broken edges of carbon brushes 6

[fiaz1404]

we have four 3300kw, 13.8kv siemens slipring motors in a cement plant, in previous maintenance shutt down one motor was dismantled due to low stator resistance. we take out rotor from stator for stator cleaning, after cleaning motor was assembled and put the motor in service.
after 15 days we took stoppage for inspection and we found that ring#1 red high lighted brushes(see attached image)are broken from edges and their leads are also melted. please see attached file. now repeatedly edges are broken and leads are melting of the same carbon brushes on the ring#1.

i think there is no problem with grade as same brushes are being used for other motors and those are normal.
Brush spring tension has been verified and the springs are constant pressure springs.
slip ring surface is smooth.
no any sparking on these brushes is observed.

please some body can advice me on this issue.

regards,

ahmed

 

[fiaz1404]

please see attached image, where red high lighted brushes are being broken frequently.

 

[waross]

Check the wiring to the brushes. You may have crossed the wires to the brushes.

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

 

[Compositepro]

The burned wires look like the damage was done by corona discharge or fretting contact with something at a different voltage. Is there something too near these wires that is at a high difference in voltage?

 

[fiaz1404]

waross
i have checked the wiring, it is ok.

 

[MikeHalloran]

I can't explain the burned wires, but that brush looks (mechanically) chipped. As if, e.g. the rotor were reinstalled with the various washers and shims and bearing springs and stuff in the wrong location, so that the brush is hit by something when the rotor spins under power.



Mike Halloran
Pembroke Pines, FL, USA

 

[panter]

Fiaz,
I agree with Mike that brushes like mechanically schiped but you should scheck resistor, all contactors and their contact expecialy if part of resistor stay in function after starting of motor.Maybe there is problem. .
Good luck

 

[LionelHutz]

Rotor moving back and forth in the stator as it shifts from it's mechanical alignment to it's electrical aligment when the motor is started catching the brushes on the edge of the slip ring and causing the brushes to get chipped? Compare the left to right location of the brushes on the slip ring with the locations of the brushes on the other phase slip rings. Also check all the pieces that hold the rotor in position as already suggested.

 

[xaviermlynek73]

Ahmed,

Your brush wear doesn't look normal and it is certainly due to a mechanical problem. There is no burn marks on the edges.

The mechanical problem might have also caused the cable ripping. I am working for this brush manufacturer. If you need, I can have someone visit your plant.

Let me know if we can help.

Xavier Mlynek

 

[RoyDoug]

Ahmed,
Looks to me like you have two problems, the mounting of the brush holders is not correctly set up. The brushes need to be radial to the commutator. If they run slightly reaction or stubbing (into the toe) vibration and chattering will occur with subsequent breakage. Second problem the brush shunts are being frayed and cut by the Tensator spring, using a suitable insulation on the shunts will negate this. Regards RoyDoug

 

[waross]

Welcome to ENG-TIPS RoyDoug.

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