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Synchronous rotary spark gap motor help.

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LAN_403

Arcstarter

Tue Mar 10 2009, 05:47PM

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

I got a rotary vane pump a year or two back, and i took the pump part off, to use the motor as a rsg.

Now i want to make a synchronous rotary spark gap with it. I know that you can make an induction motor synchronous by grinding 2 flats in the rotor (i have a 3450rpm motor). The problem is that i have no sophisticated tools and nothing to measure weight or size with. A caliper would be great though.

The motor model number is 1521-107B-G288X.

It is 1/10 HP, and 3450 RPM with 60hz.

The tools i have are very basic. Angle grinder, jigsaw, drill. I have sandpaper and a belt sander, and access to a bench grinder.

I basically want to know how i should make the flats. How 'deep' they should be is basically all that i need to know.

Herr Zapp

Tue Mar 10 2009, 11:49PM

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

Arcstarter -

Six seconds searching the web yielded:

Regards,
Herr Zapp

Timo

Wed Mar 11 2009, 12:41AM

Registered Member #1538 Joined: Thu Jun 12 2008, 07:28PM
Location: Bonn, Germany
Posts: 28

Hi Arcstarter

I will try to help you out by adding some pics showing my SRSG in a disassembled condition. I build it using a motor taken from a laundry / tumble dryer. It was used to power its fan. Maybe itâ€™s something like 1/4 hp, but I donâ€™t know. Now 1500 RPM (50 Hz). The motor turns the wooden disk (diameter 20 cm) easily and never looses synchronisation. It takes about one second to spin up, than all of a sudden the noise generated changes and the motor "locks" to the line frequency. However, it is always possible to slow the disk by touching it with your hand (carefully!) to ad some additional torch.
I noted that the heating of the coils increased a little, and can get uncomfortable to touch if the motor is left running for hours spinning the disk. But a 30min run is no problem at all.

The rotor: As you see it has 4 flats (of course, 1500RPM!)

All the Parts:

And, finally, a picture of the large number of tools required:

Arcstarter

Wed Mar 11 2009, 01:30AM

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

@ Herr Zapp:
I actually visited that site. I suppose i am not remembering what i read. I basically want to know how much i should take off. From what i read previously on that site, it states that some of the 'shorts' in the rotor should be taken out. I just need to know how much would be necessary for my motor.

@ Timo:
Thanks alot! That sure does help. I do have a file, but not a rasp >.<. I am sure i can find something abrasive enough though, i could either use the angle grinder, or maybe even my hands (seriously, my calluses are very rough!!)

UPDATE: I just noticed, there are two places in the rotor where the color changes (it is grey, when the majority is an iron color). You can see it in the photo on the first post, it was right in the middle of the rotor. It is the last or second to last picture. Should i avoid putting a flat in that part? It would make really no sense to have to, seeing as it looks to be a discontinuity already :P.

Herr Zapp

Wed Mar 11 2009, 03:40AM

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

Arcstarter -

Go back to the evolve site, and take a second look at the procedure that's presented. The instructions are pretty explicit.

There are no "standard" dimensions for the rotor flats; every different motor type and model will have its own unique flat dimensions, based on the motor's rotor and stator dimensions and winding configuration.

The critical dimension for the rotor flats is the width. Examine your stator assembly, and determine the distance between the "dead" poles. You will want the width of your rotor flats to match the distance between the dead poles on your stator.

If at all possible, find someone with a milling machine and have the flats milled precisely, rather then try to use a file, belt sander, etc. Any machine shop could do this simple machining in 15 - 20 minutes.

The flats CAN be created at home via crude methods, but the rotor balance will never be as satisfactory as on a rotor that has been precisely machined. Because of its higher speed (3600 RPM vs 1800 RPM), your motor will be particularly sensitive to unbalance caused by makeshift machining.

If you don't make the flats large enough, the motor won't achieve synchronism with the AC line when loaded with the RSG disk. If you take off too much material, the motor will run very hot, and still may not have enough torque to synch up.

In any case, after modification the motor will have less torque (= less horsepower) than it originally did. Try to minimize the mass of the RSG wheel and electrodes, while maintaining adequate strength to prevent self-destruction due to centrifugal force.

