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Author Topic: Generator Overspeed/Overvoltage protection  (Read 7127 times)
GM0ULK
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« on: April 18, 2011, 06:03:25 AM »

Hi All,

I started up my (cheap) portable generator after a service when the mechanical speed controller jammed causing an overspeed. I have since rectified the  mechanical speed controller and wondered what would have happened if my ham radio gear was connected.  The voltage shot up above 400 volts and the RPM gave significantly more than 50 Hertz!!!

Why I am writing this?  Well - I am looking for a (homebrew) trip circuit that detects over voltage (UK 230 VAC) and above or below 50 Hertz.  The generator is a 2 stroke 750VA type and just the right size for portable work. The ideal circuit would trip a relay when more (or less) than 50Hertz/230 VAC is detected and protects my gear. 

Any ideas out there amongst you all would be most gratefully accepted, the summer season is here and I want to get portable but have lost confidence on portable petrol generators..... unless I buy an expensive protected type Undecided

Best 73s

Steve

GM0ULK
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K8AXW
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« Reply #1 on: April 18, 2011, 09:13:39 AM »

Steve: There was a fine article in the June 1994 issue of QST magazine that addresses your problem.

The circuit was designed by Jerry Paquette, WB8IOW.  He used a 723 voltage regulator chip and a optoisolator chip to trip a ground fault interrupter receptacle at an adjustable preset voltage.

I built 2 or 3 of them and they work quite well.  However, I do recommend you use quality components and check the trip voltage periodically.  I found that radical temps will shift the "trip point" some.

The circuit is inexpensive and the components are common.

If you don't have access to this article; can't get it form either ARRL or WB8IOW, let me know with your email address and I will send you a copy for your consideration.

BTW, chances are if your ham gear was hooked up to the generator when it "shot to over 400V", you would also be looking for new gear!  :-(  I can't imagine this, even assuming your generator normally puts out 220vac.... to go to over 400v, the engine speed would have to be high enough to actually destroy itself!   


Al - K8AXW
aibling328@yahoo.com
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W8JX
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Posts: 6679




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« Reply #2 on: April 18, 2011, 09:32:25 AM »


BTW, chances are if your ham gear was hooked up to the generator when it "shot to over 400V", you would also be looking for new gear!  :-(  I can't imagine this, even assuming your generator normally puts out 220vac.... to go to over 400v, the engine speed would have to be high enough to actually destroy itself!   


I think you would be hard pressed to get even 220 out of it on a overspeed.
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K8AXW
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« Reply #3 on: April 18, 2011, 10:06:35 AM »

I was thinking perhaps in the UK, the generator might be putting out 220vac as a norm.  Most EU countries use 220v. 
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W8JX
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« Reply #4 on: April 18, 2011, 10:10:55 AM »

I was thinking perhaps in the UK, the generator might be putting out 220vac as a norm.  Most EU countries use 220v. 

I had forgotten that. Most modern switching power supplies can adapt to a wide range of supply voltages safely. It is transformer based units that are far more easy to damage in over voltage.
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K1CJS
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« Reply #5 on: April 18, 2011, 12:07:23 PM »

Probably that voltage reading was under a 'no-load' condition.
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K8AXW
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« Reply #6 on: April 18, 2011, 12:46:37 PM »

He 'somewhat' indicated that it was no-load voltage.  I imagine he also 'somewhat' had a pucker factor close to 10 when he reached down over that screaming engine to shut it off!  LOL.
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GM0ULK
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« Reply #7 on: April 18, 2011, 02:14:57 PM »

I had a analogue voltmeter (capable of measuring frequency too) attached to the output as the generator was running rough due to two stroke mixture gumming up the carburator when it was left standing over the winter. The generator itself is quite cheap, being of Chinese manufacture and does not have any protection.  The alternator itself is of brushless design and would, if spun up enough, produce a significant over voltage and I did see 400VAC + momentarily as I quickly switched it off!!!

I did not see the frequency, but suspect it was a lot higher than 50 cycles. And yes, there was a factor of 10 pucker factor Cheesy

Al,

Many thanks for the details of the QST article, I will send you a email separately and would be grateful for the circuit details.  The RPM on these generators is not too stable and there is a slight fluctuation of a few Hertz but not too diverse and I would be interested to see what the wave form might look like on a scope.

Thanks to all those who responded.

Best 73s

Steve

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K8AXW
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« Reply #8 on: April 18, 2011, 08:38:45 PM »

Copy that Steve.  Or you can use k8axw@arrl.com

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GM0ULK
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« Reply #9 on: April 19, 2011, 12:19:52 AM »

Thanks Al,

Just a quick one, as the meter was anologue, it is possible that needle and core inertia swung the needle to FSD!  The scale was set to 0 - 400 VAC and this type does not have any damping of the movement.  So perhaps it kicked hard when the overspeed occurred pushing the needle against it's FSD endstop giving a good impression of 400 VAC?. I did not hang about watching it as I hit the ignition kill switch and got ready to stuff a rag into the intake!

So apologies to those who say it is not able to provide that sort of voltage, retrospectively, they could be right.  Although the voltage did most likely exceed 230 VAC and most definately 50 Hertz.

Cheers all

Steve  GM0ULK
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K8AXW
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« Reply #10 on: April 19, 2011, 01:01:41 PM »

Sounds like the meter did lead you astray a bit Steve. 

After researching 50Hz generation I found that the synchronous speed of a 50Hz generator is 3,000 RPM for a 2-pole generator (which means it would have jumped to over 5,000 RPM to give you the 400V or 1500 RPM for a 4-pole generator which would have jumped to around 2,500 RPM to give you 400V.)

A reasonable guess is that your generator is a 2-pole unit and for it to go to 5,000 RPM is unlikely....possible with a 2 cycle engine..... but still unlikely.

If your generator is a 4-pole unit.... which is unlikely because of manufacturing costs, it would have had to accelerate to over 2,500 RPM which is quite possible.

I think your meter analysis is correct.

I think the "kill switch" is a great invention.  I've heard of guys stuffing a rag into the intake of a runaway diesel engine to stop it.  As I understand it, this procedure, if successful, requires two or three drinks of something strong and a change of underwear!
Al
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W8JX
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« Reply #11 on: April 19, 2011, 01:14:54 PM »


A reasonable guess is that your generator is a 2-pole unit and for it to go to 5,000 RPM is unlikely....possible with a 2 cycle engine..... but still unlikely.



Not unlikely at all.  Small 2 stroke engines can wind pretty tight given a chance too and 5000 would not be a problem. I have a 2 stroke 5 hp of a snow blower that runs 4000 to 4200 RPM normally on governor. It will easily crank higher if you override governor. But is 6+ years it has never stuck on me.
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K8AXW
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« Reply #12 on: April 19, 2011, 09:03:49 PM »

JX:  Interesting.  I know very little about 2-cycle engines, except on motorcycles.  I do know they will wind up on those things! 

My concern would also be the windings in the generator.  We had a small turbo-generator overspeed one day and blow up.  What caused the explosion was the windings in the generator pulled out of the slots at approximately 4400 RPM.  That locked the rotor in the generator and stopped (well, not quite stopped) the turbine instantaneously.  The result was the total destruction of the generator and turbine, breaking it into pieces... one weighing 90lbs that 50ft in the air, through a 3/8" fireproof sheetrock, 3" roof timbers and approximately an inch of industrial grade roofing and rolled over 20ft down the roof! 

This and the thought of a rod coming through the side of the engine was what caused me to say that Steve probably had a pucker factor up around 10!  HI.
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