DC Motor Equalizer Winding Repair Possibility

[Hesab2]

Dear All
Recently , during periodic maintenance we observe two burnt spots on edge of the commutator bars (near the risers). so we worried and perform the electrical
tests and find out the results as below : (Motor spec: DC Motor 930 kw - 1500 RPM - 800V - 1227A)
1 - Insulation Test : V(test)=1000v - result : IR(15 sec ) = 4.74 G ohm and IR(1 min )= 12.7 G ohm
2 - Measurement of Resistance = 1.1 m Ohm
3 - Measurement of Inductance = 6.6 micro H
4 - Surge Test : V(test) = 2000 - result = OK

some people declared that "Equalizer winding of the rotor " is damaged and rotor must be rewind completely and the others proposed that repairing of the damaged spots is possible and it is not necessary to rewind complete rotor . it is notable that the commutator is not V ring type and spare rotor/motor is not available .photos is attached

What's your Proposal ?
thank you for your kind ideas .

 

[dArsonval]

The commutator appears to have a normal wear on its surface. Nothing in your original post indicates there was/is a running problem with the motor.
Nicks, and burn marks show up all over pieces of equipment during their operating life span. Based on the photo alone, the motor looks fine.

One test not mentioned, and quite important is a Bar-to-Bar test.
Perform a Bar-to-Bar test on the armature. If a consistent pattern (what ever it may be) is not observed around the entire commutator, rewind the armature.

John

 

[roydm]

I may be wrong but for me it looks as though the arcing is restricted to the wire termination area, if it was a winding fault I would expect to see evidence across the commutator from the brushes arcing.
I wonder if it's possible to clean out the slot between and re-insulate it.
Maybe the solder needs re-freshing.

 

[Hesab2]

Dear John
1- Motor was in operating condition and working fine till we find out the burnt spots during periodic maintenance .
2- the question is "is it possible to repair the damaged spot without rewinding the entire rotor ?"
3- is it possible to run the motor at the current condition ?
4- what is the root cause of the problem?

 

[dArsonval]

If the motor was operating “perfectly fine” before it was taken out of service, there’s no reason
to believe it would not work after returning it to duty.

As roydm suggests, “…if it was a winding fault I would expect to see evidence across the commutator from the brushes arcing…”

The commutator is likely tig welded so “resoldering” would not be a remedy.

The marks look cosmetic to me. Puffs of brush arching smoke and dirt adhered to the insulation surface.

Have you tried rubbing, or cleaning them off with a rag?

Not discounting that there could be an electrical issue with the winding, “but” a bar-to-bar test would
provide valuable information in making a conclusion or decision.

John

 

[waross]

Is it possible that someone dropped a wire or tool onto the commutator when the motor was running?, possibly while dressing the brushes?
Try cleaning the spots with some fine sandpaper to see if this is only surface damage.
It is hard to say definitely but that strikes me as the result of a one-time flash rather than an ongoing condition.
I see similar damage when someone drops a hot wire on a grounded surface or grounds out a hot terminal with a metal tool.


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

 

[waross]

A brush neutral position test is quite easy and if the rotor is turned during the test the stator will act as an "in place" growler.
Apply an AC voltage to the stator winding. I have used 120 Volts, 60 Hz with excellent results on an exciter.
Generally the same AC voltage as the normal DC working voltage will be acceptable.
Measure the voltage across the brushes. It should be zero or very low.
When checking the brush position the brush gear is moved until the voltage is minimum.
That is done with the rotor stationary.
However, for a rotor winding test, rotate the rotor while watching the voltage across the brushes.
The voltage should be steady. There may be a slight variation as the brushes pass from segment to segment but any variation should be equal for a complete rotation of the rotor.
Any unequal windings will show up as an increased voltage at certain angles of rotation.
This test reacts to similar conditions that are detected with a growler test, without the need to dismantle the machine.
It is also useful in detecting faults that appear when the machine is hot under load, but disappear as the machine cools.
If a machine passes this test it will generally pass a bar to bar test. If the machine fails this test then by all means proceed with the bar to bar test.
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[edison123]

How did some people decide the equalizer winding is faulty? Was there any sparking during load? Majority of the equalizer windings are buried under the armature winding either on the commutator side or on the opp-commutator side. It is very rare that the equalizer winding alone can be repaired.

Do a bar to bar resistance test. If the resistance values change by more than 10%, then you have a problem in the armature and/or equalizer winding.

Muthu

http://www.edison.co.in

 

[edison123]

An equalizer winding that can be repaired without removing armature winding.

Muthu

http://www.edison.co.in

 

[edison123]

Sorry, it turned out so 'bigly'. Don't know how to resize it at ET.

Muthu

http://www.edison.co.in

 

[Hesab2]

Thank you in advance for comments .
we will do bar to bar resistance test and check the results , so if we find out the acceptable result , is there any suggestion for make a safer operation ?
again : is it possible to repair the damaged spot without rewinding the entire rotor ?

 

[Hesab2]

Dear All
we change the repair shop contractor . he check the rotor completely and perform bar to bar test . it was ok ( 0.101 to 0.098 ohm )
he believe that the burnt spot is related to moisture which has been inserted to space between bars and caused to damage the insulation bandage .
so he want to check the bars visually , hence he removed the burnt insulation material . it was ok .
he said to us the insulation must be repair again and also insulation bandage .