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Small synchronous motor for a little SRSG?

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Dr. Dark Current

Sat Jan 21 2012, 09:11PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Hi,

for a TC project I need to build a little RSG firing at 100 Hz (50 Hz line frequency), so ideally I would need a 2-pole synchronous motor running at 3000 RPM. I can get away with 1500 rpm too, though. The required motor output power is only a few Watts, but the motor must be able to reach synchronism with load of course.

The problem is that I can't find such motor anywhere so I'll probably need to make it/modify an existing one. There are little synchronous motors in timers or for powering the microwave oven turntable, but these are usually at least 8 pole motors and the output power is probably too low.

I have tried modifying a microwave fan motor in these ways:

-grinding two flats in the rotor, similar to this picture:

Results: With no load attached, the motor runs at synchronous speed. However when some load with inertia is attached, the motor can't reach synchronous speed by itself. When I quickly tilt the motor, it "snaps" into synchronism and holds it, but this is of no use in a TC.

-grinding off more of the rotor and putting two magnets in it 180 degrees apart, one with North pole facing outward and one with South pole outward.
Results: The motor can't even reach synchronism with no load attached.

I have more microwave fan motors to experiment on, so I'm open to any ideas I could try to make an usable small synchronous motor.

Any other ideas?

Steve Conner

Sat Jan 21 2012, 09:27PM

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

Some old record players, reel-to-reel tape decks and even 8 inch floppy drives, had a synchronous motor.

I've got a 1500rpm one from a dead Ferrograph Series Seven. It looks like an ordinary 4-pole motor, with a run capacitor, and an ordinary induction motor rotor with 4 flats machined on it. It works well, but my own efforts to modify 2-pole shaded pole motors failed dismally with similar results to yours. I think starting with a capacitor-run motor helps a lot, because it has a good strong rotating field. The shaded pole motor has a weak one with a big pulsating component.

Dr. Dark Current

Sun Jan 22 2012, 06:28PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

I just tried modding an universal motor by soldering diodes to the commutator. With the diodes across the whole half of the commutator, the motor has problems starting, but with the diodes across just a few pads it works OK. when the input voltage is set correctly. I think it should be usable for a SRSG.

Steve Conner

Sun Jan 22 2012, 06:31PM

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

Where did you hear about that trick?

There were people talking about it on the TCBOUK mailing list last year, but I've never heard of it before or since.

I guess there's a range of input voltages for which it'll lock. Can you adjust the phase shift by varying the voltage within that range?

Erlend^SE

Sun Jan 22 2012, 06:46PM

Registered Member #1565 Joined: Wed Jun 25 2008, 09:08PM
Location: Norway
Posts: 159

The pumps in many washing machines are synchronous,
you may be able to modify one of them into a RSG.

Marko

Sun Jan 22 2012, 07:12PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Steve Conner wrote ...

Where did you hear about that trick?

Hi guys,

this was discussed on 4hv too a while ago:

Probably the best solution around apart from getting a genuine synchronous motor!

I've also contemplated the possibility of using a car alternator as a synchronous motor, or one of those chinese RC brushless motors with a properly stepped-down 3 phase supply and field supply in case of the alternator. Not sure if it's more practical in comparison to the first though!

Erlend^SE wrote ...

The pumps in many washing machines are synchronous,
you may be able to modify one of them into a RSG.

Seriously? I've never ever seen a synchronous washing machine pump. They are all asynchronous and shaded pole!

Marko

Dr. Dark Current

Sun Jan 22 2012, 07:32PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Steve,
after thinking about the most bizzare ways to obtain a sync motor, this came to my mind also, then I remembered reading about it here so I searched it. I think the phase could be adjusted by varying the volage, but there will be a voltage at which the motor runs most stable - and I would rather change the phase by physically turning the spark gap rotor against the shaft.

I also heard about the washing machine pumps, but it seems they are only in modern washing machines and I have no idea where to obtain it. I dug out some old washing machine parts and the pump had a big asynchronous shaded pole motor in it.

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