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Synchronous rotary spark gap motor help.

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Herr Zapp

Sat Jun 20 2009, 06:59PM

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

Renesis -

The answer to your question was contained in the earlier posts in this thread.

Again, see:

for photos of a typical stator, with the "dead" poles identified. A dead pole will not be encircled by any of the "run" windings. The dead poles may be harder to see on smaller motors, because there may not be a clear difference between the "run" and the "start" windings.

Remember that a squirrel-cage motor is really a rotary transformer, with the rotating magnetic field from the stator windings inducing a magnetic field in the "shorted turns" in the rotor. The "shorted turns" are created when molten aluminum is injected into and around the iron lamination stack that is the "core" of the rotor.

Here are some photos of the rotor from a "real" salient-pole motor from a Teletype machine. It is a 3600 RPM motor, so the rotor has two flats. When machining the flats into a rotor when converting a "standard" motor into a salient pole motor, the iron lamination material AS WELL AS PART OF THE ALUMINUM CONDUCTORS are machined away. (That why a modified motor always has less torque than it originally did.)

In a "real" salient pole motor, the iron laminations are initially created with flats, and then copper or brass rods are inserted into the core laminations as well as "outside" the flats (see photos), so ALL the conductors are present in their full, original cross-section. Some of these conductors (the ones located at the flats) are free-standing with nothing but air around them.

At the ends of the rotor, the ends of the brass rods are brazed or silver-soldered into brass end-plates to complete the electrical circuit, creating the low-resistance shorted turns.

The first photo shows both the full-diameter part of the rotor, and the free-standing brass rods over the "flat" section of the rotor.
The second photo shows a close-up of the exposed rods over the flats.
The next photo shows one of the brass end plates that the brass rods are brazed into.
The last photo shows the full-diameter part of the rotor, with the brass rods buried in the iron laminations.

The fact that part of the shorted turns is machined away in a "modified" motor explains whay the motor always has less torque than it originally did, and why it always runs hotter.

Regards,
Herr Zapp

Steve Conner

Sun Jun 21 2009, 09:47AM

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

There's no reason not to use a DC motor phase-locked by a shaft encoder. Except for the practical difficulty of getting all that circuitry to function in a high-EMI environment, knowing that if it goes wrong even once it could blow your transformer. And the theoretical difficulty of the PLL loop filter needed to lock a high-inertia motor with zero steady-state phase error.

It's easier just to build a DC resonant charging coil to go with your DC spark gap motor.

Arcstarter

Thu Jun 25 2009, 09:21PM

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

I decided to go back to using the motor posted on the first post. It turns out, it *was* synchronous after the first time i put flats. However, with the 4 inch disk it would be synchronous for a few seconds, then it would lose it for a second or so, and be sync again. I made the flats just a bit larger, and it was perfectly sync! Yay! However, the 4 inch or so disk was too small, so i made a new 6.3 or so inch disk, to make sure my flying electrodes did not hit the motor. I connected it, and now it continuously 'hunts', if that is the correct usage of that word. It never locks.

My question is should i grind it down more? Or have i ground it down too much.

shows their 3450rpm motor, and it has a flat that spans about 4 entire poles, mine is almost 3 poles. The flats are aligned very well, in fact. I have two pictures, first one is showing how wide the flat on the rotor is compared to the poles on the stator. Last one is just one of the flats.

Once again, the question is take more off or have i ruined it? I think i need to take more off.

Herr Zapp

Thu Jun 25 2009, 10:37PM

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

Arcstarter -

You can't put back the material already removed, so you might as well take off a little more, re-test, etc, etc.

1. First, are you testing the motor with the fully assembled RSG disk attached? Unless it has the electrodes installed, it won't provide the "real" windage resistance that the motor will need to overcome. It won't do you much good to get the motor operating synchronously with a bare disk, and then install the electrodes and find it won't synch again.

2. As you can see on your rotor, the deeper you cut the more of the aluminum conductor bars are machined away. This reduces available torque. Worst-case, your motor just doesn't generate enough torque to synch up with your disk installed: you may need a bigger motor.

