Testing Small DC Motors

[RichT]

I need to learn how to test small shunt wound DC motors in the field. The nature of the test is to determine if a motor has failed without disassembling it. These motors are generally 3 HP or smaller and are either 180vdc or 90vdc. Is this possible to do with a typical ohm meter?

 

[bmodrow]

It is possible to use a meter to check for continuity of the field circuit only. It would also be advisable to megger the field and armature windings to ensure there is no insulation break down. Also inspect brushes and commutator.

Keep in mind that if you receive a low megohm reading on the armature circuit and you are reading through the brush box, that you may have a problem with the armature, interpole circuit or brush holders themselves. Further isolation would be needed to confirm what is causing the low meg value.

A quick way to check the armature itself would be to remove the brushes and apply power to the field. The armature should rotate freely with no drag. Drag would tend to indicate a shorted aramture winding.

 

[RichT]

Thanks for the info - very helpful to me.

 

[GusD]

Hi Rich T

Follow Bmodrow's advice for tsting the motor.
You also have to look at Protective Devices(fuses or others)for indication of problem .If fuses blow or breakers trip,most likely you have a motor problem.
I would not exceed 500 volts on the Megger test.


GusD

 

[AeroPD805]

Rich,
Another quick check [and common fail mode] is to make sure
the brushes are long enough, [not worn out]. Although, if
the brushes are badly worn there would be a noticeable drop
in typical run current due to poor [or no] contact with the
commutator.
Pete

 

[jbartos]

Suggestion: The more advanced testing tools there are, the better results and more deficiencies can be observed.
Ohmeter, Megger, Voltmeter, and Ameter are essential.
Oscilloscope, Insulation Analyzer, etc. would be more on the advanced side and reveal different defficiencies.