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rsg pulsing

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teslacoolguy

Thu Sept 18 2008, 01:14AM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792

Ok tonight i took my big coil out for a run since i havent really got a chance to tune it yet. I noticed that as soon as i got it to 30% or higher on the variac the spark gap and coil started pulsing really bad. it would shoot 6' streamers for about 1/2 second and then stop for about 1/2 second. The rsg is a dc 1800rpm 240bps. I still have no pfc so that may be a problem. Anyway why is this doing this? Any tips or suggestions would be greatly appreciated.

Myke

Thu Sept 18 2008, 03:50AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

The rsg is powered by a synchronous motor? If not, then you are around 30rpm away from what it should be. (am i right about the rpm if it were async and it pulsed for .5sec?) Otherwise I don't know... I thought that maybe they were on different circuits but they should still be in phase even if they are...

teslacoolguy

Thu Sept 18 2008, 05:06AM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
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Posts: 792

Nope it's not sync. It is a dc motor driven by a industrial dc motor controller so it is fully adjustable. I did notice that when i played with the speed control knob that it would get slightly better. I am still thinking of running it at 120bps but when i slow the motor up past 85% the spark length decreases dramatically so im not sure what to do.

GeordieBoy

Fri Sept 19 2008, 12:23PM

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

If you want to run at 120bps or 240bps (off a 60Hz HV supply) you need to use a synchronous rotary spark gap. This means that not only must the RPM be correct (usually 1800rpm or 3600rpm) but the phase relationship of the rotation to the mains waveform must also be correct and fixed. If the RPM and phase are not maintained at the correct values the system will surge very badly and will probably blow up something from over-voltage.

A DC motor gives a disc RPM that varies with supply voltage, motor temperature, windage and doubtlessly many other factors, so it is not a good choice for a Synchronous Rotary Spark Gap. However the surging does get less severe at high BPS values, so if you can get the BPS up to around 480bps (safely without the disc disintegrating!) then it will probably run with only minor surging as an Asynchronous Rotary Spark Gap.

It is possible to design an electronic box of tricks that will synchronise a decent DC motor to the mains line frequency and allow the phase to be adjusted. However it is not a project for a electronics beginner - It at least requires knowledge of how a phase-locked loop works and some power electronics skills.

So in summary the options are:
1. Run the RSG much faster, like 480BPS or higher.
2. Replace the motor with a synchronous motor or modified AC induction motor
3. Build a fancy box of tricks to lock the phase of your existing DC motor.

Good luck,

-Richie,

teslacoolguy

Fri Sept 19 2008, 02:35PM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792

I am not trying to make is sync. All i wanted to do is run it at *around* 120bps async. Unfortunately the motor only goes up to 240bps so i would have to get another motor to go any higher.

Firefox

Fri Sept 19 2008, 05:44PM

Registered Member #1389 Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346

To increase the speed, you could always get a chain and gears to gear the motor to a higher RPM.

teslacoolguy

Fri Sept 19 2008, 06:27PM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792

Yea that is a good point. The motor is 1/3hp so it will have plenty of torque.

Marko

Fri Sept 19 2008, 06:54PM

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

teslacoolguy wrote ...

I am not trying to make is sync. All i wanted to do is run it at *around* 120bps async. Unfortunately the motor only goes up to 240bps so i would have to get another motor to go any higher.

That is your problem. The *closer* you get to 120bps, your beat frequency (what you called 'pulsing') will get lower and lower, you might probably get it's period into tens of seconds but never make it go away. during periods where gap is out of sync, your tank voltage will ring up to large values and blow your transformer or capacitor.

In order to operate 120bps is to have the gap phase locked with mains, there is no other way. As Richie already said it is possible to use your DC motor as a VCO in PLL, take feedback from the rotor by an optocoupler and use a phase comparator to lock it to mains phase.

One advantage of this is that you would be able to tune your desired phase simply by turning the pot, disadvantage is of course complexity and use of electronics sensitive to EMI.

Alternative id, as said, a synchronous motor which is pretty much a mechanical PLL. Some people have had success turning asynchronous induction motors into salient pole synchronous by grinding flats onto sides of the rotor to create air gaps.

AFAIK, this only worked well with bigger capacitor run motors, but not with shaded pole motors like of MO fans or washing machine pumps.

Regarding your current setup, I'm afraid all it can do is damage your components unless you run it at really high bps (which will impact your spark efficiency severely).

Marko

Herr Zapp

Fri Sept 19 2008, 08:39PM

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

teslacoolguy -

Understand that with your current setup, you don't really have the option to run "around" 120 BPS. You either have to lock-in the break-rate and phasing relationship with the AC line frequency, or you will have the beat frequency problem and risk damaging your tank capacitor and MOTs due to overvoltage. To be slightly safer, you could add a safety gap across your RSG, but that won't cure the pulsing and poor performance.

Another really interesting option is to convert your coil into a DC resonant-charged system. This is relatively easy, requiring only a HV "reservoir" cap, a HV bridge rectifier, a HV charging inductor, and a HV "de-Qing" diode. A DC resonant charged coil is completely free from any beat-frequency problems. And, a DC coil works wonderfully with a variable speed RSG, allowing you to run at a wide range of break-rates and achieve a wide range of streamer appearances and acoustic characteristics. Actually, I think a well-tuned variable break-rate coil is one of the most spectacular to watch.

See Richie's excellent write-up on DC resonant charging at:

When you get your coil running cleanly, there is a chance that you may encounter EMI problems disrupting your solid-state motor speed controller. If shielding and grounding the controller doesn't solve the problem, just replace it with a "dumb but reliable" variac and full-wave rectifier.

Regards,
Herr Zapp

teslacoolguy

Sat Sept 20 2008, 03:52AM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792

I see what you mean now. the last thing i want to do is blow my caps or mots. I did another run tonight and i was able to make the pulsing go about 90% away from carefully adjusting the speed pot and i did have some spectacular results but now am now considering converting it to dc resonant charging. That is something i will probably do next spring because i am going back to Florida in a couple of days and when i come back home it will be winter. I have the drsstc to keep me busy for now but i will continue research on this and hope everything turns out for the best.

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