3. Part of your problem may be that you are trying to modify a two-speed motor. I am unaware of anyone who has ever successfully modified a multi-speed motor for salient-pole operation.

4. Several options:
a) Use a variac to boost line voltage slightly for starting. The motor will run even hotter, but for intermittent use, it may be OK. Try boosting the voltage until the motor achieves synchronism, then reduce voltage as low as possible.

b) Try substituting different start capacitors, with slightly different values. Watch out for excess current draw!

c) Try reducing the mass of the disk. Bore a few large diameter holes to remove material, or notch the periphery of the disk between electrodes. DO NOT weaken stressed areas!

If none of these help, get a bigger motor, or at least start out with a single-speed motor.

Regards,
Herr Zapp

Arcstarter

Fri Jun 26 2009, 01:02AM

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

This motor is a single speed motor. This is the one i wanted to use when i started the thread, you can see the picture on the first post.

Also, i decided to use this smaller disk. I will just hammer the bolts into the wood, so that they do not stick out and hit the motor. This might however crack or weaken the wood, so we will see.

So with the smaller disk, it seems to be synchronous, but if i turn it off then back on, sometimes the marks will appear to be in a different place. If i use a peice of paper, it is perfectly sync at all times.

I am thinking of how i could make a low inertia and drag rotor. I could try to cut the disk i have up, and make it just a straight piece of wood. This would reduce weight and inertia, but it might increase wind drag and reduce the strength and making the setup very unsafe. Maybe i can find something around here to use, something thinner but still tough.

Or maybe i should make the flats even bigger. Hmm, i will experiment. the motor still does not heat bad, and it is still pretty hard to make it become unsynced (no its not a word but it works) even with the disk, but like i said the spot is never showing i the right place, it changes when you stop and then restart it.

Also, i do not have a variac. I will experiment with stuff, we will see what i come up with :D

Arcstarter

Sun Jun 28 2009, 11:53PM

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

Well yesterday i put everything together, and i had perfect sync operation :D. I left it running for 3 hours, no overheating, nothing. I hooked it up to my Tesla coil, and i used 3 MOTs ballasted, and a 75nf 16kvdc tank cap. At first i was getting 2 foot arcs (i have a video of those), and then i took it outside.

The sparks where i am guessing over 3 feet, with 600 watts input with a single MOT unsorted in parallel with some choke i had, which pulled 5 amps measured. But sometimes it would not fire, even though i messed with the electrodes and got them extremely close to each other, no more than 1mm gap (im serious, i could here the electrodes scraping). After doing that, i went back because it was still not firing, to find nothing was wrong the the rsg, and it was still synced corrently. I tried to adjust again and i broke the disk in half (hehe, i am a retard). Oh well. I will have to make a new disk and probably the stationary electrode holding piece of wood thing...

Well, earlier today i cut a bit of iron from this bug zapper transformer i had for quite awhile to get more power. I only cut maybe 3mm or so from the shunted part, and i got much more power. So i figured i would make a tiny srsg from a MO fan. I just finished it up, and it is still perfectly sync with the disk attached (no flying electrodes yet, but i hope it wont matter). It heats up quickly, but oh well. This will be a cool mini Tesla coil :D.

I was at first skeptical it would work, but it was very easy. This might be the new thing for small Tesla coils, MO fan srsg's :D.

EDIT: Oh yea, and it is 3600rpm sync.

RE-EDIT: I finished the MO fan srsg, and it works absolutely flawlessly :D. It will start to fire at only 1kv. The screw are small however, because it would not take just a whole lot to bring the little MO fan out of sync. So that means it cannot do high powers (at least not for long, the electrodes would disappear!). I would highly suggest making one, it is free (i made mine from nothing but salvaged stuff, including the electrodes) and works well. Though the motor heating is a bit bad... Not too much worse than before though.

Arcstarter

Tue Jun 30 2009, 11:06PM

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

I made a video of the Tesla coil operating indoors with only ~2 foot sparks (i did not have my camcorder when i achieved the 3+ foot ground arcs). The sound is weird when the coil is operating on the video. It sounds much higher pitch than that, like a true srsg coil -.-.

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