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Tech Question – Why Do ECM Motors Trip a GFCI?
This question was submitted on the site in response to the recent GFCI tip. It's a good question with several possible answers. What do you think?
I have had a few instances where we are firing off a furnace in a new build with a temporary power pole outside with gfci outlets installed that are tripping the gfci on blower motor startup. (In order to get temporary heat for drywall, we run an extension cord to the gfci pigtailed to the furnace). I’ve never had issues doing this with PSC motors, but every time I’ve tried to use a gfci with a variable speed or true ECM motor, it trips the gfci. I’ve tried multiple motors, different furnaces.
Frustrated in Fort Collins
Howdy Frustrated (This feels like a “Dear Abby” article, but I'm just gonna roll with it),
I haven't tried this, so I haven’t experienced it (no big call for temporary heat in Florida), but it does make some sense. The GFCI is just watching for a difference in current on hot and neutral. When you add in a variable frequency drive (which is, essentially, all an ECM or X13 motor is), the circuitry/housing is exposed to a lot of “induced” electromagnetic fields due to the varying frequency/harmonics. Some of this induction (magnetic flux) may be routed to ground, causing a tiny imbalance. Here is a thread in my buddy Mike Holt’s forum that talks about this very phenomenon HERE.
Also, on a furnace with a flame rectifier, a very small amount of current (microamps) is purposely routed through the flame to prove the flame. That has nothing to do with it being ECM, though.
Thanks for participating. Great stuff!
P.S. – What do you think is going on?