Use of DC Motor for small lathe

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.

Yes, sounds like normal inertia and acceleration control in a DC motor controller.
Not a VFD, better than a VFD.
See how I retrofitted a PMDC motor for a Bridgeport milling machine:

What is the part# of the KB controller?
Grunty Grogan wrote:

or kbic -125

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.
Stan

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No. A VFD works only with induction (AC-only) three-phase motors, not with DC motors.
It should be possible -- just need to get the data sheet on the controller (chip, probably), and learn how to disable certain safety features.
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They most certainly are -- just moved into one of the "YEAR-retired-files" directories to keep things from getting too large.
Wayne Cook is the one who traced them out, and posted the drawings to the dropbox.
Good Luck, DoN.

I'm looking at DC motors/controllers for a lathe (for both the spindle and leadscrew)--could you elaborate on what makes the KB controllers good and the Minarics bad? Thanks, David
> 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 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.
Again, thanks for the various inputs.
-Bruno

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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.
Well ... I've tried to put my guesses in there.
Good Luck, DoN.

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.

I gather, then, that none of the adjustments will help deal with the delay in the motor starting up when turning up the speed.
-Bruno
snipped-for-privacy@d-and-d.com (D>>

Can you please explain "setpoint" speed?
Thanks, -Bruno

I just found a very good explanation of these controls at... -Bruno
Bruno wrote:

"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.
Ted

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.
Good Luck, DoN.

A decent DC motor control would let you choose that delay down to zero, but it sounds like in your unit it is fixed at some annoying value for the treadmill application.