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Old 12-29-2014, 09:47 AM   #1
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Motorhome Generator

I have the Kohler Model 7CM21 Generator 7,000 watt in my 1984 Motorhome. I am having issues with low voltage and incorrect hz. The Generator runs fine no surging output is 110 volts with no load and 105 volts with 1500 watt load. The main issue is the hz, at no load it is 275 hz and with a 1500 watt load it is 273 hz. When I turn off the Generator and plug the Motorhome electrical cord into regular AC outlet from the Garage I get 121 volts at 60 hz, in the same outlet in the motorhome that I measured the Generator voltage, so I know my meter is accurate. What are some of the items I should be checking to improve my output voltage and obtain the correct hz?
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Old 12-29-2014, 10:52 AM   #2
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Thats a new one on me. The voltage and frequency of the old non inverter generators was controlled by the engine speed and how they were built. Most of the big ones (like your 7000 watt unit) ran at 1800 RPM. The engine speed was always some multiple of the rated frequency of 60 hz. (60 x 30 = 1800). The frequency you have measured is so far out of spec that it simply makes no sense to me at all.

The voltage you show of 110 volts with no load and 105 volts with a 1500 watt load are a bit low. I would expect to see more like 125 volts no load and 115 to 118 with a load. That would indicate your engine governor is set a little low and I would normally also equate that with a slightly low frequency (like 55 to 58 hz). But a hugely higher frequency is too strange.

Although you measure the normal 60 hz on line power the vastly higher frequency from the generator still leaves me wondering if you don't have a metering error rather than an actual problem. But I can't imagine what that would be.

I will be following other's comments with interest.
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Old 12-29-2014, 11:02 AM   #3
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The high hz had me stumped also that is why I started and stopped the Generator several times to make sure the reading didn't change. It stayed the same each time.
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Old 12-29-2014, 03:16 PM   #4
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I did speed up the engine considerably, all it did was increase the hz but the voltage stayed the same.
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Old 12-29-2014, 04:16 PM   #5
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Stranger and stranger results. Kohler went their own way for many years and had some unusual designs but I can't imagine what they could have done which would produce the results you are seeing.

I have an old 4000 watt Kohler from a Holiday Rambler Motorhome in my basement which uses the generator windings themselves to start the engine (no separate starter) and has some other unusual features. It has never been an easy generator to understand or work on.

I wonder if there are any Generator Forums, or Kohler Forums? My '83 310 had an Onan generator in it, and I think most Airstreams had Onan's, so you may not find too much help here.
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Old 12-29-2014, 04:22 PM   #6
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I am trying to join the Kohler Generator forum, I am waiting for the Administrator to send me an email that will allow me to post on their forum. I believe my regulator is keeping me from increasing my output voltage, don't know if it defective or if the hz is affecting this. The hz has me baffled.
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Old 12-30-2014, 10:48 AM   #7
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I would suspect your meter is responding to 'noise' on the output of your generator. Some frequency meters will read any minute spikes impressed in the 60 hertz sine wave as a higher frequency. You could try loading the generator higher and see if the erroneous reading clears up. On some generators the AC must pass thru some slip rings and brushes which may introduce such noise on the output especially if it hasn't be run in a while, corrosion on the slip rings. Keep us posted.


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Old 12-30-2014, 11:03 AM   #8
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The Kohler forum advised on the same noise issue and told me to load the generator with a minimum 500 watt load. I loaded it with 1500 watt (heat gun) and it changed from 273 hz to 228 hz. They couldn't understand the high hz either said it was impossible.
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Old 12-31-2014, 12:59 AM   #9
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So random thought here... in military and industrial applications generator frequencies can be adjusted. There is something that controls the frequency of your generator. It may be integrated into the generator's voltage regulator. However there is something that controls it, that part is definitely bad from what you are telling us.

Have you contacted Kohler customer/technical support with your model number, serial, year, and the description of what it is doing?
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Old 12-31-2014, 08:41 AM   #10
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I have not contacted Kohler at this time, one reason due to the age of the Generator. On the Kohler forum website I was advised to plug in a transformer and measure the hz on the output side of the Transformer with the belief that the transformer will eliminate any noise in the output voltage. I am trying to determine what to take apart to find a transformer I can use to determine this.
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Old 12-31-2014, 09:17 AM   #11
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I know nothing specific about the Kohler generators but in general the frequency is controlled be engine RPMs. Those high frequencies must be from noise induced by the mechanism that excites the armature. Possibly brushes or a commutator ring. That's my guess.
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Old 12-31-2014, 09:19 AM   #12
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Fluorescent light?
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Old 12-31-2014, 01:41 PM   #13
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Quote:
Originally Posted by Kota View Post
I know nothing specific about the Kohler generators but in general the frequency is controlled be engine RPMs. Those high frequencies must be from noise induced by the mechanism that excites the armature. Possibly brushes or a commutator ring. That's my guess.
It appears to be noise, I found a small transformer, one I took out of my low voltage lighting system for my house, and I also used a battery charger transformer. I was not able to reduce it to 60 hz.
With the battery charger transformer the hz went from 278 to 210
With the small transformer the hz went from 278 to 144
I don't understand why a very small transformer would clean up the noise better than the large transformer. I am still looking for a analog clock to measure time vs. 60 hz to confirm the generator is running at 60hz.
Now I need help to raise the no load voltage from 110 to 120.
A ballast from a fluorescent light did not work.
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Old 01-01-2015, 07:29 AM   #14
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That's what transformers do. They pass time varying current. (AC). The ratio of primary to secondary windings determines the relation of input voltage to output voltage.
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