# Three phase Motor rotation

Is it possible that a three phase motor still spin in the same direction it was even after the phases are interchanged.

• I would like to think that it could spin stably at twice nominal speed in the opposite direction if you could force it into that state, but I wouldn't like to try it. Maybe arrange for a motor inverter to suddenly switch to half frequency supply at opposite rotation would be a safer way to experiment. Maybe that would only apply to PM rotor motors, and not induction types? – Neil_UK Feb 10 '16 at 8:14

If you swap any two of the three phases, then it should spin in the opposite direction.

I believe that I once heard that a motor with an open phase may start as a single phase motor if the other two phases are good. That can only happen if the load, or at least the starting load is very light as it would be with a fan. There is nothing to determine which way the motor might start except some characteristic of the load like a little air moving through by convection or something. If the two good phases are interchanged, that load characteristic would tend to make the motor start in the same direction regardless of the change.

• +1 Here is a Youtube video illustrating this. The motor also tends to draw excessive current as long as it is stalled. Protection devices (lost phase detectors, overload relays) are sometimes fitted to prevent this. – Spehro Pefhany Feb 10 '16 at 6:55

If you swap any two phases, it should spin in the opposite direction.

HOWEVER if you swap any two phases instantaneously on a running motor with a high inertia load, it could conceivably continue to run in the original direction.

I can't swear to this with a 3-phase motor but it's certainly possible with a single phase motor with a run capacitor (which phase shifts a second winding).

So if such abuse tripped the reversed phases, the remaining phase could indeed power the motor in the original direction.

"Plug reversing" a running motor is not a good idea for other reasons. Braking to a stop then reversing will safely work as intended.

We need to know more about the context of the question : has this been suspected in an accident situation? is it part of a safety case? is plug reversing a running motor involved? etc.

Is this a theoretical swap because open circuiting a charged inductor is only a good idea if you want to generate an arc

If however the phasing was swapped "electronically" via control laws of a V/f controller, the motor would very quickly stall and start rotating in the opposite direction