commutation problem 2

[Neil1677]

Hi everyone

We have a problem with a DC motor driving an air compressor in the UK. Motor rating is 4.75 HP running at 550V DC. I've uploaded a video of it running.

Any thoughts as to the problem? The coils all check out OK, the comm has been skimmed, the brush pressures and brushes appear OK and the neutral point has been checked.....

any help appreciated

Link

 

[edison123]

Looks to me like wrong interpole polarity. Try moving all 4(?) brush leads to the next brush holder. i.e. Move brush holder 1 lead to brush holder 2, move brush holder 2 lead to brush holder 3 and so on and see if the sparking goes away.

Probably whoever did the skimming did not put back the brush leads in the original position.

Muthu

http://www.edison.co.in

 

[waross]

Won't that reverse the motor Muthu?
Should the field be reversed at the same time?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

If the interpole's polarity is wrong. It will help. And that is definitely what it looks like. It is a common mistake, also in "good" winder shops.

Gunnar Englund

http://www.gke.org

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

 

[waross]

Jumping the brush connections over to the next brush will reverse the armature. The inter-poles will now be correct, but the motor will rotate in the opposite direction.
Either reverse the inter-pole connections or change the brush connections and reverse the field connections also.
I hope that this is not a compound motor.
If this is an inter-pole issue, the sparking will get worse as the load is increased.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

If the connections got wrong at the shop, the rotation is probably wrong now. It will be OK if you correct the connections. And the series winding will automatically be right. Restore the connections to the original ones. That is usually the best and lest complicated thing to do.

There is also the possibility that the bridge has been moved. Put it back on the mark. If there is no mark - connect an AC voltage (mains should work) to the excitation winding and adjust for minimum armature AC voltage. Look for a few volts and adjust to minimum. Put a mark on bridge and bridge holder.

Some motors then need to be adjusted for minimum arcing when loaded. But not all. If you got one of those then make the neutral mark white and the "running/loaded" mark green.

Gunnar Englund

http://www.gke.org

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

 

[edison123]

Bill
Yes, shifting the leads to the next brush holder will reverse the rotation, which then can be corrected by reversing either the field supply or the armature supply but not both.

Muthu

http://www.edison.co.in

 

[waross]

I once had a problem with similar symptoms.
The arcing only started once the unit was running and hot.
Heating the armature in an oven did not show any issues.
We stopped the generator and used Gunnar's method after the unit was running and the arcing had started:

If there is no mark - connect an AC voltage (mains should work) to the excitation winding and adjust for minimum armature AC voltage.

The problem was in the armature but the differential expansion of the core and the windings was apparently different when the unit was heated in an oven as compared to being heated by the field current when in service.
With AC applied to the field, the voltage across the brushes should be zero or close to zero, and stable.
A short in an armature winding will show up as a varying voltage on the brushes when the armature is rotated.
Speed of rotation is unimportant. Only one rotation is required to show a fault.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dArsonval]

I've been sitting on the fence with this thread waiting for the Op to return.

It seems that may be un-likely, "but" one never knows.

Initially, "Brush Entry" (the connections made to the brush holder) is suspect when a reconditioned Direct Current Motor
is placed back into service exhibiting brush sparking.

In a miss-connection situation, the arching is typically more pronounced then exhibited in the video.

When I first viewed the You Tube video link a while back, I was curious about the
compressor noise being in unison with the arcing.

I'm looking more carefully at the You Tube video and note an obvious "tell tale"
sign of a commutator that was not turned (skimmed) parallel and concentric.

The commutator is not round.

The brushes are hopping in unison with the [out-of-roundness].

Look carefully at the screen shot at the beginning of the video.

Take note how the commutator bar that never saw the edge of a cutting tool.
It is still "burned" at its edge. And look carefully how the chocolate color of the commutator in that place is missing...
because the brushes are not making contact in that portion of the comm.

Here's my guess.

The commutator was skim cut out-of-center from the armature's shaft.
That would certainly cause brush sparking.

John

 

[flexoprinting]

Yes yes!,I agree with that, My first thoughts were based on my experience with an identical old motor we had, the brushes appear low to me and I think with age those clock springs weaken causing bounce although the OP did say brush pressures were ok. Motor is very clean and appears to have been rebuilt justifying Gunnar and Bill's explanation of the problem. Truely a great tip for tuning of bridge position.

You folks on that side of the pond probably get better vacations than us in the US so maybe the OP will tune back in.

Chuck