Low generator output

[richard collette]

My 1971 ONAN 7.5 gen. has decided to only put out 60v -seems to run good but not enough juice. Happened once and increased rpm solved the problem-not this time--anyone have any ideas before I call for service?
By the way -the Hatteras school was great--Thanks Roger!!

 

[REBrueckner]

Hi Richard, A few things to check, but, alas, no firm suggestion:

When did you last boost RPM a bit?? For just a few volts, right?? Just recently?
A few volts low can typically be the result of the need for a minor governor speed adjustment, but not 60 volts low. Thats sounds like an electrical component issue.

First thing to decide is it a voltage or RPM issue....,,,,,but just in case....

RPM
Did maybe something come loose (maybe a lock nut on the spring connection) there where you last adjusted, I presume a governor adjustment, and can you tell from the engine sound the genny is running at normal (1800?) RPM. If the genny is running slow, check for a spring at the governor speed adjustment...maybe it popped off or slipped? Exhaust sound outside is smooth....no missing, right?? Can't imagine all that voltage loss is an RPM issue, but not impossible.


VOLTAGE
I assume you are looking at a reliable ac voltage reading...always a good idea to double check the voltage reading if you have any doubt. And maybe also a good idea to restart and run the generator again to be sure the problem is still present...and not intermittent.

60 volts is just half of 120v, so maybe one of two circuit components has either gone bad or is disconnected, as from a broken wire....You could be getting a half have output instead of a full wave (smooth and continuous) sine wave output. You might be able to find that section of the ac circuit if you have the wiring diagram for the electrical end of the genny.

Question: Expurts...Nothing in the control box that could cause this,right? I can't recall anything on my own....different model Onan .....but fortunately I never had to work the ac specifically.

Good luck, let us know the final resolution.

 

[Pascal]

My 1971 ONAN 7.5 gen. has decided to only put out 60v -seems to run good but not enough juice. Happened once and increased rpm solved the problem-not this time--anyone have any ideas before I call for service?
By the way -the Hatteras school was great--Thanks Roger!!

Never adjust RPM without a frequency meter as RPM directly affects Freq not Voltage. Obviously voltage will be affected by an rpm adjustment but it will throw your Hz off.

 

[Boatnut]

Richard,

If you know the model number let us know, or if you or anyone has a link or copy of the wiring diagram they can send me I'll be glad to try and assist with solving the problem. Without a wiring diagram it's like shooting in the dark, and can waste a lot of time checking things that don't need to be checked.

btw: you can also check the speed with a hand help tachometer, 1800 will give you 60Hz frequency, gauranteed.

 

[BUSTER]

Never adjust RPM without a frequency meter as RPM directly affects Freq not Voltage. Obviously voltage will be affected by an rpm adjustment but it will throw your Hz off.

To adjust a mechanically controlled diesel generator;
If a meter is not available find a 120 volt electric clock.preferably with a second hand. As you adjust the RPM's it will be helpful to have a second person with a stopwatch or just a watch to compare the second hand on the clock with the watch as you increase or decrease RPM's.When they match,you will have 60 cycles,58 cycles loaded to 62 cycles with no load is acceptable for all but the most sensitive electronics. Courtesy of circa 1969,US Army on the job training.

BTW/To the original poster; Make sure your linkage is tight and the jam nuts are locked.Those old 7.5 shakers are notorious for loosening the linkage.

 

[rsmith]

My 1971 ONAN 7.5 gen. has decided to only put out 60v -seems to run good but not enough juice. Happened once and increased rpm solved the problem-not this time--anyone have any ideas before I call for service?
By the way -the Hatteras school was great--Thanks Roger!!

I had the same problem If increasing rpm does not raise the voltage its a electrical problem in the windings or the exciter. My armeture was loaded with salt after a particulary nasty trip where I was pulling spray thru the intake vents. The fan in the gen side moves a lot of air over the windings for cooling if that air has salt in it funky things start to happen with the output voltages. I had to have the gen end removed and baked out. They test it with a megger.

 

[jojaanright]

This is a Porter Cable 5250 watt brushless alternator (generator) with low output, It will lite lights and other tools with low amps, But it dose not produce full output, I see it has been stored in a wet area and I see lite rust on the stator, How do I test the condenser

 

[pl5898]

I had the same problem on my 15kw 1989 Onan MDL3. The Onan guys knew nothing about these older units. He thought it was the voltage regualtor, but $500 later, not it. I did my own diagnosis on-line and through my local generator guy and here is what worked: A generator guy called it on the first shot-rectifier plate. I pulled off the bell end covering the rotor to get access to the rectifier assembly. These are 2 small plates, semi circle about 3" long. I removed the plates and tested for resistance. No resistance on one of the plates. $40 from ebay later, I reassembled the unit with the new plate and has worked great ever since. Good luck.

 

[krush]

It's all about getting excited..... excitation is the key

 

[mr hatt]

I had my voltage regulator go out and I had a similar issue. Mine had a lima generator, but my understanding is that both onan and the old phasors use the same voltage regulators. I got mine from flight systems for around $200. You should be able to google them to get their info if you need one.