operate on only two wires out of three. If the power demand remains the same, the current will go up by three wires divided by two wires.

However, without a rotating magnetic field due to the absence of one of the phases, the motor may stall. And we all know that a stalled rotor takes about 5 times the full load current, don't we? At this point, you'd better make sure your overload protection equipment is capable of disconnecting it, otherwise there may well be smoke, fire, a rewind, and so-on.

"... the current will go up by three wires divided by two wires."

Well, actually the current goes from 3 phase current to single phase current, so it goes up by the square root of 3, or 1.732 times the 3 phase current (3/2 = 1.5).

But also, the voltage effectively goes down by the same amount, becuase the same voltage from the single phase source is divided by the 3 phases, so it is the equivalent of 58% of the line voltage. Then since torque in an AC squirrel cage motor is a factor of the square of the design voltage, the peak torque reduction is .58 x .58 or it is now 33% of the full load torque capability. If the speed has not changed (because frequency and number of poles is still the same) and torque is reduced to 33%, then so is the mechanical kW(HP) capability of the motor. So if the load does not reduce to 1/3 of what it was before the phase loss, the motor increases slip, which causes it to draw more current as it attempts to accelerate back to design slip and if nothing else changes, it over loads and hopefully your overload relay takes it off line.

But if the motor is unloaded at the time, you may never know there is an overload. If that happens and the motor continues to run on single phase power, then you can still lose the motor. When you have a current imbalance on a 3 phase motor, that creates what is called "negative sequence current", which in turn causes counter rotating torque in the rotor. This opposes the normal torque and provides no useful work, so it is all converted to heat. That heat can build up inside of the motor without ever exceeding the FLC setting of the overload relay and the motor can actually burn up without ever tripping the OLs. That is what a Phase Loss Protection circuit does for you.

We can offer you a job in a sunny part of the world if interested.

If you use symmetrical components you can understand it better

I like to use this Bussman information to explain this problem. The drawing make it easier to see what happens.

https://www.progress-energy.com/assets/www/docs/business/motor-protection-voltage-unbalance.pdf