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Old 16-03-2013, 18:27   #1
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Poor AC Generator power quality

I have an interesting problem that I am trying to track down.

I have a 6kw 115V, 60Hz Northern Lights generator on my boat. I recently installed a new dive compressor that draws about 16 amps at 120V. The compressor has a frequency drive on it and can be configured to run on 230V or 115V power. When I run it on 240V shore power (my current location is Australia) the compressor runs fine. When I run it on the 115V generator, the compressor is not happy at all. It won't come up to full speed and cycles on and off. A voltmeter, ammeter and frequency meter on the AC supply shows the generator is pretty solid, 120 volts, 58Hz, 10-16Amps, but there are some voltage fluctuations that correspond with the compressor's erratic operation. The fluctuations don't seem very large, around 10 volts, but with a DVM it's hard to tell.

At this point, I suspect the generator is not maintaining a constant, stable AC voltage, but I can't test it very well with just a multimeter. I also measured across the line and neutral wires, but when I test between line and earth or neutral and earth I get 59 volts. So the generator AC supply is not grounded at the neutral leg, is this the way it should be?

Any ideas on how I can effectively test the quality of the power supplied by the generator, what measurement equipment is needed?

Any thoughts on what causes power quality issues in generators?

Thanks all in advance.
Doug
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Old 16-03-2013, 18:40   #2
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Re: Poor AC Generator power quality

What is the locked rotor amps on the dive compressor? It may be many times the 16 amps running load. The inrush when the compressor comes on may be too much for the genset. BTW, the Northern Lights 6kw is a fine unit. Mine has given me no troubles and taken lots of abuse.
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Old 16-03-2013, 19:12   #3
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Re: Poor AC Generator power quality

The Air Compressor uses a variable frequency drive, so it soft starts the motor. When it turns on, the amperage is about 4 or 5 amps, then it ramps up to 16 amps at full speed. There are periods where the compressor will run fine, and then it will start dropping out, so this is not an inrush current issue that I can detect.
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Old 16-03-2013, 20:37   #4
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Re: Poor AC Generator power quality

I'm guessing that the dive compressor is a tough load, where the current is not strictly in phase with the voltage. This might be a problem for the generator load regulation.

Contact the compressor's manufacturer and tell them your issue. They would likely be aware of any compressor issues when used on a dive boat, including when run from a generator.
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Old 17-03-2013, 00:43   #5
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Re: Poor AC Generator power quality

If you have one of our MarinePak compressors, 16 amps is about right for an unloaded compressor. But at 115 VAC, it will climb to about 29 amps at pressure.
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Old 17-03-2013, 01:06   #6
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Our old generator was producing dirty power ie not a nice sine wave type. Any new fangled electronic equipment didn't want to know about it and wouldn't run. Solution: electrician to test with spiffy tester and ultimately a new generator. Hope you don't have to go to the same lengths.
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Old 17-03-2013, 04:09   #7
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Re: Poor AC Generator power quality

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Originally Posted by Teeto View Post
Our old generator was producing dirty power ie not a nice sine wave type. Any new fangled electronic equipment didn't want to know about it and wouldn't run. Solution: electrician to test with spiffy tester and ultimately a new generator. Hope you don't have to go to the same lengths.
I fear replacing the generator might be the solution, our genset has just under 6,000 hours, so it's certainly had some use. Those spiffy testers are amazing, but they cost over $2,000 (for a Fluke 43B) so I'm not too keen on buying one. I'll see if I can find a local marine electrician that can test the power quality for me.
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Old 17-03-2013, 04:12   #8
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Re: Poor AC Generator power quality

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Originally Posted by Lake-Effect View Post
I'm guessing that the dive compressor is a tough load, where the current is not strictly in phase with the voltage. This might be a problem for the generator load regulation.

Contact the compressor's manufacturer and tell them your issue. They would likely be aware of any compressor issues when used on a dive boat, including when run from a generator.
I have given them all the information about the problems and am waiting to hear back. They have many units being run at sea under generator power, so they believe the problem is on my end, but they are looking into it.
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Old 17-03-2013, 05:08   #9
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Re: Poor AC Generator power quality

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Originally Posted by Poozer View Post
I fear replacing the generator might be the solution, our genset has just under 6,000 hours, so it's certainly had some use. Those spiffy testers are amazing, but they cost over $2,000 (for a Fluke 43B) so I'm not too keen on buying one. I'll see if I can find a local marine electrician that can test the power quality for me.
That Northern Lights is the Rolls Royce of gensets, and 6000 hours should not be a problem if it has been well serviced. I believe those generators have been known to go 20,000+ hours without a major overhaul.

