View Full Version : Help with DC motor speed control?

Doc Nickel
02-22-2006, 07:47 PM
I happened across a small, used treadmill the other day, and hauled it home for the cost of tossing it in the truck.

It turns on, but I can't get the motor going. There's a spot that looks like some magic smoke escaped from the heart-rate/speed-readout board, but that appears- key word- to be seperate from the actual motor speed controller.

Anyway, just from a casual glance at the speed controller, it looks like it might be as simple as stabbing in a pot in place of the heart-rate board. Lower left corner:


If that's the case, it looks like it might almost be easy to use this thing. Yank the motor, controller and transformer, plug in a pot and away we go. Maybe?

Here's the PN of the speed controller:


Another board that I'm not entirely sure what it does, but it's 'between' the heart-rate board and speed controller:


And for good measure, one of the motor data:


I haven't unplugged or unwired anything, other than pulling the plug off the heart-rate board so I could dismantle part of the stand. If it's as simple as connecting a pot to those two terminals, what value and capacity pot do I need?

And is this thing really a 1HP? The motor is only a bit bigger than a can of Fosters' beer- I have a hard time believing it's three times the power of the four-times-larger 1/3rd HP 110vac motors on either of my lathes...

And last- any guesses on the max RPM? Doesn't seem to say anywhere.


02-22-2006, 07:59 PM
the three legs labled speed pot, should do the trick for a pot. I will look for you when i get to work in the am for a pot rating from a similar machine. I sell and service this stuff for a living. For what it is worth about the motor, they usually put out rated horsepower untill just after they are unboxed http://bbs.homeshopmachinist.net//smile.gif. the manufactured for weslo is our industries version of the harbor freight crap. Not bad if you are not going to use it much.

02-22-2006, 08:27 PM
The typical pot value for one of these controllers is 5K.
Pretty much any power rating will work.
Just as easy to use as you surmise.


02-22-2006, 08:50 PM
I got a few treadmill motors laying around,most I have are accurate hp wise,but the rpm is in the 4500-6000 range which expains the size.

J Tiers
02-22-2006, 10:42 PM
That appears to be a reasonable controller, it has all the right adjustments, min& max speed, IR compensation, and current limit.

I notice that apparently they made some changes to the wire colors to obey US standards... no green power wire!

The power is indeed developed at high speed, whch is the major drawback of the motors. Slowing them more than a little will lose power, since they have little torque, and not much overload torque, especially with the current limit set right.

Probably 3:1 or 4:1 speed range in general, more if you do not need low speed torque. You will need some back-gear type arrangement of pulleys to get the 4000+ rpm down to the usual 1750 or 1150 rpm motor speed. Then you can talk about slowing from that point.

The "extra" board is to power the heart rate board and display etc, as well as possibly part of the controller "brains", looks like it may have a voltage regulator, as well as a relay that may start the motor.

A pot might well fix it. Be aware that the pot and wires are likely at line voltage unless there is an optocoupler on that control board.

[This message has been edited by J Tiers (edited 02-22-2006).]

02-22-2006, 11:06 PM
there's an optocoupler.

U1 looks like.

What is the part # on U3?

5K is about right too for the pot.

still treat it like line voltage.

02-22-2006, 11:29 PM
I've got one of those driving an 8" 4-jaw chuck in a vertical setup. Granted it doesn't have much punch on the low end but it sure spins that chunk of semi-steel just fine. I'd have to measure the pulleys but I set it up with about a 3:1 reduction.

Your PWM board looks more advanced than mine. I don't have all the fine adjustments. I used the original slide pot that was mounted in the console. The distance, calorie and heart rate monitor console was standalone.

J Tiers
02-23-2006, 12:05 AM
Ah-yes, I finally found U1.

Only opto-couplers are typically available in white packages.

Toshiba makes a series of opto gate drivers, I want to say they are TLP-26 or a similar number....memory escapes me.

Good to 25kHz switching frequency for PWM. NOT good for 250 kHz, as a competitor of ours seems to have found out.........

That may well be one if the controller is a PWM type. Otherwise they have some that would be good for SCR or triac drives also.

[This message has been edited by J Tiers (edited 02-23-2006).]

02-23-2006, 12:11 AM
Quite likely that you have to turn the pot to minimum speed before the thing will start the motor. I got tricked by this until I did that. I guess I was expecting the motor to start right up at the speed that was set, but no.
My pot was also 5k.

02-23-2006, 12:16 AM
The opto on there is white. However all the ones I have in my stock are black. The controller I have is low volts DC to the pot, 10-12vdc I believe.

