Re: Microwave motor runs backwards.

That is normal operation. Many microwaves will have a carousel that alternates directions periodically.

Dave.

When I found the first new motor behaved this way, I told my supplier what it was doing and I was told to return it. I have now told them the second motor is doing the same thing, and they suggest there is another problem in the oven that is causing this to happen.

They suggested ringing Sharp for help.

It may be normal, but I have repaired many microwave ovens and have never seen this happen before.

Even if I power the motor out of the oven, the direction it turns is quite random each time I apply power.

Russell.

Synchronous motors start and run in a random direction unless there's a mechanism in them to force them to only go one way. A Google search indicates that lots of microwave oven turntable motors are of the synchronous variety.

This is kind of funny that they don't know:-) Anyway,

I noticed the direction thing many years ago and I found it a great idea. Imagine the plate getting stuck (limited friction between the plate and motor of course also provides for safe operation), it just changes direction. This provides for (limited) even cooking even in awkward situations. In worst cases it may even prevent a motor from burning out. And no brushes to wear. Brilliant:-)

Yep, my Sharp does exactly that. I can't see any great benefit, but presumably there is a reason.

MrT.

shaded pole motors in particular. I've only repaired a dozen or so microwaves, but they all had shaded pole motors. very cheap.

for a fun, analagous exercise, take a vacuum cleaner apart, and remove the fan. Then re-assemble it, stand well back and turn it on. no-load speed tends to infinity, so it spins until it destroys itself.

Cheers Terry

"Terry Given" wrote

shaded pole motors in particular. I've only repaired a dozen or so microwaves, but they all had shaded pole motors. very cheap.

****Shaded pole motors will start in the same direction at each start and they are NOT synchronous!Shaded pole motors area variant of an induction motor.

for a fun, analagous exercise, take a vacuum cleaner apart, and remove the fan. Then re-assemble it, stand well back and turn it on. no-load speed tends to infinity, so it spins until it destroys itself.

**** Vacuum cleaners are driven by a brush and commutator motor,commonly called a universal motor.Universal motors are series wound and are NOT a variant of the induction motor.

Brian G.

The IEEE dictionary of electrical & electronics terms defines a shaded pole machine as:

"a single-phase induction motor with a main winding and one or more short-circuited windings (or shading coils) disposed about the air gap. The effect of the winding combination is to produce a rotating magnetic field which in turn induces the desired motor action"

and they are synchronous. If you looked at wikipedia you would notice that unidirectional shaded-pole motors use a mechanical device to prevent it starting in thw wrong direction. without said device, the direction is "arbitrary" (OK, it depends on the rest position of the rotor)

besides,

yep, its a shaded pole motor, and turns in an arbitrary direction.

who said they were? I just said its no load speed was, theoretically, infinite (it is). Try the experiment I suggested, its fun (albeit dangerous fun). Perhaps you might want to look up the word "analogous" in a dictionary. And a bit of electrical machine theory wouldnt hurt, either

Cheers Terry

"Terry Given" wrote >

The IEEE dictionary of electrical & electronics terms defines a shaded pole machine as:

"a single-phase induction motor with a main winding and one or more short-circuited windings (or shading coils) disposed about the air gap. The effect of the winding combination is to produce a rotating magnetic field which in turn induces the desired motor action"

and they are synchronous.

***** Absolutely incorrect! Shaded pole induction motors are NOT synchronous! Shaded pole synchronous motors ARE synchronous! They are two different types of motors. Shaded pole induction motors use a squirrel cage rotor! Shaded pole synchronous motors use a permanent magnet rotor! They are two different types of motors. Shaded pole induction motors are unidirectional at startup. Shaded pole synchronous motors are non directional at startup. They are two different types of motors.

If you want to play at electric motors,at least get your facts correct (or at least,learn how to read the very wikipedia entries you quote).

Brian G

should say "shaded pole machines may be synchronous or induction"

Hi Brian,

I wrote (in response to Bob):

you started talking about induction motors; I was referring to synchronous motors. the relevant section out of the IEEE definition is that regarding shading coils.

furthermore, my comment referred to microwave oven motors, and was intended to be read as "lots of microwave oven turntable motors are of the synchronous variety, shaded pole motors in particular"

I suppose you could interpret it to mean "shaded pole motors in particular start and run in a random direction..." but thats not what I meant. isnt reading comprehension fun.

we can, however, conclude a couple of things:

- shaded poles can be used with both squirrel-cage & PM rotors and can therefore be either synchronous or induction

- the IEEE definition neglects PM rotors. naughty them.

- most microwaves use shaded pole synchronous motors, and hence start in an arbitrary direction (because they dont care which direction it rotates, only that it does)

- SP synchronous machines start in an arbitrary direction (unless a mechanical device prevents this)

- SP induction machines start in a known direction.

the reason for the last 2 is fairly simple. The sinusoidal distribution of windings in an induction machine (along with supply phasing) controls the direction of rotation of the stator H field; being induced, the rotor H field is always "pointing the right way", so an induction machine always starts in the same direction. Whereas for the PM rotor synchronous machine, the rotor H field is absolute, and is created by the magnets glued to the rotor; the direction of rotation is a combination of the stator H field and the rotor H field, which depends on where the rotor sits wrt the stator when energised.

interestingly enough, for very large machines, induction motors look just like synchronous motors (large machines have extremely narrow slip

- once slip is very small, it starts to look like torque angle).

ultimately, there is nothing wrong with the OPs motor.

Cheers Terry

That is correct. I bought some synchronous motors from Oatley Elect. years ago and only recently found a use for them. I got hold of a laminator machine with a stuffed motor, so I thought I'd put one of the Oatley motors in. It fitted alright, had the right speed, but would start up in a random direction. OK for a microwave but a little annoying for a laminator !!!! A web search turned up some Asian supplier and there were two types of synchronous motors for microwaves etc., those that start in a particular direction (labelled CW and CWW I think) and those that started randomly. There is a gear wheel and a pawl inside the motor of the directional types that kicks it backwards if its starts going the wrong way on power-up.