stange brush problem 1

[bigheadted]

I have a problem at work that was wondering if anyone else has encounters before. The 400A DC motors that we use at work seem to consistantly blow the tags off the brushes at the point where they enter the brush. This occurs far to early in he brushes life for to be normal and the manuafacturers of the brush say they are correct for the motor.
The Connecting tags then arc out on the motor frame with obvious consequences any ideas?

 

[aolalde]

I assume that you name "tags" the connection of the shunt wires to the carbon brush.
If that is the case, the problem is "manufacturer quality" of the connection point. Some manufacturers have better electric connection procedures than others. Poor connection means high resistance and for high current (>100 amps) the heat generated destroys the joint, triggering a potential catastrophic failure. We have almost eliminated that problem replacing those problematic brushes with equivalente grades manufactured by “National". Make emphasis to the technical advisor on the connection problem for the shunt wires.

 

[UKpete]

I agree with aolalde. I spent a number of years working on brush trials on similarly rated motors (traction) and never saw that problem. The brush makes were generally either National or Morganite. Brushes often showed signs of over-loading, it usually caused the pigtails/flexibles/shunt wires (whatever you want to call them) to discolour, and occasionally burn through, usually part way along but never at the tamp - the point where it is connected to the brush. Even if there is a brush vibration problem it is unlikely to fatigue at that point if it is properly designed. Brush vibration can be spotted by looking for wear marks on the side of the brush where it slides in the brushbox.

I think you ought to approach another brush manufacturer.

 

[mc5w]

In one plant where I worked a 3 year old DC 4 pole motors had both of the brushes hop out of one of the brush holders. I discovered this while cleaning dust out of the motor for the first time after I was hired. I put the brushes back in and wrote out a work order to change out the motor on the basis that "1/2" of the rotor was not carrying any electricity and the motor had been in slow cook mode. About 1 or 2 months later this motor burned up.

Theoretically, a 4-pole 4-brushholder motor should have an individual motor overload relay for each brushholder. If 1 brushholder fails the other brushholder(s) will register excessive current.

The application for this motor was for an elevator at the end of a paperboard corrugating line. There was no excessive vibration or other problem to speak of. Could have been sabotage given that later on I was falsely accused of taking a bath or shower once a week and of changing my underwear once a week. Amazing how much people can malfunction while under the influence of greed.

 

[mc5w]

I forgot to add that copper oxidation where the brush shunts connect to the busbar can cause uneven current distribution between brushes that are in parallel. You should use #240 garnet abrasive paperfor cleaning the connections before putting them together, not silicon carbide paper which I use for cleaning building wire because SiC paper tends to shed semiconducting particles. Aluminum oxide sandpaper is not good for cleaning electrical connections. You should then use electrical grease such as Ilsco Deox or Anderson Versaseal to put together the connection.

I have encountered quite a few instances of a 120 volt outddor light fixture that has stopped working and all I have to do is to take apart the wire connections, get rid of the copper oxide, and put it back together. There is a reason why there are 2 antioxidant compounds of the market that have copper particles in them.

 

[motorsdirect]

Hi Bigheadted
Maybe the brush holder itself isnt carrying enough current, and all the current to the brush is therefore being carried by the tail...thus overheating the cable.
just a thought..
Jeff

http://www.motors-direct.co.uk

 

[oftenlost]

I have noted that in paper mill applications, particularly on the wet end of the machine or on fan pumps that the commutator develops a sticky film which creates brush chatter. Typical symptoms are: brush pigtails start to fray, brush sides show markings of movement on the box sides and in wafer brushes, the brush opposite rotation tends to wear against the adjacent brush and wears in a trapezoidal shape with the bottom being smaller than the top. Clean the commutator with a canvas rag or use a sligthly abrasive brush or call your brush manufacturer as there are brushes specifically designed for chemically altered, sticky films. I have seen similar effects in textile dying operations.

 

[TMD]

A high over-current will tend to blow away the weak connections. This was demonstarted to me during fault testing with dramatic results. But the brushes in this test, and many other we've done, never resulted in brush/shunt falure. We buy copper graphite brushes from Helwig Carbon.

 

[GopelT]

Have you checked brush current of each brush and verified amps per inch squared? Depending on brush grade this should fall between 40 and 100 max. Are all brushes in a rig even with regards to amps?