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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I have a Lennox furnace that does not always run when my Generac generator is running. The frequency and voltage are within the tolerance of the furnace. The sine wave is not clean and some suggest I need "clean power". Is there anything other than a power conditioner that could smooth the sine wave? I don't want to get into any inverters. I have even read that an LCL filter might work. I'm open to suggestions.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714
Good to see you here. For the benefit of overseas readers, you're talking about a central furnace to heat your house, right?
I bet most of the amps on the furnace nameplate are for the main air-circulating blower. After that, perhaps for an induced-draft blower that's part of the firebox system. Hard to imagine either of those having trouble with unclean power.
Do you think the trouble lies with the solid-state electronic control module? In my old furnace, that gets power via a 24 volt control transformer. Newer units might power the control electronics with a little 120-to-12 volt flyback converter built into the control board. I've seen that on some microwave oven controllers.
Passively cleaning up AC power for the controller could be easier / smaller / less expensive than doing it for the whole furnace unit. As a preliminary test, you could remove AC power from the controller and substitute an independent DC power supply. Can be connected to the same place where you measured the DC control voltage before choosing a supply. I guess it's possible that AC line frequency is used as a time reference by the controller, to save a few pennies of cost.
Generac is a reputable brand, and it came as a surprise that it's delivering a distorted sinusoid. Does yours incorporate an inverter, perhaps as part of a "UPS" or "No interruption" feature?
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I've looked at the power in the past with a scope and it's messy. I'm going to check it again tomorrow. I have a generator from around 2003, so maybe they're better now. There is no inverter or clean-up unit to the best of my knowledge. My freq is about 62.5 Hz. I'm going to turn it down to about 61 and see if that makes a difference. From everything I've read online, it is a "clean power" issue. When I have my winterization service I'm going to talk to the technician. The strange thing is that it has worked in the past.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
If the problem is due to noisy power to the electronics, it may be sufficient to add electrolytic capacitors across the low voltage dc (rectified ac) supplies to prevent intermittent power problems.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
So I put a scope on the outlet and noticed that the power is actually clean. However, the frequency was slightly outside the tolerable range for my Lennox furnace. The frequency was 63Hz while service frequency is 60.2Hz. I adjusted it to 61Hz incase the generator gets too loaded. The voltage also appeared to be slightly above tolerance. I adjusted both on my generator and now everything works.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714
That's good news.
When I mentioned inverters, it was not in the sense of being there to clean up the power. Rotating alternators inherently generate clean power, when the engine speed is governed to get the right frequency. Could say governated, but that would suggest that I'm Californian and not very young.
The worry was that inexpensive _inverters_ can make dirty power out of clean input. The most prominent issue IMHO is "modified sine waves" that come from H-bridges running at mains frequency. Lots of harmonic distortion, that can go up to pretty high frequencies because of fast-rising steps.
Yes, your Rigol waves appear to have a bit of distortion, but not problematic. I bet one could get a good fit with just 3rd harmonic at the right amplitude and phase. Where might that come from?
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conditioning power from generator
