How to / DIY > Generators

5 KW "ST" acting up & down


Good Day,

It's been a while since I've been around as things have been working fine till now. When a power outage occurs I've been using my small one cylinder Lister and 2.9 KW gen set but today I needed more power and fired up the 2 cylinder Lister with the 5 KW, "ST" generator. I ran it for a couple hours and the voltage and cycles kept going up and down. Voltage from 109 to 125 and cycles 57 to 63. The volts and cycles would hold steady for several minutes then fluctuate again.  This happened the last couple of times I fired up the "ST".  I'm wondering what the "safe" perimeters for volts and cycles are but mainly what the problem might be and how to fix it with my limited knowledge. I've heard of a "automatic voltage regulator", is that what this generator needs?  My cheap little"Northern" 2.9 generator works like a champ. Any help would be appreciated.  Steve

If it was just the voltage fluctuating I would say it is the generator head but with your frequency fluctuating as well that means your engine isn't holding a steady rpm. Could be a governor issue or a load issue.

What loads are you running with it?

+1, Its a genset rpm variation. Intermittent belt slip might also also be a cause of the frequency change. 
The ST head with stock harmonic excitation will vary voltage with rpm change. 

You will sometimes get bogus frequency readings from an ST head due to the "harmonic hump" distortion of the waveform caused by the harmonic winding in the single phase ST heads.  It will show as a frequency way, way off from 60Hz nominal so is quite unlikely in your case, Steve.  It varies wildly between units.  Another good reason to have one of the cheap hand held digital oscilloscopes in your tool kit.  Seeing a waveform can really help with troubleshooting all kinds of things.

Best Wishes,

Thanks for the replys.

broncodriver99,  Not much of a load, a couple of freezers and friges, lights, small bathroom heater, TV. I think the engine RPM is steady. Wouldn't an RPM fluctuation in a two cylinder Lister be noticeable? I got my "ST" owner's manual out and I will open the "cover window", blow any crud out and clean the slip rings. I will do that as soon as the temp up here in the PNW comes up a little.

BruceM, I'll check the tension of the belts (2) when I open the cover window. I'm not sure I have enough knowledge to troubleshoot anything if I had an oscilloscope.

  thanks much for the help guys,  Steve


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