Mains AC motor speed control

Periproct 17 years ago

I am playing around with a couple of CNC mills. One is a tiny HDPE thing with a Dremel 300 series drill and the other a larger homemade creation with a Trend T4 router.

Both the Dremel and the Trend are 240 VAC variable speed but my problem is that their minimum speed is 10,000rpm and I'd like them a lot slower. (With a really slow feed rate I can engrave mild steel front panels but I think

10,000 rpm isn't good for my cutting bits). I'm thinking of using an external variable speed controller (after bypassing the internal one, if possible). Is it possible to do this with a brushed
240VAC motor and still get reasonable power at low speeds? Can anybody point me in the right direction? (Velleman K2636)? If someone was to tell me how to control the spindle speed directly from Mach3 CNC software I'd be a very happy bunny indeed.

Regards, Nigel

Dremel 125W Trend 850W

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Sjouke Burry 17 years ago

I use a variac transformer for those, and am quite satisfied with that.

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Tim Wescott 17 years ago

Universal wound motors like that one have an odd torque/speed curve; at low speeds the torque is very very high, and while it decreases monotonically as the speed increases, the armature torque never goes to zero, it just gets overcome by wind and bearing resistance.

So if you want the thing to go at a constant speed with decent torque you pretty much have to drive it with closed-loop control.

I don't know if there's a published circuit out there, but I would expect that someone has done this somewhere.

Tim Wescott Control systems and communications consulting http://www.wescottdesign.com Need to learn how to apply control theory in your embedded system? "Applied Control Theory for Embedded Systems" by Tim Wescott Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

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Periproct 17 years ago

Thanks for the info. I'm not very knowledgeable where cutting speeds are concerned but the software utility I found would indicate both my Trend and Dremel are way too fast for mild steel. I've found a very low feed rate and improvising with oil as lube/cooling is stopping my cutting bits turning blue. I'll have fire up google and look for closed-loop control.

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