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Is microwave turntable motor reeally a synchronous motor?

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cavemen

Sun Apr 11 2010, 06:08PM

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

Is there any IC chip or a circuit that I can use to obtain this kind of function?

How do I build something like that?
it will prolly work on the rectified 120V

radiotech

Mon Apr 12 2010, 01:10AM

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

If you want something simple that will work off 120 VAC, and you accept that the positive on time can approach a maximum of 8.88 mS then use a SCR with a phase control firing circuit simple rc will do. Now for the negative part of the wave, repeat, but connect the SCR in rerverse. You would have
2 potentiometers, one for the positive half duty cycle, ond one for the negative half.Turrned fully up you would have the equivalant of a triac dimmer in series turned all the way up.

If you expect a motor to yield a torque pulsation it had better have a very low mass rotor to do that over a 16.66 mSec rotation of 2 pi radians. Ceiling fan motor triac speed controls make the motor buzz (torque pulsations).

Btw loudspeakers try to do that all the time; the result is a harmonic mix of F1 +fn. Its called timbre.

cavemen

Mon Apr 12 2010, 04:50AM

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

Two ways to achieve the same result.

Wouldn't the bridge one burn out working like that?

1271047813 2008 FT86764 Bridge Controller

1271047813 2008 FT86764 Thrystor Controller

radiotech

Mon Apr 12 2010, 05:26AM

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

Your use of the two scrs, oppositely connected is on the right track, but they work best if gated with a cr network rather than just r.

But you have shifted the focus of your stated problem from an AC syncronous sub-cycle machine to an asymetrical rectifier.

Simply state the function of the device mechanically.

cavemen

Mon Apr 12 2010, 05:39AM

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

How do I state it mechanically?

I just realized that SCR circuit gave out the same waveform as the asymmetrical rectifier.

Not sure what you mean by the CR network, since i came up with the thyristor circuit myself.

cavemen

Tue Apr 27 2010, 05:25AM

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

Are microwave oven synchronous motors that turn the table TWO POLE motors?

I need two pole light duty motors for my project.

Since those motors don't really turn anything, a tiny coil and a magnetic ring from a microwave motor should do the trick.

(i am thinking of those motors)

Thank you for help.

radiotech

Tue Apr 27 2010, 07:38AM

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

AC motors have pairs of poles, 2 pole motors turn at 3600 RPM
if they rotate develop torque without needing a slip to produce
current in the rotor.

2 pole induction motors can be syncronous or non-syncronous. A syncronous induction motor has saliences in the rotor. A hysteresis syncronous motor magnetizes the core of the rotor as it pulls into step.

A good example of an induction - syncronous motor is the one used to run AC power Teletype machines. A hysteresis syncronous motor can be found on certain tape recorders.

Whatever motor you have, if you want to know if it is syncronous, use a tach to find out. If you have no tach, use a neon lamp connected to the AC supply, and observe a mark on the shaft or the rotor illuminated by the neon lamp. A non-floating strobe will indicate the motor is syncronous, and also if it stays that way under a load.

cavemen

Fri Apr 30 2010, 04:04AM

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

I didn't organize my thoughts very well. I am sorry.

I just received a PAIR synchronous motors in a shipment and here is how they look from the inside:

A motor has all the gears coated with lithum grease and a PAIR of coils under those gears, one stacked on top of the other.
This is the part I want to harvest, but I don't need two coils stacked one, over the other with a 15 degree shift from one another, as i noticed by the appearance of the alignment bracket and the 0.68uF capacitor separating the two.
the rotor consists of a barrel magnet that fits through two holes.

Or this barrel receives the magnetic charge when it passes through a phase.
who knows

(All I need is for my machine to speed up and slow down two rotors in opposite directions.)

How do I need to arrange my coils for the motor to do the speedup/slowdown job based on what my asymmetrical rectifier circuit will give out?

What kind of rotor do I have?
Is it a permanent magnet or a piece that gets magnetized/demagnetized?

My device will have a new case anyways.

klugesmith

Fri Apr 30 2010, 04:46PM

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

cavemen wrote ...
What kind of rotor do I have?
Is it a permanent magnet or a piece that gets magnetized/demagnetized?

You could answer that, even count the poles, with a paper clip or some iron filings.

Thank you for the pictures. Maybe an understanding of this motor's normal operation will help. Not hard to find with Google, instead of waiting for answers here. But since I'm started...

Your rotor is a permanent magnet with 6 north poles and 6 south poles. You could re-magnetize it differently.

Look at the sheet metal stampings that bracket each coil, and extend into interdigitated pole fingers next to the rotor. When the coil is energized with DC, one set of fingers become north poles while the other set becomes south poles. The rotor will be stable in one of 6 rotational positions, 60 degrees apart. (try it with a battery!)

If the coil is energized with the opposite polarity, the rotor stable positions are halfway between the first set. The second coil, which used by itself generates stable positions 15 degrees away from those of the first coil. You can see that if you energize both coils with AC, a quarter-cycle apart (that's what the capacitor is for), then the rotor will want to turn 60 degrees in each electrical cycle.

cavemen

Fri Apr 30 2010, 07:32PM

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

Thank you for explaingnig. I couldn't find that explanation with google.
So for my application all I need is to remagnetize the rotor to two poles and mount the coils one over the other dead on.
Right?

How do I remagnetize a rotor to have two poles?
Place it inside a big coreless coil that is powered by rectified 120vac?

My device will have it's own frame.

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