[SOLVED] Do synchronous motors change the direction under heavy load?

[adnan_merter]

Hello

Even though i am not sure this is the correct place to ask this question but i hope someone helps me.

Today one of my friends said some synchronous motors change its rotation direction under heavy load. I just read as many document i can read but haven't seen anything about it. Is it motors property or some kind of electronic device detects the over current and change the direction?

 

[dick_freebird]

I have never seen a single phase AC motor do this,
all the way down to stall. The phase slip should
always be in one (lagging) direction and repeats
across line cycles.

Not to say I've seen every rotor and stator style,
by a long shot, but this does not match my limited
experience with abusing motors.

 

[BradtheRad]

I recall working on a few motors in electric clocks made 1940-1970 or so (120 VAC). Usually the whole thing is in a flat metal case, roundish figure-8 outline. One output gear is visible and it turns very slowly. I can barely get a good enough grip with my fingers to slow it down. I have stopped it by grasping it firmly with pliers. It produced a tap from within, then turned in the opposite direction.

Maybe that's the effect you're asking about?

By stopping its movement repeatedly I made it go one way, then the other. It resumed the correct direction when I removed power briefly.

 

[betwixt]

Experiment:
Keep starting a microwave oven then stopping it. Note the turntable rotation. It will run clockwise or anticlockwise, apparently randomly. I would say SOME single phase AC motors are likely to restart from a stationary position in the same way.

 

[FvM]

It can't happen with two- or three-phase synchronous motors that involve a rotary field. There's however a type of small single phase motors with an only oscillating rather than rotating field and a mechanism that achieves starting in the right direction. The clock drive mentioned by BradtheRad is of this kind.

 

[adnan_merter]

Thank you all answers, I read the wikipedia page again and you are right single phase syncoronius motors can start any direction unless prevented doing so.

A single-phase (or two-phase derived from single phase) stator winding is possible, but in this case the direction of rotation is not defined and the machine may start in either direction unless prevented from doing so by the starting arrangements.[29]