Doc Nickel
02-23-2006, 12:26 AM
Okay, I'm keeping up with about half of what you've said so far... http://bbs.homeshopmachinist.net//biggrin.gif

A little more info, if it helps: The heart-rate board is programmable. It has (had?) selections for multiple users, and several LED bar-graph elements so you could program a "cycle". IE, light workout for a few minutes, then faster, then slower, etc.

I'm assuming that board fed a signal through the "unknown" board to the speed control, which did the actual voltage regulation.

There's also a speed readout, an LCD display that shows time, speed in MPH and a couple other things. It's actually a self-contained module, "snapped" into the heartrate board. Not designed to be removable, but it was obviously a seperate module they already produced, and made a place to snap it into this larger board. It has it's own case, connectors and mounts- I'm assuming it was a smaller speed/timer readout for cheaper versions of the treadmill.

In the picture of the unknown board (tread3.jpg) the small connector with the back wires, in between the wide white connector and the black relay-looking thing, leads to an inductive pickup on the motor flywheel, which reads a magnet embedded therein. That cable/plug pops out and plugs right into the back of that MPH box, into a socket labelled "speed".

Also on tread3.jpg, the two-wire white connector on the right edge leads directly to the two connectors on the speed controller board marked "speedpot".

Would a rough wiring diagram help?


02-23-2006, 12:50 AM
The motor controller is fed a varying voltage from the console board. The controller requires a voltage that varies from zero to +10 or so for the complete range of speeds. This only takes two wires, one for ground and one for the voltage input to the motor control speed input.

If you use a pot the motor controller has a 10-12vdc reference output that the pot is connected to. One side of the pot goes there, the other side goes to the ground terminal and the center tap of the pot goes to the variable voltage speed input. 5k to 10k should work just fine. Use a linear taper pot for best results.

02-23-2006, 01:13 AM
Doc, the standard is black (L), white (W) and red (H) from the circuit board, you can see those labeled on the controller heat sink.
Hook the wiper (center terminal) of your 5K pot, to white (W), then the two remaining connectors to either of the outer terminals on your pot.


Doc Nickel
02-23-2006, 04:40 AM
That seems easy enough. I'll try digging up a potentiometer tomorrow and see what I can find out.

Power comes in through a big transformer screwed to the frame, as well as that smaller transformer and the black relay looking thing. Hopefully that too can be reduced somewhat...


02-23-2006, 05:37 AM
The two wire xformer screwed to the frame is a choke, you'll want that in what ever application you find for your motor and controller.
as for the other board, that is not a part of the motor controller.

All you need is 120 AC fed to the controller (with the hot line through the choke and switch/fuse/breaker of your choice), two wires going from the controller to the motor, and three wires connecting the 5K pot to the appropiate lugs.


02-23-2006, 07:14 AM
Keeping in mind what darryl said, and returning the pot to minimum to get the motor to start. You can, by momentarily breaking the wiper connection start the motor at the speed you have set the pot for. However I would give some thought about that.

Don't let the size fool you, these a very powerful motors, strap it down before you try running it.

J Tiers
02-23-2006, 08:13 AM
<font face="Verdana, Arial" size="2">Originally posted by Evan:
The opto on there is white. However all the ones I have in my stock are black. The controller I have is low volts DC to the pot, 10-12vdc I believe.</font>

Yes, the white package has somewhat different properties in many cases.... but in general only optos are available in white.

When it comes to the wiring, rather than believe the color codes, which are wrong, incidentally, make a diagram of what is there before you unplug any more.

That way you will have a reference later.

BTW the opto does not guarantee the conreol is not "hot", since it depends how the thing is made.

With a separate small power transformer, you MAY get control power through it, OR it could be for the display only. The control pot could still be hot. Many, even most small motor controls operate with no transformer, because that is cheaper.

J Tiers
02-23-2006, 08:16 AM
dup deleted

[This message has been edited by J Tiers (edited 02-23-2006).]

02-24-2006, 10:37 PM
can you possibly give me the part numbers off the 5 TO220 mount devices. Probably two different devices. Three of one, two of the others. The items in question are the top most parts that are screwed into the block of aluminum. (little metal tabs built in to the plastic epoxy with three terminals coming out of the bottom).

I seem to have accidentally stepped on my motor controller http://bbs.homeshopmachinist.net//frown.gif Broke off the 2 of the diodes and the 2 SCR's at the tabs. A $200 motor controller crapped out. (my parts are obsolete)


02-24-2006, 11:04 PM
NO ****, the pots on some dc drives are 180 volts..

