Generator Problem

[Karl Welhart]

My 2002 Patriot Onan 7.5kw Quiet Diesel generator has recently been over revving during additional loading.  The other day it blew the breaker on the genset panel and upon reset has worked fine, but still over revving.  I checked transfer switch, and the neutral load wire was loose.  Tighten all the lugs, but still have this revving issue.  Voltages seem OK, and appliances work with genset power.  Any ideas on what may be causing this problem?

Karl

[Bill Sprague]

I have the same genset.  It provides 30 amps to each leg.  Shore power provides 50 amps to each leg.  So, there is a 40% reduction in power available.  You could even say that the genset provides "about half" of what shore power provides.  Because of the way it is wired it can be thought of as two separate 3600 watt generators.  The circuit breaker limits don't even allow the full 7500 watts the the label says it will do.  

Other than my ear, i have no way to tell what the RPM is.  So, in other words, the only measure of RPM is "loudness", which changes as the load changes.  But, I do have voltage and amp readings in the Aladdin system.  So, normal operation is constant voltage and variable "loudness" as the amp load changes.  

On my motorhome, it is easy to pop a breaker.  The inverter/charger is set to take full advantage of a 30 amp supply when it senses that the batteries are low.  The inverter/charger is on the same 30 amp leg as the front air and fridge.  If the house batteries have been used or are discharged in any way, the inverter/charger will take most of the available 30 amps on that leg for several minutes.  If anything else is on it will exceed the 30 amps, the genset will "surge" and the breaker does what it is supposed to.  Leaving the fridge on automatic will cause it to switch to electric and be part of the problem.   If the charger is working hard and I turn on the air it will blow the breaker too.  

After several minutes, the amp load of the charger will drop and stabilize.  Then I can run most of what I want, but not as much as when plugged into shore power.  

Perhaps your surge and resulting breaker pop are a normal function caused by too much electrical load on one of your "legs'.  

[Edward Buker]

Karl,

Bill has some very good insight for you in his well stated post. To add to that, these generators are voltage regulated by maintaing a constant RPM. As the elecrical load increases more fuel is injected to compensate for the increase in load. You will hear the exhaust "note" get a bit louder and the engine labor a lttle harder. However, the engine RPM should never increase by design. If it did you would be putting out something other than 60 cycle AC which all of the load items in your coach have been designed to operate on. On your monitor panel you should see the voltage vary a few volts +/- from 120V depending on the load and that is normal. If the 60 cycles is varying or the voltage does not stabilize under load and no-load between roughly 116V and 124V then you need to not use the genset and have it serviced. The genset should be running at a constant 1800RPMs to produce 60 cycle AC. Hope this helps.

Later Ed

[Karl Welhart]

Ed annd Bill,

Thanks for the replies.  I will look closer at the 60 cycle issue during these over revving periods.  The voltage is staying within those numbers you listed in your message.  I have owner this unit since new (9 years last April) and did not notice this revving issue under any load.  Also, never had the breaker blow before this last trip.  The genset has 680 hours, only thing that has ever been done is oil, oil filter, fuel filter, air filter and coolant change.  What other issues could be considered under "serviced"?

Thanks,

Karl

[Brad Burgess]

Hello Karl

Our 03 Monterey has the 7.5 Onan - it is an inverter type generator and the engine drives a DC generator (or maybe alternator) that provides DC current to the inverter section of the genset.  As loads are added to our genset the noise level increases with the additional load.  I do not know if this is just the noise created by the additional load on the diesel engine that powers DC gernerator or if the engine actually increases in speed to accommodate the additional call for more DC current from the DC generator to the inverter - the change in noise level, and perceived increase in rpm of the diesel engine, is very noticeable.  Nevertheless the unit works fine.

Kindest Regards,
Brad Burgess
03 Monterey #800460    

[Edward Buker]

I think you have it well covered. My genset(2002) needed a belt replaced this year that drives the alternator and fan. It caused the temperature sensor to shut the generator down due to poor airflow. The first symptom was the shutdown. Somewhere along the way you will probably need that belt replaced. You may want to get an estimate for that job if it concerned you to have the problem surprise you while traveling or dry camping. My fan is belt driven and I'm assuming yours is also.

Later Ed

[Bill Sprague]

Karl,

Bill has some very good insight for you in his well stated post.....

Thank you!