2000HP DC Motor Flashover 1

[Aaron88]

Hi Guys,

I've recently replaced two DC Drives (ABB DCS800) driving a single DC motor. The plan is to upgrade the motor to a bigger one to cater for production increase - however it hasn't yet arrived so it's only the new drives installed at present.

The current motor is an old GE 2000 HP machine rated at 750V, 2300A. Historically there have been problems with this motor regarding flashovers and the motor rewinder tells me the commutator is out of round (don't know how much or if it's still within spec). Based on this history the motor had been derated to 2000A and they were still experiencing infrequent flashovers. Whilst replacing the 2 DC Drives my boss (the senior EE at the time) recommended I limit current to 1840A, which i did.

Now 5 months down the track: Another minor flash over. Again the "solution" is to lower the current to 1750A. I am somewhat sceptical of this - however maintenance is persitant that their routine inspection & replacement of the commutator and brushes is adequate.

Is Flashover caused predominantly by overcurrent? I was always under the impression that Flashover was predominantly a problem due to Carbon buildup or broken brushes, however my experience with large DC machines is quite limited and I've never had to deal with flashover before.

Any advice would be much appreciated.

 

[waross]

I understood that excess voltage caused flash over. High current causes higher voltage.
BUT,
How about scoping the applied voltage to see if the drive is putting out any high voltage transients.
I would investigate the possibility of limiting the voltage rather than the current. That may inherently impose a current limit.
Another thought. Flashovers that I have seen were not to the brush holders but from segment to segment around the commutator. We called it "Ringing the com."
Have you changed brush grades prior to issues developing?
Is there any dust in the atmosphere? Do the brushes tend to stick if not serviced on schedule? Do you check all brushes for freedom of movement at each service interval? How often? In a very dusty environment we cleaned the dust out of the brush holders weekly. If the cleaning was skipped the brushes started sparking in about 10 to 12 days.
Does your service team sometimes do the paper work and then not have enough time to do the actual service?

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

 

[jowang]

Just see the pic and i strong suggest to re-surfae your commutator, flashover may come from the out of roundness.

 

[Aaron88]

See attached graph for current:
This is based on 1 hour average over the past 9 months.
Point 1 was the shutdown during which the 2 drives were replaced from DCS500 to DCS800
Point 2 was when the flashover had occured.

The interesting thing is though if I arrange the current from highest to lowest the top 10 is all in this September (when the flashover occured) so perhaps there is some merit in the overcurrent theory.

waross:
Regarding the voltage i haven't look at this yet. We do record the drive voltage so I'll be able to generate a similiar graph as below for the current. However the sample time is quite slow (5s).

Brush grade has not been changed to my knowledge (I am relying on information of others as I have only been on the site for a year). The atmosphere isn't very dusty (it is fitted on a sand dredge in a freshwater pond). However during an inspection today i rubbed my hands inside the air ducts and found them to be a little bit dirty (ie some amount of dirt rubbed off on my hand). The brushes are monitored and replaced on a 3 monthly basis.

jowang:
Agreed, however as a 2MW upgrade motor is currently being procured I will not be able to justify the expense.

 

[mikekilroy]

just some more thoughts on why flashover occurs, or what the limits are: volts, amps, and power. All 3 limit the commutation capability.

there is a max amp limit of course you are familiar with - more produces more sparks.

there is a max voltage limit that is based upon the maximum bar-bar voltage that can be switched with that size commutator before excess sparking. typically around 70v IIRC.

then there is a power limit. Dc motors have a peak current rating (mostly based on commutation capability - see above), a max voltage rating (mostly based on commutation capability - see above), and a max power (HP) curve between the two (purely based on commutation capability).

I assume at your current levels your drive is SCR based not IGBT based: If it was IGBT based, the max bar-bar voltage can easily be exceeded with the dc bus switching causing 2-2.5x higher voltage at the pwm freq at the commutator, and these spikes if exceeding the motor max volt limit will cause excessive sparking.

Lastly, is there any chance your brush ring is out of phase? I dont have experience on that large a motor, but some larger ones I have seen had adjustable Brush holders to adjust for min spark by aligning them properly?

 

[rhatcher]

The picture does appear to show that the commutator is out of round. This may cause flashovers. Other causes for flashover are:

- improper brush grade
- improper brush tension
- improper brush neutral setting *
- shorted interpole(s) *
- shorted shunt field pole(s) *

Improper brush neutral setting, shorted interpole(s), and shorted shunt field pole(s) are a common source of flashovers.

I assume that you have brush arcing all of the time. Do all of the brush arms show arcing or is it only one?

 

[Skogsgurra]

You haven't mentioned what application it is. If this motor is actually braking (running as a generator), there are quite a few possibilities that a flash-over occurs.

Gunnar Englund

http://www.gke.org

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

 

[ScottyUK]

There are a number of images and some suggestions on possible causes of problems with brushgear in this document:

http://www.mersen.com/uploads/tx_mersen/514_1-brushes-for-electrical-machines-en.pdf.

You can judge the actual shades and any variation in surface wear or colour better for yourself looking at the machine than we can from a picture.

 

[ScottyUK]

A better link:

http://www.mersen.com/uploads/tx_mersen/5-carbon-brush-technical-guide-mersen.pdf

- see chapter 5. Oh I wish we had an edit function...

 

[Aaron88]

I think I've found the answer to my problems:

The original General Electric GP200 motor comes with angled brushes. Many years ago somebody decided it was too hard to change brushes with this configuration and perpendicular brush holders were installed. The angled brushes had a much bigger surface area on the comm compared to the straight brushes.

Unfortunately there is no great engineering solution to this since the motor will simply have to last until February 2013.

Thanks everyone for your responses.

Skogsgurra - the motor drives a Warman centrifugal gravel pump for a sand dredge. Being a pump it only spins in one direction (unless removing impellor for maintenance - field reversal & very slow). Since it's part of a mining operation it operates almost continously at near full load - only being stopped once or twice a week for 2-4 hours of plant maintenance excluding major shutdowns and breakdowns.
ScottyUK - that's a very guide for commutator wear.

 

[Skogsgurra]

Good feedback, Aaron.
A pump that works the way you describe it is not prone to flash-over due to regen and mains dips. So, forget about that. The angled vs straight brush is a very probable cause for the sparking problem. Thinking a little bit further, it is possible that the change from angled to straight actually did offset the neutral position and that you can get a few more hours out of a set of brushes if you adjust the bridge position (shall not be on its mark any more).

Gunnar Englund

http://www.gke.org

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

 

[djs]

Another element is that the angled brushes have more surface area in contact with the communtator. So fewer bars are in contact with the brush, this is bad news.