Testing a pm dc motor

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That's the problem trying to measure motors. If it's ok it's always going t o read low ohms, but maybe it should read 3.2ohms (eg) and has shorted turn s (eg), how would you ever know? If on the other hand it measured K ohms it would be faulty. An insulation test from windings to armature would be mor e appropriate. If that's ok give it a spin (I mean apply power).

Paul Conners prodded the keyboard with:

If it isn't turning, then check that it has actually got voltage applied to it. You have to consider both the motor and controller !

Can you spin the shaft and see how much voltage it generates?

Ohm it to ground, too.

Perhaps if we knew in what way the motor was acting up we could better suggest a course of diagnostics. Like does it make any noise, does it turn slowly or not at all, is it popping the breaker, etc.

The faq,

I think Sam did write something about checking CD player motors, and I'm sure he gave tips on figuring out whether it was good or bad.

Michael

Not likely that his info is pertinent for 180 volt motors of that size, when his info is for miniature DC motors.

The motor types likely differ as well.

That's the problem trying to measure motors. If it's ok it's always going to read low ohms, but maybe it should read 3.2ohms (eg) and has shorted turns (eg), how would you ever know? If on the other hand it measured K ohms it would be faulty. An insulation test from windings to armature would be more appropriate. If that's ok give it a spin (I mean apply power).

The resistance sounds too low. Can you rotate the armature and re measure at a different point?

A 180 volt DC 1 hp motor should require around 5 amps or less at full load. I would expect somewhere around 40-50 ohms when running under full load.

But as others have said, you want to check from the armature to frame ground. Any resistance might be bad.

Regards

those kind of motors tend to have mechanical issues..

Bearings is common which causes a tight turning shaft and may cause the armature to move around on the brushes.

Also, a bearing could be completely blown out and the motor will still make attempts to turn if the brushes are still making contact.

Another issue is the PM's some times fall off, lose their bond to the outer shell and come in contact with the rotor.

Then you get the occasion of weak magnets due from too many hours of running hot, which causes more Arm current and lack of torque but with higher RPM abilities.

Now and then, the armature windings may short to each other but not to ground. Most of the time megger meters will not reveal this defect but usually signs of over heated wire is a good bet.

Also, if you manually spin the motor it can act as a generator..

Disconnect the leads to the motor and see if it turns freely, if so then you most likely do not have mechanical issue.. Now connect the ARM leads together from the motor and see if you get heavy drag (Dynamic Braking) due to energy being generated and short via the A1 and A2 leads. If you do get a got drag on it, chances are it's of.

When checking the brushes via the DMM, you need to turn the motor very slowly so that you can account for all the armature bars, you may have a set of scorched brushes.

Jamie

Motor out of the equipment. Turning armature slowly, the resistance reading between the armature leads varies from a low of 2.3 to high of 10; most are around 6-7 ohms. Resistance of each lead to motor frame is minimum 150K up to over 1 meg.

What's normal for such a motor?

Thanks.

Open it up and see if it has discolored windings. Does it smell burned? It would seem to me that the resistance readings indicate compromised winding insulation.

Now you can wind a bit of string round the shaft and pull it to see if it generates any DC output! Of course if there's another motor you can link it to with a bit of hose or similar (as a flexible coupling; no water!), so much the better.

Mike.

Will check *for* DC output...

Will check or DC output.

Ah, now that you mention it, this equipment experienced a flood which probably submerged the motor fully for 48 hours (late last year). Not operated when submerged (!) but probably some moisture still in it when first operated after the flood. (2 other equipment suffered damage: a VFD and a contactor burnt out.)

Thanks.

That is something that should have been noted up front. You have a bad motor.

Something killed the motor but didn't affect the measurements discussed here? What could that be?

Something killed the motor but didn't affect the measurements discussed here? What could that be?