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?
Thanks in advance.
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
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
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.
[ ... ]
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
[ ... ]
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.
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?
> 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
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
Again, thanks for the various inputs.
[ ... ]
Yep -- it was a particular surplus outlet which had a lot of
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
"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
"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.
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
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