Get me up to Speed on DC Motors

[SteveHGraham]

I want info on DC motors.

I got a small belt sander, and a decent AC motor will run $75, minimum. A VFD and 3-phase motor will be maybe $250. I saw Frank Ford's DC-driven sander, and it got me thinking about DC. It would be nice to have variable speed and good torque without springing for a VFD.

I've been reading up. If I understand things correctly, what I want is a constant-duty permanent-magnet treadmill motor that goes about 1 HP. Those are cheap. I can get one delivered for between $50 and $100. But you have to get the juice to it and control the speed, so you need a control board with a potentiometer.

I figured the power supply and control had to be the expensive part, but it's starting to look like that isn't correct. Surplus Center has a unit which APPEARS from the specs and photo to do everything for $85. It has a transformer right on the board.

Can it really be that simple? Is there something I'm missing here? It looks like the whole mess would run maybe $135.

I wish I were one of those lucky people who have stuff given to them all the time. Used treadmills cost a hundred bucks on my local Craigslist!

http://www.surpluscenter.com/Electric-M ... 1-2269.axd

[chucketn]

The best treadmill controller I have found is the MC-60, used on many brands. I lucked out and got 2 treadmills off the local Freecycle that had these controllers and nice 2.5HP motors. I have one powering my X2 mill, and one on my homemade sand muller. I needed another, but the Freecycle source has dried up. I found a controller on ebay for $35 and snapped it up. The seller had several available at the time.
All you need is a 5Kohm linear pot. I even reused the ac cord to the treadmill and the 15a circuitbreaker as a power switch.
HTH,

Chuck

[SteveHGraham]

I guess if it will run a 2.5 HP motor, it would have no problem with a smaller motor.

I found a 1.25 HP Weslo motor on Ebay for $50, but it doesn't say anything about RPMs.

Do you know how many volts you can get out of that controller? The Weslo is rated for 90, but I saw another one rated for 130.

[SteveHGraham]

I will tentatively answer my own question. Assuming all MC-60s are created equal, and assuming all Proform 585 treadmills have the same motor and controller, the MC-60 will go up to 130 volts. I just found a Proform 585 motor with that rating, and current Googling suggests the Proform 585 uses the MC-60.

[WJH]

Make sure it matches the type of motor. Some treadmills use brushless motors, a bit different from your typical brushed 2 wire dc motor.

[SteveHGraham]

That would suck. The one I'm looking at now is an Icon (Nordictrack) 1 HP motor.

[JackF]

Steve, I have gotten 5 or 6 working treadmills off the Craigs list free section. Some I gave away, some I kept for future use and one I put to use on a Rottler boring bar. Working tread mills mean both the motor and power supply work. The one, and maybe not the only drawback is having to make or buy a housing to fit the power supply. Anyway free is hard to beat.

Jack.

[chucketn]

The MC-60 and motor combo on the mill came from the same treadmill. Give me a minute or two and I'll go to the shop and check the motor info plate. I think it's more than 90 volts, and 2.5 HP, brushed. I did play with the controller and several different PM treadmill motors, and they all work fine with it. Lately while cutting gears, the motor has gotten quite warm, so I added a fan blade to the armature shaft, which sticks out of the end bell about an inch. I just made an adapter on the lathe and loctited it on the shaft.
Seems to help, plus blows the smoke from cutting fluid away...

Chuck

Edit: Just checked the motor plate, 2.5 HP, 120VDC on the motor. I will take my multimeter to the shop in the morning and measure the volts to the motor at max RPM. Will that help?

[SteveHGraham]

I don't think there is any need. It looks like that controller is more than adequate. Thanks.

[atunguyd]

Steve, most DC speed controllers are PWM controllers. Unlike your VFD they are not inverter based and do not generate the voltage but rather just switch it on and off at the correct times.

So if you feed one that is rated 110V with 90V you will not get 110V out but rather 90V.

Note however that the motor is rated at 130V which is probably the peak rating, over there I am sure you rate your mains as 110V rms which will give you a peak voltage of around 150V.

[Torch]

Steve, most DC speed controllers are PWM controllers.

A lot of DC speed controllers, especially for higher power motors, are actually SCR based rather than PWM. They are cheaper, but are simply rectifying the 60hz AC signal. Voltage is clipped to reduce the speed. Motors tend to run noisier with a typical AC hum and hotter. There is a significant loss of torque at low speeds since torque is proportional to voltage.

By contrast, PWM controllers supply a fixed voltage at high frequency (eg: 16,000 hz), in variable duration pulses. Pulse duration (and therefore current) is reduced to reduce speed. Low speed torque is still reduced somewhat, but remains considerably higher than with an SCR controller.

I swapped out the SCR controller that came with my 1.5hp Chinese machine with good quality PWM controllers and the difference is like night and day.

[SteveHGraham]

I ordered an Icon motor and an MC-60 with a power supply. We'll see if it blows up.

Some dude in West Palm is selling another Rockwell sander with a neat old stand and Rockwell motor. I'm going to buy it if I can. The stand will be great, and I can put the DC motor on it. I guess I can put the best parts from the two sanders together and keep it, and I can put the worst parts together and sell.

Or I could have two sanders. No law against it.