Goodell Pratt 125 Bench Lathe #3: Motorin' - or How to wire a 3 speed AC motor.

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Blog entry by Woodknack posted 01-06-2013 06:40 AM 4227 reads 1 time favorited 0 comments Add to Favorites Watch
« Part 2: Ye olden lathe Part 3 of Goodell Pratt 125 Bench Lathe series Part 4: The Stand »

You’re motoring
What’s your price for flight
In finding mister right
You’ll be alright tonight

My choices were between a single speed, 1/3 HP, 1750 RPM motor or a three speed, 1/4 HP, 1050 RPM motor. Wow, 3 speeds! The 1/3 has a very short shaft meaning I could only fit one pulley meaning I would have to slide the motor to each lathe pulley (or I could have used a jackshaft but I was too stupid to know it). The 1/4 has a longer shaft to fit a step pulley and did I mention 3 speeds! Who could resist the the promise of 9 speeds over 3?

And 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 this switch would kill kittens and cause my untimely death or at best, burn out in short order. OR, it was perfectly okay to use and sure, maybe it would only last 100,000 cycles instead of 200,000 but big deal. Back and forth came the advice. I had 3 people telling me it was a bad idea and 3 telling me it was fine. The only thing all 6 could agree on was that toggle switches would be safe®. 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 motor use from Home Depot, they had no problem whatsoever with me buying their switches. In the end, of the 6 people actively giving me advice, I’m pretty sure only one of them was happy I went with the toggles and that was a fellow LJ’er.

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

Make sure you turn off the main power switch before changing speeds.

If you know of a more elegant way, feel free to post it in the comments below. If you believe my way will be the doom of us all, I don’t want to hear it.

-- Rick M,

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