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Synchronous rotary spark gap motor help.

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LAN_403

Arcstarter

Sat Mar 14 2009, 05:52PM

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

lbenson wrote ...

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

Actually, if your tank cap is correct, the static gap and synchronous rotary work the same. The both fire at both peak positive and peak negative. The difference is quenching. The rotary spark gap has very good quenching compared to a few carriage bolts or richard quick (copper tubes) gap. If you do not have the stuff or ability or time to make a sync gap, you can get an old vacuum to rip apart and take the motor complete with the 'fan' (more like a turbine :P) out. Then make this :P.

It has very good quenching.

...

Sat Mar 14 2009, 07:04PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

But he was asking about an asynchronous gap, which is not a static gap :p And yes they can be made to work, but you need to use more electrodes to get a much higher break rate (I have seen in the range of 600-1000bps!) with the idea being that chances are one of the electrodes will be in the right spot when the tank voltage is at maximum. Then you just need to move the electrodes far enough apart that they can only arc when the voltage is at a maximum/minimum and it will at least quench well.

Arcstarter

Tue Jun 16 2009, 10:02PM

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

Well, i once again feel like making a sync rsg. I have two more motors that might work, induction and 60hz. One has a long shaft, 4 speed, and 1/3hp. I also have a washing machine 60hz (no driver, this was an old washing machine, it is induction) motor, 1/2hp, but 7 wires i need to figure out. The 1/3 hp motor states the RPM is 1075rpm/4spd so i do not think it will be good to use. The washing machine motor says the RPM is 1725/1140. That makes the think it would be perfect, but im not even sure how many speeds it is, or what wires to use. There one wire connected to the capacitor, who's other terminal goes to the motor. That makes me think this motor used a relay to connect and disconnect the capacitor upon startup. The washing machine motor part number is 3363736.

My main question is can a 4 or what ever speed motor be turned into a sync rsg (i am fairly certain, but i am in no mood to kill some good motors!), and which of the speeds should i use? I am guessing the one closest to 1750rpm or 1750 rpm, no?

I plan to use a 5/8 inch thick plywood disk, i have used it at up to 20,000rpm with 8 flying electrodes, for around 5500bps and it never flew apart.

EDIT: the motor is reversible, as most clothes washing machines are.

MORE EDIT: I took the little box thing on the motor apart and found that there is a centrifugal switch that indeed disconnects the capacitor as the motor starts spinning fast enough. The switch will also disconnect one of the motors internal windings from one of the input wires, and then attach the input wire to another of the wires. Confusing! I might be able to figure it out.

Coronafix

Wed Jun 17 2009, 12:25AM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

Another way is to make the width of the flats 1/4 the diameter of the armature for 4 flats. Can't remember what it was for 2 flats, a bit more.
You can check the synchronicity by putting a white plastic rod on the shaft in conjunction with a fluoro.

Herr Zapp

Wed Jun 17 2009, 04:54AM

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

Arcstarter wrote:

"Actually, if your tank cap is correct, the static gap and synchronous rotary work the same. The both fire at both peak positive and peak negative."

Unfortunately, this is not at all true.

The static gap will ALWAYS fire when the gap breakdown voltage is reached.

The asynchronous RSG can ONLY fire when the electrodes are (approximately) in alignment, so if peak voltage happens to occurr when the electrodes are not aligned, then extremely high voltages can occur if a near-resonant value is used for the tank cap. This is why a static safety gap is so important in an asynch RSG system.

Trying to use a standard fluorescent lamp as a stroboscope is generally useless, as the phospher coating on the tube (the actual light source) has relatively high "persistance" and stays glowing well after the AC sine wave has passed through 0 volts. A zenon flashlamp (or a spark gap) will give the best results for "freezing" rotor position and verifying that the motor is actually synchronizing with the AC line frequency.

Make sure that you "test" for synchronism with the fully-assembled RSG disk installed on the motor. If the rotor flats are too small or too big, the motor will not have enough torque to bring the rotor up to speed and synchronize, and may continuously "hunt" (RPM oscillates up and down).

Don't waste time trying to modify a multi-speed motor; fixed-speed motors of the correct RPM rating are easily found, and will give the best chance of success. For a given motor size, the multi-speed motors will generally have less torque at any given speed than a single-speed motors.

Making a RSG disk out of plywood, soft polyethylene, brittle acrylic, etc is foolhardy. The only safe materials are fiber reinforced composites like glass-reinforced epoxy (G-10, G-11, etc) or cotton/linen reinforced phenolic (grade CE or LE). Especially at speeds like 3,000 or 3.600 RPM, a fragmenting RSG disk or a broken-loose electrode could be EXTREMELY hazardous. Making the flats on the rotor is child's play compared to making a safe, well-balanced, accurately machined disk, and mounting it securely on the motor shaft.

If you don't have the money to buy or the facilities to build a safe, well-designed RSG, stick with a good static gap, a triggered static gap, or go solid-state.

Regards,
Herr Zapp

Arcstarter

Thu Jun 18 2009, 03:13AM

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

@ Coronafix, i will look into this some more.

@ Herr Zapp, thank you, that was very helpful :D.

