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quick synchronus motor question

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Smiley

Fri Jul 03 2009, 07:14AM

Registered Member #1249 Joined: Mon Jan 21 2008, 07:18AM
Location:
Posts: 10

Hi all, I've got a synchronous motor from an old record player that has no name badge on it. I'm hoping to use it for a synchronous gap in my tesla coil provided its powerful enough but I need to know the RPM of it, which I hope to work out with s=f120/np. Can anyone tell how many poles it may have from its dismantled state in the pictures below. As can be seen, the rotor has the numbers 1-6 on it, not sure if this is the number of poles. I cant really tell from the stator.

03072009264

mod edit: oversized pix

Arcstarter

Fri Jul 03 2009, 07:37AM

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

I would say 1800rpm. The stator has 4 copper shorts it looks, and for 3600rpm operation there are 2 of them, but this is a non sync 3450rpm motor from a microwave fan. But it seems most likely.

Here is a simple way to find it out. You will need to do it anyway to get the flying electrode disk to line up with the stationary electrodes as the voltage approaches the peak and trough of the sine wave.

Get a neon bulb, or even anti-parallel LEDs with their own diode and 15kohm few watt resistor in series with 120v. Glue a piece of paper that is like 1cm wide and around 4 inches long to the output shaft. Bring your light source close to the paper with the motor running in a dark room. If you see 2 stationary 'figures' that seem to 'fade in and out' (due to the sinewave reaching higher voltage making the lights light bright enough and blah blah blah) then it is 3600rpm. 4 would be 1800rpm.

Any more questions, feel free to ask.

Smiley

Fri Jul 03 2009, 07:46AM

Registered Member #1249 Joined: Mon Jan 21 2008, 07:18AM
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Posts: 10

Hey, that helps a lot, just like using a timing light on your car. :) I'll see what I can do. Thanks.

Smiley

Fri Jul 03 2009, 09:39AM

Registered Member #1249 Joined: Mon Jan 21 2008, 07:18AM
Location:
Posts: 10

now that i've thought about it a bit more, based on your information, i dont even need to know the the rpm, just need to set my flying points wherever I see the makeshift arm light up.

Arcstarter

Fri Jul 03 2009, 07:43PM

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

Smiley wrote ...

now that i've thought about it a bit more, based on your information, i dont even need to know the the rpm, just need to set my flying points wherever I see the makeshift arm light up.

Correct. I am very surprised you understood anything i said :P.

EDIT: Well, if you do the light thing and you see 4 figures, so to speak, you will need either 4 flying electrodes and 2 stationary, or 2 flying 4 stationary. connect all the flying electrodes together, and if 4 stationary electrodes are used, you need to connect the two electrodes on each side of the srsg together.

Herr Zapp

Fri Jul 03 2009, 08:45PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Smiley -

The bare copper rings in the stator indicate that you have a "shaded pole" motor.
There are two types of shaded pole motors: shaded pole induction motors, and shaded pole synchronous motors. Shaded pole synchronous motors are used in clocks and phonographs where precise speed control is required, regulated by the AC line frequency. In shaded pole synchronous motors, the rotor core is permanently magnetized. Check if the exposed iron core laminations in your rotor are magnetized; this will confirm that it is a synchronous type motor.

These shaded pole motors don't develop much torque, and you may find that your motor may not have enough torque to spin up even a small, lightweight rotor.

You might be better off starting with a physically larger motor with a larger HP rating.

Regards,
Herr Zapp

Arcstarter

Fri Jul 03 2009, 09:55PM

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

These shaded pole motors don't develop much torque, and you may find that your motor may not have enough torque to spin up even a small, lightweight rotor.

You might be better off starting with a physically larger motor with a larger HP rating.

I made that modified MO fan motor srsg, and it stay perfectly sync all the time. Uses a small disc, bgut it is possible and very simple in fact to make one. I bet this motor has more torque than a modified MO fan motor, if did not have stuff taken off of it.

Steve Conner

Sat Jul 04 2009, 08:56AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I have a sync motor of about the same size that I got from an old tape deck. The rotor looks very similar to the one in the original pictures, except it has four flats milled on it. The stator looks similar too, but it has two sets of windings and a capacitor, rather than shaded poles.