29 amps at 115 volts is only 3.3kW, which should not be a problem for that generator.

Maybe you have a wiring problem? Is the dive compressor connected directly to the genset? Is there any undersized wiring between the genset and compressor? Or any dodgy terminals, connectors or sockets? As an experiment, you might try wiring directly to the generator and see if that makes any difference. Always try the cheap stuff first -- a lesson I like many others learned the hard way!
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Old 17-03-2013, 05:19   #10
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Re: Poor AC Generator power quality

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That Northern Lights is the Rolls Royce of gensets, and 6000 hours should not be a problem if it has been well serviced. I believe those generators have been known to go 20,000+ hours without a major overhaul.

29 amps at 115 volts is only 3.3kW, which should not be a problem for that generator.

Maybe you have a wiring problem? Is the dive compressor connected directly to the genset? Is there any undersized wiring between the genset and compressor? Or any dodgy terminals, connectors or sockets? As an experiment, you might try wiring directly to the generator and see if that makes any difference. Always try the cheap stuff first -- a lesson I like many others learned the hard way!
That's excellent advice, thank you. I'll definitely give that a try.
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Old 17-03-2013, 06:47   #11
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Re: Poor AC Generator power quality

Do you have an isolation transformer?

If so, rewire the generator to run through the transformer, it'll clean up a lot of harmonic distortion (which I assume is the root of the problem).

Just need to put the shorepower/generator transfer switch on the input side of the isolation transformer.
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Old 18-03-2013, 06:22   #12
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Re: Poor AC Generator power quality

I bypassed the AC wiring in the boat, by connecting a cable between the generator output and the air compressor variable frequency drive terminals and tested it. Same problem. It slugs along at low speed/current and never comes up to full speed.

I talked to a Fischer Panda rep about the problem and he said that some of the smaller marine gensets have trouble running rectified loads such as variable frequency drives. I've read too many horror stories about unreliable Fischer Panda generators to want to go that route. It seems that my 6KW Northern Lights genset is as good or better than an FP genset, and I would like to find a way to get it to run this compressor.

I'll try calling the Northern Lights folks in the USA tomorrow, to see if they can provide any advice on a solution to the problem. I also asked the dive compressor manufacturer to verify if they have experience with their compressor being powered by a Northern Lights genset. Hopefully I'll hear back from them tomorrow.

We had to postpone our departure date until we get this sorted out. Very frustrating!
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Old 18-03-2013, 06:46   #13
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Re: Poor AC Generator power quality

"I also measured across the line and neutral wires, but when I test between line and earth or neutral and earth I get 59 volts. So the generator AC supply is not grounded at the neutral leg, is this the way it should be?"

I am a little surprised nobody commented on this. Yes the neutral ground should be connected at the source, the generator in this case. Sounds like you do have a wiring issue you should not be seeing that 59 volts there. I would suggest you have an electrician who knows boat AC look at it, could be you have a dangerous situation there.
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Old 18-03-2013, 15:45   #14
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Re: Poor AC Generator power quality

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Originally Posted by sailvayu View Post
"I am a little surprised nobody commented on this. Yes the neutral ground should be connected at the source, the generator in this case. Sounds like you do have a wiring issue you should not be seeing that 59 volts there. I would suggest you have an electrician who knows boat AC look at it, could be you have a dangerous situation there.
Thanks for the info about the ground connection. Next step is to get the electrical pros involved, so I'll be sure to go over this with them.

The folks at the compressor manufacturer are being very responsive. They assure me these compressors are working with many, many generators of all types. They are contacting Northern Lights directly to discuss the possible causes.
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Old 18-03-2013, 17:34   #15
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Electrically, it is unlikely that a simple AC generator would produce anything but a nice clean sine wave. That's the most natural thing for such a device. But if the current demand and power factor must match the specification. Not only RMS current but peak. I can imagine some soft start electronics manic having an huge power factor (bad).
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