Wound Rotor Secondary Current vs Brush wear 1

[Steamintrains]

I have a couple of 6,000 HP wound rotor motors that are using a lot of brushes. Besides the cost and maintenance of the brushes, the resulting carbon dust is killing my motors. I have had several failures of these motors over the past ten years or so. We also have to pull the motors and do full PM's on them on a regular basis to mitigate the carbon from the windings
We did hook up a CT with a recorder and got about 600-700 Amps on start and 50-60 running. Meanwhile we have 11 brushes per phase and I believe each one is rated about 200 amps. We have started to remove some of the brushes but there has to be a reason why I have so many brushes. Brush life is only a few months.

Why do the motor manufacturers provide the motors with such a high brush load? I have motor from two different manufacturers and both have about the same amount of brushes..

any thoughts?

 

[eeprom]

From my experience, that's a fairly normal brush load. There's a lot of reasons you may be burning through brushes. If you have the wrong brush type (too soft), or the brush holder spring is not set correctly, or if the commutator ring is not smooth, brushes are not being seated correctly, changing too many brushes at a time...etc. The way the brushes are failing will tell you a lot, and they way the brushes look when running will also tell you a lot.

I had a similar problem on a wound round some years ago. I worked with Helwig and they recommended a different brush which lasted much longer.

Attached is a guide from Helwig. This might help. They have a lot more troubleshooting guides on their website. It would also be helpful to send a used brush into Helwig for review.

EE

 

[LionelHutz]

What is the rated secondary current of the motor? I find 50-60A running very odd unless you mean current per brush which is around 600A total secondary current and that still seems low for a motor that size. Also, having 10X the current on start-up seems odd. Typical is no more than 1.5-2X rated current. What is the starting method?

 

[Steamintrains]

I believe the secondary is rated at 1,200 amps
I out a CT on one phase of the buss connections feeding the Liquid starter (there is only one per phase)
Starting method is a Liquid Starter

 

[panter]

Steamintrains,
when the motor with wound rotor is working secondary or rotor current is almost DC and frequency is in Hz parts so it can not be measured by traditional AC instruments .That is why I think the results of measuring currents are fake .
Usually such a big motor have a device that upon completion of the start raise the brush and make a short outputs of winding rotor but your motor unfortunately do not have it .
Good luck !

 

[jraef]

The usual reason for having so many brushes is so that you do not need to shut down the motor for brush maintenance and replacement. This is especially true on continuously running machines, like gas compressors. You have more brushes than are needed so when you remove some there are still plenty left to carry the load.

What happens sometimes is that this information gets lost as personnel changes and new people are never properly instructed on that issue. Then if the operation changes where the motor is no longer running continuously, the new crew changes ALL of the brushes at the same time, then let's them get too low before changing again because of the cost. Waiting too long befor changing can accelerate the brush loss toward the end, which results in extreme carbon release.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[ScottyUK]

On-line brush changes on a big turbo-alternator are nerve-wracking to say the least when you have a 6 MW rectfier feeding the field. The system is floating so 'in theory' it is safe to do the change, provided nothing goes wrong. These days I'm not sure whether I would entertain doing this job with the machine running.