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generator problem

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Posted by Cat 22 on October 24, 2010, 8:16 pm
I have a 5700 watt Mcculloch FG5700AK generator. When its under a
good load its ok,but when the load is light/none the freq jumps way
up. The speed sounds about normal, but the frequency measures 180 HZ
at very light/no loads and under quarter load or so the freq drops
back to close to 60 HZ.Output voltage is ~120vac and it does vary
some depending on load. I discovered this when we had a blackout and
i tried to run my new furnace on the generator, it would show fault
led's if the fridge kicked on (momentary voltage sag). or if nothing
else was running besides the furnace.
I measured the noload freq with a fluke meter t ~180Hz-  and at first
i thought i just didnt have a connection to the meter probe, but
under load (the lowest load i tried and still got 60Hz was 700 watts)
it drops back to right about 60Hz
Its a new Coleman CP9C Echelon furnace, my old carrier weathermaker
8000 worked fine on the generator. One big difference is the Coleman
draws very little power.

I had thought abut putting this on the furnace circuit:  
  "Tripp Lite LC 1800 Line Conditioner / AVR System LC1800"
or maybe this:
  "Tripp Lite LCR2400 Line Conditioner - Automatic Voltage Regulation
with Surge Protection"
Would an ordinary Computer 1200VA UPS do?

Any ideas? Why does the freq act like that? Time for  new generator?


Posted by Ralph Mowery on October 24, 2010, 9:41 pm

This seems to be a standard generator and not one of the inverter types.
There is no way for it to really be putting out 180 Hz.  It is determined by
the speed of the generator which should be about 3600 RPM.  To get to 180 Hz
it would have to turn 3 times that fast which would be around 10,000 rpm.

It is probably putting out some spikes in the voltage and your frequency
meter is picking that up.

If the generator still powers up most loads that do not have any electronics
in them, the generator is probably OK.  If the voltage falls way down under
loads like the water heater or lots of lights or even if you have one a load
such as a portable heater or hair dryer and does not come back up in a
second or so , then you may need to look for a new generator.  It may help
to replace teh brushes in it if it has any.

Posted by Cat22 on October 24, 2010, 10:47 pm
 On Sun 10/24/2010 2:41 PM, Ralph Mowery wrote:

I think you are right, the fluke is reading harmonics or something when
there is little to no load. All my other stuff (2 fridges a freezer some
cfl lights and a microwave) run fine, its just this furnace that seems
sensitive to the generator output.
What do you think about either of the two items i listed above to help
clean the power? Would I be wasting my money?

Posted by Ralph Mowery on October 24, 2010, 10:57 pm

I don't know anything about the items you listed so can not advise either

Posted by Martin Riddle on October 25, 2010, 12:02 am

Try a resistive load in parallel with the furnace. Such as a 100w
incandesant bulb.
If you have a Kill-a-watt, see what the Power factor is of the furnace.
The 100w bulb might pose enough of a resistive load to the generator to
make things stable.
I agree with Ralph that the brushes or slip rings may be worn or dirty
causing havoc with the generators exciter circuit.  But it could be that
furnace motor has lousy PF.  Is it a capacitive start or does it have a
start winding?



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