DC: Treadmill Motors Damaged by Removal & Replacement of Armature?

I apologize if this isn't the best subforum for this question, but it seemed like the correct choice.

I have a 2.75 HP treadmill motor and controller, which I got for nothing. I'm thinking of using it to power a buffer. It has two ratings. One is 2.75 HP for "treadmill duty" (until the fat person faints, I guess), and the other is 2 HP (1492 watts) for continuous duty at 130V. I don't yet know the voltage the board puts out, but 2 HP would be a God's plenty for buffing little doodads in my shop.

Anyway, a thoughtful person who wants to be helpful gave me some advice which sounds unreliable. First he said "treadmill motor horsepower isn't real horsepower." I don't know what to make of that. I don't know why a manufacturer would pay a motor supplier for mislabeled motors. Anyway, he claims the motor will only be good for 1.3 to 1.5 HP. Is this actually true, or is it just a machinist myth?

Second thing: he said that if I removed the armature from the motor without putting some steel in the motor to fill up the space, the magnets would instantly go bad, and it would thereafter run weakly, drawing huge current. This is my second treadmill motor. The first is now on a belt grinder. I removed the armature so I could machine a keyway on the shaft, and the motor now runs fine and pulls well. It's rated for 9.1 amps, and I installed a 15-amp fuse on it, and if the current was "huge" compared to 9.1, one would expect the fuse to blow, which does not happen even if I stall the grinder.

Can I conclude that he is misinformed?

I've seen people on the web saying the magnet problem only applies to old motors with magnets made of obsolete materials. I think that must be true, because if my grinder motor runs this well with the magnets destroyed, it must have been suitable to power a Cessna before I ruined it. I'm no EE, though, so I could be wrong.

Wiping the magnets only applies to servos and steppers which use alnico magnets (perhaps samarium cobalt as well). the ceramic magnets in your motors cannot wipe themselves as you pull the rotor out.

Some treadmill motors will burn up if run at their max load without a fan, the fan blades are usually on the flywheel, which is usually taken off the motor when re-purposing them.

As far as I can tell there is little practical difference between my baldor 90v 1750 rpm dc motors and my treadmill motors aside from the treadmill motor running at 2 to 5 times the rpms.

my baldor and daton 90v 1750 rpm motors do not have internal fans, and they are TEFC. so the rotor is cooled by radiation and air churning around internally. if i were to cut slots in the case and put a fan internally, i'm sure the hp could be doubled, even at the same rpm. this would be 4 times the copper losses in the rotor, which means the efficiency would drop significantly.


There appears to be little practical difference in the size or diameter of the commutators on my better designed treadmill motors. the cheap small one i have, runs at 6700 rpm, 130v ~2.5hp, the commutator is perhaps 1.2 inch diameter, brushes are half inch by .25 inches. -this one probably won't last long. it was thrown out because the 608 bearing locked up, probably due to heat from the commutator.

this link to a photo of a dayton 90v 1750 rpm 3/4th hp motor, the bearing at the commuator is a 6203. this motor could probably be run at twice the hp and rpm without much life reduction, my other baldor motors have nearly double the commutator diameter for the same rpm, no idea why.
http://johansense.com/bulk/dayton90v34hp.JPG
It has 2 coils per slot, and twice as many as normal commutator slots. there should be no sparks visible at the brushes. my guess is the two coils per slot dump out the leakage inductance into the resistance of the brush, so there are far fewer or no sparks at all.

I found the answer. Alnico was the first alloy used in magnets. It lost magnetism very easily. Even the shaking from use in machinery killed Alnico and pure iron magnets. Motors and magnetos using Alnico required "keeper" bars across the poles to prevent demagnetization.

These days, barium-ferrite ceramic is commonly used in cheap DC motors (like mine), and it's much harder to demagnetize. It takes a pounding, and it doesn't need a keeper. So if I need to machine the shaft in my latest scrounged treadmill motor, I won't have to worry that removing the armature from the motor will kill the magnets.

Rare earth magnets are better than barium-ferrite, but it looks like I won't need them to run my buffer!

Couple of sources:

Magnetic Healing: Advanced Techniques for the Application of Magnetic Forces - Buryl Payne - Google Books

http://www.rexresearch.com/monus/monus.htm#2

Thanks for the information, johansen. I didn't see your post when I replied to myself.

I think I got off track when I said my little grinder didn't blow a 15-amp fuse. The motor is rated at 9.1 amps, but that's DC, and I don't know the RPM level or corresponding DC voltage at which the amp draw was measured, so I guess I can't say the motor is okay because it doesn't blow the fuse. I mean, it IS okay, but the fuse doesn't prove it's not drawing abnormal amounts of juice.

Do you think it's useful to reduce the size of drive pulleys on treadmill motors in order to be able to run them at higher speeds and get more torque out of them? Seems like it should work.

Regarding the fan problem, when I made the pulley for my grinder, I machined vanes into it so it would move air just like the original pulley.

when permanent magnet dc motors are demagnetized, they will have higher, in some cases far higher no load rpms.

so, i have no idea what the no load current should be for your motor. do you have a variac? a bridge rectifier and a variac should drive any dc motor fairly smoothly.

torque is linear with amps, the 9.1 amps on the nameplate is the amps at full load, you can run the motor at whatever speed you want, but no load rpm is proportional to dc volts at the terminals.

9.1 amps at 130vdc is more like 1.58 hp theoretical, probably 1hp actual shaft hp.

amps squared is proportional to watts lost in the rotor.

so unless you provide more aggressive cooling then you can't simply overload the motor. also the magnets may not handle more than 50-100% overload. increasing the rpm also increases iron losses in the rotor, but for small motors i suspect that is not a significant issue.

As I understand it, speed in a DC motor is a function of applied voltage. If that is correct, then I don't know what the voltage is when I'm running my motor, because the average voltage provided by the PWM circuitry is variable.

I do have a variac. I never thought about using it to run a motor. Man, that would make setting these motors up easier. When I got my first MC-60 board, I blew it up because some electronics "experts" advised me to ground the board, which instantly destroyed it. I had to take it apart and put new rectifiers in it. It would be a whole lot easier to turn a knob on a variac hooked up to rectification and a pi filter.

I don't think the grinder motor is putting out 1.58 HP. It's only rated 1.0, peak.

Yes, speed varies with voltage for a permanent magnet motor. Can be different for a wound field motor, depending on how the field is connected. Is VERY different for a series DC motor.

The motor generates voltage when it runs, and current through the motor makes torque on the shaft. It runs at the speed which generated just enough that the difference between the generated voltage and the applied voltage allows exactly the amount of current to flow to maintain that speed.

If you remove load, the motor speeds up because it now has excess current and torque. At a higher speed, the voltages again balance to allow just the needed current to flow.

If you raise the voltage, the motor must run faste to generate enough to balance the new higher voltage.

If the magnet becomes weaker, the motor must turn faster to generate the same voltage, so the speeds increase.

The speed the motor runs when producing rated power at rated voltage is the "base speed".

The reason the treadmill motors are said to be over-rated in power is that folks try to run them slow. They are high speed, low torque motors, that must turn fast to produce rated power. they have a high "base speed". When people try to run them slow, they notice the low torque, and tend to equate that with an overall lack of power, but it is really just a misuse of the motor that is causing a problem..

Thanks for the info. Sounds like a small drive pulley would go a long way toward making the motor more useful.