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Trying to understand why a DC motor feels this way.

Trying to understand why a DC motor feels this way.

Trying to understand why a DC motor feels this way.


This motor is ill. It's 4 years old and lives in spraying coolant in a large CNC machining center grinding ceramics.. It's nasty location is because it runs a large rotary table on the deck.

It recently quit. I'm trying to understand why. It's 90VDC with only an armature as the stator has only permanent magnets. The brushes look fine, the commutator looks fine as I can't even see a ridge on it. The gaps in the commutator bars are deep so the bars don't look like they've worn down to the gap filler, probably an 1/8" at least.

Meggers infinite at 250V.

Measures about 15Ω at the two motor terminals.

I dragged out my HP 40V 50A supply and tried ponying it up. It turned about 90° and stopped at 35V 50A.

Now here's what's puzzling to me: If (unconnected) I try to turn the shaft it turns with perhaps a little more effort than I might expect but after going perhaps 180 degrees it will suddenly bang to a stop. It feels like it hits a brick wall or something. If you back it off and try again it might hit the stop again or it might not and might turn another random-seeming partial revolution then bang to another stop. There may be a few mini-stops in a full revolution too where it stops but then relents and passes thru the stop to somewhere random before it bangs to another hard stop. None of these stops may actually be random but I've not yet eked out any pattern.

Can anyone explain what might be causing this motion?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Trying to understand why a DC motor feels this way.

PM motors CAN have external parts that got into them. Or some other loose part that feel the attraction from the magnets. If they get into the air gap, it usually means a new motor. But they mostly stay somewhere else, but always where the magnetic field is intense. The stray field is highest at stator ends and near the air gap, so that is where they like to sit. And they are influenced by the rotor flux so they change position slightly. Sometimes serving as a brick wall. And sometimes not.

I guess that is what happened.

If you try to get rid of the debris, do not open the motor if you can't guarantee total freedom of any magnetic particles or even dust. Some manufacturers recommend a clean-room environment. Not the super ones you are used to. But still. Not the ordinary work-bench. That's for sure.

Also, the PM magnet sometimes demagnetize when you remove the rotor. Read the service manual carefully before opening the machine or doing ANYTHING like that. Or you will have a motor with higher speed and lower torque.

Gunnar Englund
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Trying to understand why a DC motor feels this way.

A guess:

One of the permanent magnets has worked loose.

Another guess:

A portion of the armature winding has come out of its slots.

There should not be much concern in dismantling this motor to inspect
what's going on. Obviously [match mark] the end brackets in relation
to the permanent magnet field before prying it open.

It looks like a commonly available D.C. motor.


RE: Trying to understand why a DC motor feels this way.

Bad bearings?


RE: Trying to understand why a DC motor feels this way.

I agree with John, the glue on one of the magnets came loose. It's toast Keith.


RE: Trying to understand why a DC motor feels this way.

I have seen similar action with a bad bearing. The race broke and the balls would bunch up and jam. Reversing the rotation would free the balls, but with continued rotation they would quickly jam again.

"Why not the best?"
Jimmy Carter

RE: Trying to understand why a DC motor feels this way.

I have also Bill, odds are the failure would be on load side of motor and that would be felt with the wiggle meter on shaft. I found the dang magnet would float on top of rotor when free and catch at various points with each turn. I have tried to re-epoxy one once but the opposing forces were smarter than me.


RE: Trying to understand why a DC motor feels this way.

Thanks for the suggestions! They all sound very plausible. I was just going to return it with a 'beats-me' response but you've all got me curious so I have to see what it is.

One other thing:
There is a lot of loose corrosion inside. Out of a brush access I poured about a heaping tablespoon of what looked like designer salt. It was cubic crystals (0.04") that were off white with about 30% institutional green ones mixed in. The motor casing has aluminum end-bells and the cylinder is steel. (Great for wet environments!)

Keith Cress
kcress - http://www.flaminsystems.com

RE: Trying to understand why a DC motor feels this way.

Wet environment calls for a truly sealed motor, this one isn't.
And the full Stainless version would likely help also.
But the failure, it has to be either a magnet loose or a stator slot fill strip that has dropped out.
Of course depending on the type of magnets one could have broken...

I used to build PM rotors, Lots of fun.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

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