Tuesday, January 20, 2015

How to wire an HVAC fan motor for 3 speeds

Disclaimer, I am not an electrician or in any way qualified to give electric advice. Following my instructions may result in a fantastically painful death and/or total loss of property and innocence,  but probably not

It's man's nature that if you have a motor capable of three speeds you will want to use all of them. Such was the case when I was looking for a motor for a small shop project. My choices were between a 1/4 HP single speed motor or a 1/4 HP three speed HVAC fan motor. The fan motor was designed so that you pick one speed, wire it, and that would be your speed forever. But does it have to be that way?

Probably not but then I had to figure out how to wire it. Simple, I found a rotary switch on Grainger: off/on/on/on. Not simple, I was advised by the internet this switch would cause my untimely death or at best, burn out in short order. OR, it was perfectly okay and would only last 100,000 cycles instead of 200,000. Back and forth came the advice with lots of technical mumbo jumbo about snapping that was over my head. I had 3 people telling me it was a bad idea and 3 telling me it was fine. And all of them seemed credible. One fellow had an engineering background and had worked for a switch manufacturer so I gave his advice more credence. The only thing all 6 could agree on was that toggle switches would be safer. So I called Grainger, ’We don’t sell switches rated for ac motors. Don’t use any of our switches!’ You think I’m kidding but that’s almost word for word what they told me. It’s also a bald face lie, they do sell switches rated for ac motors. So I threw in the towel and bought toggles rated for AC motors from Home Depot. They had no problem with me buying their switches and even walked me directly to them in a rare display of eptitude. In the end, of the 6 people actively giving me advice, only one was happy I went with toggles, the other 5 were disappointed in the lack of apocalyptic outcomes.

So you want to know how to wire a 3 speed HVAC direct drive AC motor? Here’s how I did it.

1 each – SPST (on/off) Gardner Bender toggle switch
2 each – SPDT (on/off/on) Gardner Bender toggle switch
wire, motor, and appropriate electrical housing w/ cover

Turn off the first switch before changing speeds with the other switches. If you know of a more elegant way, feel free to post it in the comments below. If you believe my way will be my doom, well I'm here writing about aren't I?

Update: for clarity: Green = ground, white = common, black = high, blue = medium, red = low. If you have more colors I don't know what they do.

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