Regards,
Herr Zapp

GeordieBoy

Wed Mar 11 2009, 01:41PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Timo, that motor is just like the one I used for my first Synchronous RSG. It also came from a clothes dryer and had the same mounting cradle and clips that let you rotate the motor in the cradle. Very important for finding the best firing angle of the spark gap.

I'd say that the amount of material you took off with the hand file is just about right or maybe slightly on the cautious side. A good way to test the salient pole synchronous motors locking ability is to mount the RSG disc and electrodes, spin it up to speed and check it locks. Then slowly decrease the supply voltage to the motor using a variac. You want to be able to get well below the worst case low mains voltage before the motor starts to pole slip (accompanied by that strange "weeeoooouuuw... weeeoooouuuw,,, weeeoooouuuw..." noise!) If the motor pole slips with only a slight reduction in mains voltage, or with slight mechanical loading you need to take off more metal to increase the synchronous locking torque.

The important thing is that if you take off too little metal the motor will only barely lock synchronously with the line. Then when you fire up your high power tesla coil the extra current draw from the line will might make the motor loose synchronicity or will at least shift the firing phase significantly. On the other side, taking off too much metal will cause the motor to run hot because you are increasing the air gap in the magnetic path. For a marginally rated motor it can also reduce the low-speed torque making the motor unable to actually get a large rotary disc up to synchronous speed!

-Richie,

Arcstarter

Wed Mar 11 2009, 09:35PM

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

I have now ground some flats now, but it is still not sync i dont think. When i slow it down enough, i can hear the sound change just like i expected, but this means nothing. I put a dot on the shaft with a sharpie, and put it near a regular 60hz light (the only source of light). I could not even see the dot, no matter where i looked. also, the is a hole from side to side on the shaft, and no matter how i looked through it i could see through it, but with some blur.

Update: Now i tried under a florescent light, and it was no better.

Timo

Thu Mar 12 2009, 01:22AM

Registered Member #1538 Joined: Thu Jun 12 2008, 07:28PM
Location: Bonn, Germany
Posts: 28

Richie, thatâ€™s funny. The same motors all across Europe. Btw, I actually tried its ability to lock to the mains frequency exactly as you said. I had to disassemble the motor twice, because I filed down to little in the first try. Now the motor locks at about 180V input with the disk installed and stays locked until the voltage is lowered significantly .
To avoid problems with an unbalanced rotor, I filed as flat and strait as possible (no curvature) and measured the width of the flats (pic 4 in my previous post). Same width gives roughly the same mass. Trivial statement, at least easier than filing straight. But, however, it is a lot less critical with a motor that 1) runs slower, 2) is bigger (12mm shaft, tough bearing), and 3) has a rotor that is heavy compared to the material you need to file away. After all, I think that my wooden disk is a lot worse balanced that the rotor

@ Arcstarter: Maybe your motor runs synchronous already. I was never able to see any sign of synchronous rotation with light bulbs or florescent light tubes. To check for proper operation I used a bright LED driven by a short puls (~ 100-500us) coming from a monoflop (monostable multivibrator? translation problem >.< ). This was triggered by the output voltage of a small transformer once per mains period. A sharp picture of something spinning with the motor can only be seen with very small duty cycles (100us << 20ms).
The easy method would be to run it from a variac and listen to the sound. It will become steady if the motor has locked, and changes with a relatively low frequency if it dose not lock. But on the other hand a motor without flats produces a steady sound, too. So? I guess you have to know what it should sound like, to be able to identify synchronous rotation just by hearing its noise.
If you like to I could try to get a record of my motor running â€œlockedâ€ and â€œnot lockedâ€, so you can get a feeling for the sound, but I will not be able to do that before Thursday evening.

lbenson

Sat Mar 14 2009, 03:35AM

Registered Member #1756 Joined: Tue Oct 14 2008, 07:12AM
Location:
Posts: 32

What happens if you use a non synchronous motor on a rotary spark gap? Or perhaps what does not happen?

Gunboat

Sat Mar 14 2009, 11:57AM

Registered Member #338 Joined: Thu Mar 23 2006, 12:28PM
Location:
Posts: 41

lbenson wrote ...

What happens if you use a non synchronous motor on a rotary spark gap? Or perhaps what does not happen?

you won't get sync and your bps will be off. Meaning,that you will probably suffer spark loss

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