You get your arm into one, surprise.. makes you winkie jump.. *normally mount the pots on doors, they swing shut on you, surprise..

Not all speed pots are the sometimes 10volts. plus and minus,, thou this one looks it since they are pumpin the refrence voltage in on the wiper and the common..

looks like you didn't need my input on this one doc..

Doc Nickel
02-25-2006, 03:31 AM
Thanks guys, it works!

I took the small sub-board out of the system, and so it's just AC to the speed controller, wires out to the motor (the two-wire transformer is on these lines, not the AC-in lines; that's the way it was when I got it, is that correct?) and a salvaged 5K pot my brother had in a box.

I plugged it in, no magic smoke escaped, and the speed control goes from "it can almost manage to pull the belt" to "holy crap, nobody can run that fast!". (Motor is still in the treadmill.)

So it looks like I have a handy-dandy variable-speed 1HP motor and speed control. Might be time to get back to that CNC conversion of my old mill-drill. http://bbs.homeshopmachinist.net//biggrin.gif


Doc Nickel
02-25-2006, 04:16 AM
Oh, the other question: Just where, exactly, does it turn from AC to DC?

The motor is DC- says on the tag shown in the pic above, but it's taking AC direct to the speed controller.

Is the transfomer, on one line between the motor and speed control, the point where it changes?

Pardon my lack of electrical knowledge- I can usually get the batteries into the flashlight the right way, two tries out of three. http://bbs.homeshopmachinist.net//biggrin.gif

Things like transformers and such, while I may know a vague manner in which they work, I'm nowhere near up to speed on the practical application thereof.


J Tiers
02-25-2006, 08:59 AM
It is entirely possible that that unit has SCR control, in which case of the 5 items on the heatsink, 2 would be diodes, and 2 SCRs. The 4 together would form the full-wave rectifier, with the SCRs controlling the current as well as rectifying (Silicon Controlled Rectifier= SCR).

If it were a mosfet PWM type, there almost certainly would be a separate rectifier.

02-25-2006, 11:53 AM
as standard, 3 of them are diodes, 2 of them are SCR's.


Doc Nickel
02-25-2006, 03:06 PM
That raises another question: What's the chaces this speed controller might be undersized?

Right now, if I turn the new pot above, say, 2/3rds of it's travel, the tradmill belt by that point is moving quite a bit faster than a person is going to be able to use. I haven't tried it, but it looks like you'd have to be young, healthy and running full-tilt to keep up. http://bbs.homeshopmachinist.net//biggrin.gif

And it can go higher still. Presumably the now-removed console board, or the smaller daughterboard, had a speed limiter or limitation. It did have a speed sensor, an inductive pickup on the motor flywheel.

The question is, does the speed controller have the capacity to run this motor at "top speed"- especially in a high-draw situation like in a mill and taking a deep cut at high speed- or could it be undersized (to be cheap, presumably) since it was never designed in this application to exceed, I'm guessing, about half the motor's speed range.

I recall looking at DC motors and speed controllers in a surplus catalog, and a 1-1/2 HP 90VDC motor needed a speed control with an optional heat sink. My controller has no sink other than the aluminum case the SCRs are screwed to.


02-25-2006, 03:25 PM
well...what are the part numbers on the SCR's?

You'll need to know that to know if it's got the balls or not.


02-25-2006, 03:50 PM
Your controller has a min and max speed pot. Those do work. It also has a current limit pot and I would assume it works also.

If you put to much of a load on the motor, the controller will cut out until the current level drops to a set point. This is to not only protect the motor, but also the controller.

Whether it has enough poop to run what you want to run with it depends on how much poop you need. http://bbs.homeshopmachinist.net//smile.gif

02-26-2006, 09:58 PM
Just to put in my two cents worth, I put one of those motors and speed control (from Surplus Center about 10 yrs ago)on an old Delta 14" bandsaw, and have loved it. Had a couple of good metal blades made up, just recently put on the second one.Running 2" motor to 8" driven pulley, can almost count the teeth going by at slow speed, but it doesn't stall cutting 2" steel. You do have to turn the pot to zero then back up to make it go. (this was to keep Ditzie from getting on the treadmill and starting at 40 MPH) Iffen I did it agin, I'd try a switching pot, might make it automatic.

Forgot to say, it will also really RIP turned up.

[This message has been edited by MikeyR (edited 02-26-2006).]