Reading Armature Current with a Clamp On Amp Meter

[BearB]

I have read Armature Current with a Clamp On Amp Meter many times on AC and DC Motors. However on DC Motors I have generally read a lead to see if it looked normal. Recently I had a 7.5hp DC Motor that seemed hot when I shot it with a Pyro, 167 F. I then checked the Armature leads at the Drive with a DC Clamp On Amp Meter, Results
A1=.3A, A2=3.3A, both leads together in the Clamp=1.6A.
Can anyone explain to me the readings? I do not understand how this readings separately are so different than when read together.

 

[roydm]

Did you realy mean 0.3 for A1 current?
I expect your Hall effect meter is effected by the nearby field poles, but just to be sure check for a ground fault on the armature.
Try flipping your meter over so the wire goes through the other way, you might find the +/- are different.
Roy

 

[BearB]

OK I will try flipping it over. Yes .3A
The only leads at the bottom of the Drive were the Armature leads, no other wiring close.

 

[Skogsgurra]

Excuse me - an impolite question, perhaps.

It couldn't be one of the excitation wires you measure on?

.3 A seems to be in the right ballpark.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[roydm]

As you know Hall effect clipons work by measuring the magnetic field around wire, I was wondering if it was efected by the strong magnetic field from nearby field pole.
I think Skogs is onto something.

Roy

 

[BearB]

I flip the meter and read the armature leads again and did not see a difference. Perhaps the the magnetic field disturbance is the answer. It helps to discuss a strange reading. Thanks

 

[LionelHutz]

I would check for a ground fault. I doubt it's the magnetic field if you've got the meter in about the same location while checking both wires.

 

[BearB]

Follow up to this problem. It has been found the Comm on the motor is very bad shape and we are going to use it until it fails or replace it on the next Shutdown.

 

[electricpete]

Thanks for the feedback. So, what do we conclude?

The sparking at the brushes is causing very large current to ground?

Is there any ground protection or indication associated with this motor or supply?

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[electricpete]

I guess the sparking also can cause very erratic waveform. Makes processing of the signal important (is it reporting a peak or rms).

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[edison123]

BearB

By bad commutator, do you mean mechanically bad (grooves, worn-out etc.) or electrically bad (Ground fault though it could be the winding too) ?

Also, you cannot just replace the commutator alone. It is either a rewind with new comm or a new rotor altogether - both of them have a considerable lead time.

 

[BearB]

Motor will be changed out, the Comm. is sparking and badly worn. There is a Ground Fault connected with the DC-300 Drive but it is not indicating a problem.