I was given a treadmill recently that I took apart for the DC motor, which I had heard could be used for a small lathe with variable speed control (better than belt switching). I want to use it on my Atlas
618, which now has a 1/2 hp motor.
What I found was a 1hp DC motor, which I removed with the transformer, control unit, etc. When I put it back together on the bench, I find that the motor does not start instantly. Seems I need to turn on power and crank the speed dial up a ways and after a bit of delay the motor will start. When I reduce the speed control, it sounds like the motor stops taking power until it slows down and then I hear the slight hum again. Is this behaviour normal? My main concern in hooking up the lathe is that I can turn it on and not have to wait for the motor to start. Is the treadmill control unit basically a VFD? Can I make it work for me?
I am running my South Bend with a 3/4HP treadmill motor. Go to E-Bay and buy the KB Penta motor controllers they sell all the time. The controller for the treadmill has to allow for a lot of mass to accelerate, plus the lawyer fear of the runner turning the control up too fast and being slung off the treadmill. The $70 Grainger controllers also have a four second delay, unless you buy an add-on board for them. The KB Penta powers have acceleration adjustments, etc, and are going for around $50. They are the best PMDC controllers I have ever found. I keep buying them just to have them around. Forget the Minaric ones as well. I had a couple and threw them away. I have to say that changing from an AC induction motor to a variable speed PMDC is one of the best things I ever did for the lathe. I did it for my miller, too.
A few years back, there used to be a lot of treadmill motors sold as surplus and there was a lot of interest in modifying the controller to get rid of the need to turn the speed control down to zero before the motor would restart. I think some of the mods and circuit diagrams made it into the dropbox but I don't know if they're still there. The controllers back then were PWM controllers, not VFDs, VFDs are only for multi-phase motors, not DC at all. I've got a couple of those surplus units, there is always a slight delay when they start up. I think it's like the "soft-start" on woodworking routers, keeps the startup current limited to something reasonable and also reduces the torquing of the motor in its mounts. For the original use, you wouldn't really like to start trotting on the treadmill at full speed from a dead stop either. The mods to the controllers were pretty easy to do, you might see if you can scout up the circuit diagrams and see if they match anything you've got.
I found the files in the dropbox (1998), but they don't seem to be applicable to what I have. They apply to a specific available motor and controller that apparently was available in quantity some years ago.
I did look more at the controller though. It's a Samsung Motor Speed Controller Model MC-20. It has four small adjustments, I believe. They are round plastic things that look like they should be turned with a screwdriver. They are labeled "Min Speed", "Max Speed", "I.R. Comp" and "Current Limit". Min and max speeds look they had had some glue/wax/something put on them to keep them from being adjusted, although I'm sure it could easily be removed. But I don't know what the I.R. Comp or Current Limit would do. Can anyone shed any light on this. I googled the controller, but could not find this item. I thought it might be good to know what to expect before I start readjusting everything.
Yep -- it was a particular surplus outlet which had a lot of them.
Potentiometers -- resistors with a sliding tap which can be set from totally connected to one end through somewhere in the middle, and all the way to the other end. They are designed to produce an adjustable percentage of the overall voltage across the end terminals.
Common practice to set these things with a paint so they won't shift with vibration -- and so tampering is obvious if there is a a safety question.
"I.R. Comp" is most likely "Internal Resistance Compensation". It is tuned to the motor so the speed control has a better idea of what the motor will take under various current loads (and how much voltage you should see across it under each set of conditions). Since the controller and the motor came together, you should not have to change this one. If it is off, you will have poorer speed regulation with load changes.
"Current Limit" is because with permanent magnet DC motors, current above a certain level risks demagnetizing the permanent field magnets to some extent -- weakening the motor's available torque. Secondly, for long runs, the motor current needs to be kept below a certain level or it will overheat. This can be a special problem when running at low speeds, where the current is high (proportional to torque), but the fan (on the motor's shaft) is running too slow to provide adequate air. I would suggest leaving this one set as it was, too, as it was set to protect the motor.
The "Min speed" probably is a good idea to keep set as it is, too, unless you *really* need slower operation.
"Max speed" may have been set with the safety of the person using the treadmill in mind, or with the chance of the motor rotor blowing up from overspeed.
The two of them control the overall range of the speed control pot(entiometer) which is probably the only rotary control which is available to the user in the treadmill configuration.
No. IR compensation is a poor-man's not-quite-closed-loop control algorithm. I = current, R = motor resistance, I*R is added to the nominal setpoint voltage to better regulate setpoint speed. Works surprisingly well, but must be custom adjusted for each motor's characteristics.
The Pentapowers have always had very good load/speed response. I have had too many failures with Minarik controllers. Possibly the KB units have better transient suppression. As usual, your mileage may vary, but at least here, from 1/2 to 1 HP, the KB's are on my machines, and every Minarik has left in the dumpster, sooner or later. Maybe they are better for constant load...In which case a Variac and diode bridge would be a simpler answer! I added big heatsinks to them, but even with feeding 0.1" chips, the heatsinks stay cool.
"setpoint speed" is the no-load speed as determined by applied voltage and back EMF.
Think of IR compensation as adding a negative resistance to the output voltage characteristic. If you drive a PM motor from a perfect voltage source, under no-load the motor will run at a speed where its back EMF (the voltage it would produce as a generator) is equal to the supply voltage. As you apply load, the speed will drop a little due to the motor internal resistance. Vapplied=IR+EMF where I is current and R is motor effective internal resistance. You can improve speed control with loading by making the power supply voltage rise somewhat as current is increased. Since this behaviour is opposite what would occur if a real resistor were inserted, it is equivalent to a negative dynamic resistance.
Correct. There is probably some RC (Resistor-Capacitor) circuit in there to slow the ramp up to the set speed. Given a schematic of the controller, I might be able to discover what is doing it, and how to disable it, but lacking that, and lacking the actual controller to trace or experiment with, I don't have much chance to figure it out other than theoretically.
Note that a low enough current limit could slow the motor's ramp-up -- but it would also make it difficult for it to drive much of a load, too.