Date: Thu Oct 09, 2008 7:58 am. By: Neon John
On Wed, 8 Oct 2008 19:08:18 -0400, "Rob.....in Courtice" wrote:
I have a 900W 2-stroke "King Canada" generator. (Inherited it). On my day off decided to see what it would run. I have a six circuit manual transfer panel hooked up to the house service. And a Kill a watt meter.
Running a trouble light, 60 watts @ 61- 62 Hz and a Pf of 1 @ 120 - 121 v.
Running the fridge drawing about 350 watts @ 61-62 Hz and a Pf of .9 @ 120 -123 v, it sounded about right, about 1\2 of what the gen had to offer.
Running the furnace (20 yr old, 1\3 hp blower I think) drawing about 300 watts @ 69 - 73 Hz and a Pf of .6 @ 130 - 135 v (it sounded as if the generator was putting out about all it had).
Because the Pf is so low, would that cause the voltage\Hz to go so high? Also, is running the furnace at this voltage/Hz gonna wazzoo the motor?
The problem here, I suspect, is that the crappy waveform from the cheap generator and moderately low PF are fooling the KAW. It is almost impossible for that little generator to run that fast. That would be 3600 * (69/60) = 4140 RPM. It would have been obvious to you if the generator had been screaming like that. If this is the kind of generator that I think it is, the field would probably have lost excitation, cutting off the output.
On the other end, the fan motor would have been over-speeding, at minimum, making a racket and at worst, the blower wheel would have come apart.
A couple of things you can try. One is to connect a resistive load of a couple hundred watts to the generator. This may clean up the waveform enough that the KAW is no longer being fooled.
Another thing you can do is to run the furnace blower for awhile (hour or two) while monitoring its temperature. If it were really operating on power that far out of spec, it would likely overheat rapidly. If you can lay your hand on the motor casing without getting burned, then the motor is OK. Be sure to leave the cover on the air handling unit except for when you're checking. Letting the blower pull ambient air instead of from the return duct will overload it.
The generator has to supply the "wattless current" that the motor draws because of the low PF even though it's not "real" power. The term is "lagging VARs" (Volt-Amp Reactive). Leading VARs can also be supplied by a capacitor, the process being known as "power factor correction." A capacitor of suitable size is simply connected in parallel with the motor.
One can calculate the size cap to supply the appropriate amount of leading VARs but it's easier to simply substitute a variety of caps while watching the line current upstream of the cap. Select caps until the line current drops to the minimum value.
A motor run capacitor just like the one already on the fan works fine. I'd start out with about a 25 microfarad unit. Using utility power, watch the KAW and see what the PF and amps do. Add another unit of capacitance, say, 5 or 10 uF. If the PF improves and the amps drop, you're on the right track. If they don't then you already have too much capacitance.
Shop around for the caps. Many HVAC shops will give them to you from removed old units. So will electric motor shops. New, the cap shouldn't cost over $20. I've been working with almost the same problem with someone else via email and I've been amazed at the rip-off prices some outfits quoted him. One was >$100.
The reason that you want to do this is that it will unload the generator so that you can run other loads simultaneously. 300 watts is about right for a 1/3 hp blower motor. With the PF adjusted to 1, that leaves you 600 watts for other loads. As it is, even though the real power is only 300 watts, supplying the necessary VARS is heavily loading the generator, leaving little room for other loads.
John -- John De Armond See my website for my current email address http://www.neon-john.com http://www.johndearmond.com <-- best little blog on the net! Tellico Plains, Occupied TN Some people are like a Slinky .. not really good for anything but you still smile when you shove them down the stairs.