I am going with the washing machine motor, i was being stupid apparently >.<. It is single speed, and reversible. The reverse direction is just slower.

I need to find out how to connect for 1725RPM operation. I can almost figure it out by the little box on it, but not quite. Inside the box, there is a switch and on the motor shaft there is a centrifugal force 'thing' to activate the switch. When it is not activated, the capacitor is connected to one of the internal windings and the other terminal of the capacitor goes to one of the 7 wires hanging off the motor. This is disconnected when the centrifugal force is high enough, meaning the capacitor needs to be connected to some sort of power source to get the motor spinning, and when the motor is spinning fast enough to spin without the starter windings, it is disconnected. When that happens, it connects one the same wire to an internal winding.

Also, there is another pair of contacts that connects an internal winding to one of the seven wires, and when the RPM's are high enough, it disconnects it. There is a wire that connects to the box, which connects it to a thermal shutoff switch, which connects it to an internal winding. There is 4 other wires that are connected straight to internal windings.

Quite complicated, i cannot find a hookup schematic.

Herr Zapp

Thu Jun 18 2009, 07:54PM

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

Arcstarter -

There are a number of slightly different AC induction motor designs that utilize start capacitors and centrifugal switches.

A very good "white paper" on AC Induction Motor Fundamentals (with schematics) can be found at:

Generally, on a single speed motor there will be two sets of stator windings, of two different wire gages. The thin wire will be the RUN winding, and is connected directly across the AC supply.

The heavier winding will be the START winding, and will be connected in series with the start capacitor and the centrifugal switch. This winding carries much greater current to provide high starting torque to accelerate high-inertia loads, but is only used for a few seconds (at most) until the rotor achieves "synchronous" speed, at which point the centrifugal switch opens and the synchronous speed is maintained by the RUN winding.

Because the start winding carries high current and heats up quickly, most cap-start motors will be rated for some maximum number of "starts" per hour. Failure of the centrifugal switch to open (due to contamination, etc) with resulting gross overheating of the start winding is one of the most common cause of failure in cap-start motors.

Herr Zapp

Arcstarter

Fri Jun 19 2009, 12:00AM

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

Thanks again, Herr Zapp

.

I got it figured out, finally. I first just looked at where the wires where connected to the motors windings. I found two and hooked them up to an 80 volt transformer, spun the shaft, and it spun. So, then i looked at the centrifugal switch connections and hooked the start capacitor up. Also, i found which wires are high and low speeds.

I checked to see if it was operating anywhere near synchronous speed. I took a neon light and hooked it in parallel with the motor, and shined it on the shaft. You can easily tell that it is off by just a small bit, it looks like the shaft is spinning a few rpm in real life. I have a short video i will upload to youtube. You can sort of tell that it looks like the reflection on the shaft is blinking. This is because the shaft has two flats.

UPDATE: Video of the 'slipping'.

klugesmith

Sat Jun 20 2009, 07:12AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

Couple stories & couple questions here, from my own enthusiasm for sparks & synchronous motors.

S1. Thanks for posting your video of neon-lamp-strobed motor slippage. Along those lines, I've made two line-synchronous stroboscopes, first with a camera flashlamp and more recently with a big white LED, for a portable demonstration of AC motor types. The dishwasher pump motor hits 3594 RPM without its cooling fan. I think it's cool that induced torque at 0.1 Hz can overcome the bearing friction & windage.

S2,Q1. Partial success after filing 2 flats on the armature of a MWO fan motor (following SRSG threads here a few years ago). It pulls in to synchronous rotation, but only after warming up for a minute or two. Pre-warmed, even with a butane torch, it spins up & locks within a second. The temperature dependence is not from changes in lubricant viscosity. I think the variable-reluctance torque is insufficient when the stator and rotor iron is cold. Anyone here got a simiar experience or better explanation?

S3,Q2. A line-synchronous stroboscope makes it easy to distinguish different kinds of synchronous motors, by braking the motors just enough to slip a step. Are hysteresis motors considered useless for SRSG's because the mechanical phase of their rotation is not repeatable?

Q3. This forum has lots of advice about modding induction motors for synchronous rotation. But how about using a common motor with speed control, phase locked to the power line using simple shaft position feedback (e.g. Hall effect switch or optical interruptor block)? Too complicated or too noisy? Cordless drill drive? Belt-driven SRSG rotor? I found one account on 4hv here:

but it ends inconclusively.
Check this out: phase locked color wheel in commercial "poor man's" color TV in 1955:

That product contains another TC-related oddity: its color subcarrier regenerator has no CW oscillator. The color burst "rings up" a crystal, which keeps ringing through entire horizontal line. Not without damping, but it's levelled by a limiting amp.

Good night!
Rich

Renesis

Sat Jun 20 2009, 11:26AM

Registered Member #2028 Joined: Mon Mar 16 2009, 08:13PM
Location: Norway
Posts: 319

But how about using a common motor with speed control, phase locked to the power line using simple shaft position feedback

Why not use a DC motor, with a PWM driver controlled by an accurate tachometer? Is the phase positioning really that important, or is the speed important?

Herr Zapp:

What do you mean by a dead pole?

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