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Is microwave turntable motor reeally a synchronous motor?

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cavemen

Sat Apr 03 2010, 03:56AM

Registered Member #2008 Joined: Tue Mar 03 2009, 05:11AM
Location: USA, Frederick, MD
Posts: 118

I am trying to build a machine that would require two synchronous motors that can maintain the timing.

I was looking for a light-duty synchronous motor and I see a lot of microwave turntable motors being sold as synchronous.

I had one in my house and I ripped it apart to see what is inside. There are layers of stamped plates with gears and there is a motor. A small ring-shaped winding stator and a thyroidal magnet for a rotor. I need that part.

Before I buy two identical motors of this kind, I want to make sure those are really
synchronous. Would those motors loose their synchronization if I run them without a load of the reducer mechanism that I pull out of the case?

I will probably find some way to machine moving parts for those motors. So I will just use the winding and the round magnet, since those motors will be used for higher duty application.

Thank you.

Sulaiman

Sat Apr 03 2010, 05:54AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

If the rotor is a permanent magnet then it's a synchronous motor,
it will be synchronous with or without gears, with no-load to full-load.

Arcstarter

Sat Apr 03 2010, 06:19AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

If this does not work out for some reason, you can always go for a dryer or washer timing motor i am sure.

But i am fairly sure MO's use sync motors. I remember i used to have one that i turned by hand and it would zap you a little bit because of the induced voltage. Things where simpler back then

If the motor is not syncing then you will hear it. The sound will change, you will know it when you hear it.

And again, if that does not work because you need more torque, you can make a MO fan motor into a 3600rpm sync motor, by sanding two flats 180 degrees from each other (or 4 flats 90 degrees from each other for a 1800rpm motor). You just shave it off until it starts running sync. I made a srsg that way once.

klugesmith

Sun Apr 04 2010, 12:50AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

In my experience most MWO turntable motors are true permanent magnet synchronous motors. The rotor turns once every 4 to 8 cycles of the AC power.
This is geared down to, usually, an integer (like 4) revolutions per minute.

So if you run the MWO for a whole number of minutes, the turntable stops in the same orientation as it started (e.g. with coffee mug handle facing the operator's favored hand).

Often they save a penny by omitting the anti-reverse pawl, and of course they would never dream of including a capacitor or pole-shading loops. So the direction of rotation is randomly chosen each time the motor starts.

Back to original question: I think there isn't much you can do to greatly increase the torque of the rotor. (just cheap ferrite or plastic permanent magnet material). You can determine the number of pole pairs by inspection of the stator.

cavemen

Sat Apr 10 2010, 11:19PM

Registered Member #2008 Joined: Tue Mar 03 2009, 05:11AM
Location: USA, Frederick, MD
Posts: 118

Thank you for telling me a lot of useful information.

Do they make small motors with two poles that make 1 revolution/ AC cycle?
Can i convert a microwave motor into one?

What is under that single coil of wire that surrounds the rotor, that defines what is an rpm/ac cycle ratio?

I need 1rpm/cylcle, because i want my motor to speed up on one half of the way and slow down on the other, but that is a long story of what the mechanism does and how it overcomes inertia of the rotor.

I just want the power to pulse, so one half of the wave would go through a resistor and diode, the other one will go straight.

radiotech

Sun Apr 11 2010, 01:27AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Look into a type of AC stepper motor called a SLOSYN- High Torque, Slow speed. They were available cheaply as surplus.

IamSmooth

Sun Apr 11 2010, 02:15AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

If you need a synchronous motor you can find them on Ebay. I have one for my TC.

cavemen

Sun Apr 11 2010, 03:16AM

Registered Member #2008 Joined: Tue Mar 03 2009, 05:11AM
Location: USA, Frederick, MD
Posts: 118

i mean that my moter needs to accel for the half of the revolution, decel on another half

radiotech

Sun Apr 11 2010, 04:11AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

I just want the power to pulse, so one half of the wave would go through a resistor and diode, the other one will go straight.

To do this on 1 AC cycle, you could look for a 'syncronous' chopper.
It vibrates at whatever coil frequency is specified and has a switch contact that moves from one contact to the other at syncronous speed. They are sealed, and plug in.

Here is An OAK chopper

Sulaiman

Sun Apr 11 2010, 03:12PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

If you have a fixed frequency drive you can't slow down the motor for 1/2 cycle by reducing voltage/current/power etc. it's a SYNCHRONOUS motor!

What's the application?

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