Re: Microwave motor runs backwards.

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Does it uncook the food ?

But seriously, I have seen uwaves that each turn on alternate platter
direction - presumably by design.

geoff



Re: Microwave motor runs backwards.
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That is normal operation.
Many microwaves will have a carousel that alternates directions
periodically.

Dave.

Re: Microwave motor runs backwards.

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.

Re: Microwave motor runs backwards.
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    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.






Re: Microwave motor runs backwards.
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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

Re: Microwave motor runs backwards.

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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.



 


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Re: Microwave motor runs backwards.
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This is kind of funny that they  don't know:-)
Anyway,


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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:-)

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Re: Microwave motor runs backwards.

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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.



Re: Microwave motor runs backwards.
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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,

http://hansen.en.alibaba.com/product/50148876/51086487/Shade_Pole_Motor/Shade_Pole_Motor_for_Microwave_Oven.html

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

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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

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Cheers
Terry

Re: Microwave motor runs backwards.

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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




Re: Microwave motor runs backwards.
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should say "shaded pole machines may be synchronous or induction"

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Hi Brian,

I wrote (in response to Bob):

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

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

Re: Microwave motor runs backwards.

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

MrT.



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