Mains AC motor speed control

[Periproct]

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

 

[Sjouke Burry]

Periproct wrote:

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


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

 

[Tim Wescott]

On Mon, 24 Nov 2008 00:18:09 +0000, Periproct wrote:

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

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

 

[Periproct]

"Tim Wescott" <tim@seemywebsite.com> wrote in message
newsPedndBHQ51abLDUnZ2dnUVZ_tzinZ2d@web-ster.com...

On Mon, 24 Nov 2008 00:18:09 +0000, Periproct wrote:

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

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.

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.