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Old 01-19-2014, 07:44 PM   #15
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Originally Posted by Algoma View Post
chboone, I can only assume you have never owned an Onan QD generator.
Doesn't matter diesel, gas, natural gas, propane or which manufacturer if the rpm's are fluctuating the frequency will follow. RPM'S = frequency. Voltage by voltage regulator, the lower the volts the higher the amps, the higher the volts the lower the amps, OHM'S law.

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Old 01-19-2014, 07:54 PM   #16
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[moderator edit]You appear to have a gas RV and presumably a gas generator. The Onan QD generator is a variable speed generator and the output is provided by a solid state inverter. This is not a synchronous generator (as is the Generac). [moderator edit]

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Old 01-19-2014, 07:54 PM   #17
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Originally Posted by chboone View Post
Doesn't matter diesel, gas, natural gas, propane or which manufacturer if the rpm's are fluctuating the frequency will follow. RPM'S = frequency. Voltage by voltage regulator, the lower the volts the higher the amps, the higher the volts the lower the amps, OHM'S law.
Well...this is what I thought before. My gasoline powered Onan 5.5Kw generator worked as you stated. My 7.5Kw Onan Quiet Diesel works differently. The RPMs vary while the output is constant. I don't understand the system at this point, but I'm researching. It's a fantastic generator!
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Old 01-20-2014, 04:48 AM   #18
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Chboone/Koop: As algoma mentions above, the QD7500s and above are inverter based generators. The RPM of the engine will vary from idle to 3525 rpm based upon the load/demand on the AC side. The QD5500's and below (with one exception) are conventional generators and will attempt to sustain a fixed rotor speed regardless of load.

Jerry: In the majority of cases with a QD hunting around at different RPMs unrelated to load you will find it's fuel delivery related. In the majority of those cases it is the fuel filter that is needing replacement, but it can be cracked fuel lines allowing air in the lines or a failing fuel pump. Cummins/Onan has a recommended fuel delivery test as follows:

FUEL PUMP/Fuel Flow Test

1. Disconnect the fuel return hose from the line to the supply tank and point the end into a quart (1 liter) or larger container that has volume graduations. There are 2 fuel lines entering the unit. 1 is the supply, 1 is the return line. Make sure all testing is done from the RETURN line which will be clearly marked on the cover.

2. Prime the genset by pushing the Start/Stop switch to STOP and holding it there for the duration of the test. Flow rate into your container should not be less than 200 ml (0.4 pints) per minute.

3. If flow is less than specified, replace the fuel filter. Re-test. If that does not help, look for other issues in the fuel system such as wet or leaking fuel lines especially at unions and connections. Last, replace the fuel pump as the pump is not serviceable.

If the unit passes the delivery test, I use a separate powered small 12V fuel pump and a 1 gallon container of fresh fuel. Using my own supply/return lines I'll run the unit on my "test" pump and fresh fuel to see if the problem still exists. If the problem clears up then I'll look at the 12v wiring to the pump and/or replacing the fuel pump.

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