So, I'd guess this is an induction motor, but it could be modified for sync operation at 1800rpm by grinding four flats on the rotor. Unless it's already a hysteresis motor, in which case the rotor will be made of hard steel that can be magnetized, as Herr Zapp suggested. Try touching it with a strong magnet, then removing the magnet and see if the rotor still holds some magnetism.

The only downside of a hysteresis motor is that it doesn't always sync in the same position, so you may need to reset your spark gap phasing every time you run the coil.

I tried modifying MO fan motors too, but could never get them to sync.

PS, the numbers on the rotor are for use with a balancing machine in the factory, as far as I know.

Arcstarter

Sat Jul 04 2009, 06:00PM

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

The only downside of a hysteresis motor is that it doesn't always sync in the same position, so you may need to reset your spark gap phasing every time you run the coil.

I tried modifying MO fan motors too, but could never get them to sync.

The MO fan motor always syncs in the same position, and they are 3600rpm, so if you have problems with that motor i suggest an MO fan motor. What i did for the disk was take one of the fan blades, chop the actual blades all the way off and grounds them flat. That is your disk. Then i drilled two holes in the sides, where the blades attached, exactly opposite of each other. Then i worked a small bolt into each hole, with the head facing inward (cause otherwise you have to use bolts to hold it in, and if they come lose you have to worry about being hit by a bolt). Then one bolt on the outside to keep it from moving around. Electrically connect the two somehow, making sure it does not because the least bit unbalanced, vibrations will bring the tiny motors out of sync.

When these motors go out of sync, you can hear it. I cannot explain the sound, though. Since the rotor is so small, the flats are very easy to machine. A file and half and hour will do. Just two flats perfectly centered. Grind them until you can see the aluminum shorts, then grind a bit more. Keep putting the rotor back into the motor and testing it with your disk until it *stays* sync. Once sync, it will always sync in the same position if you ground the flats correct and did the deep enough but not too deep.

Have fun.

klugesmith

Sun Jul 05 2009, 12:21AM

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

Steve McConner wrote ...

I have a sync motor of about the same size that I got from an old tape deck. The rotor looks very similar to the one in the original pictures, except it has four flats milled on it. ...
So, I'd guess this is an induction motor, but it could be modified for sync operation ... Unless it's already a hysteresis motor,
...
I tried modifying MO fan motors too, but could never get them to sync.
...
PS, the numbers on the rotor are for use with a balancing machine in the factory, as far as I know.

After taking apart -my- synchronous record player motor, I bet Smiley's motor is synchronous with repeatable phase, in spite of 1) no salient poles and 2) apparent squirrel-cage conductor slots. Picture time!
Already knew mine had 4 shaded poles:

The fan end of rotor shows evidence of balancing, as Steve suggested:

Side view of rotor shows laminations (probably with buried squirrel cage bars), stacked with what looks & behaves like a plastic or ceramic permanent magnet. I think there's one in Smiley's picture too.

Not having time to study the magnet any more on 4th of July (could not find the magnet viewing film) , I put motor back together & repeated an old check of mechanical phase. Here's the painted flag on stopped motor, in normal light:

and on spinning motor lit by 60-fps synch. stroboscope. (Uh, a still photograph fails to convey the apparent lack of rotation. Blur is from strobe duty cycle).

Braked by hand, the motor shows a fair amount of synch. torque. When it slips, or is re-started, it always goes to one of two steps separated by 180 mechanical degrees = 360 electrical degrees. Without more data, I think this gadget has a permanent magnet with 2 north & 2 south poles, and a conventional induction rotor for better starting torque.

My experience with filing flats on little fan motors is intermediate between Steve and Arcstarter. This unit turns synchronously, but only after warming up for a minute. Along with other flattists here, I lament not having a sound & video recording of the transition from almost-locked to locked.

Finally, a new curiosity. (should this be a new thread?)
This gearhead motor has a 3600 RPM synchronous rotor, with -two- stable phases. Not consistent with a permanent magnet, so probably based on variable reluctance with salient poles on rotor.

Because of the gearing, I can brake it or speed it up with a twist of the fingers. The phase stiffness seems to be higher when the torque tends to speed it up, but that could be an illusion, or bias due to friction in the gear train.

Now off to activities more appropriate for our national holiday!
-Rich
[edit] Just made & posted a webcam video of my flatted MO fan motor. Does this link work? File size is 2.2 MB.

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