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DC Motor - Sparking

08/06/2017 7:16 AM

We have one Seperately Excited DC Machine of 60 KW, 440 Volt Armature Voltage, 220 Volt Field Voltage, RPM 1500.

we have observed sparking beneath the Brushes, we have done Neutral setting by Centre Zero Galv Meter, removed all carbon from the Motor also done undercutting of commutator segments manually, winding resistance and insulation resistance of all Coils of Main Pole, Interpole are ok, even the armature resistance and Insulation resistance is alo ok.

Unfortunately the above efforts has not reduced the Sparking..

Please let me know, what is to be checked

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#1

Re: DC Motor - Sparking

08/06/2017 9:51 AM
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#10
In reply to #1

Re: DC Motor - Sparking

08/07/2017 10:58 AM

Hmmmm

One thing that article seems to have OMITTED.....

[Having once-upon-a-time been "blessed" with this discovery...]

When I worked in GE's DC Motors & Generators Department {30+ years ago!}, I learned a "thing-or-two". But, FAR MORE was learned when I subsequently spent a few years "working the Walk-in-Counter", and performing outside Service Calls, for a "Rewind Shop".

Improperly-trained maintenance personnel are entirely capable of "finding" and installing new brushes that are of the wrong composition (e.g., insufficient copper content). IF this was done concurrently with a "sanding" of the commutator, and "dragging-out" of the slots (more-often-than-not also accompanied by improper, or totally omitted SEATING of those 'new' brushes)... well...

...the sparking might seem "tolerable" when the unit is first returned to operation (depending upon its loading), but, will get real intolerable quickly...(!)

Not suggesting that THAT could be the case, here, but, anything is possible. And, I am surprised WHENEVER I see such "well-written" (or, well-meaning) articles OMIT lessons that have been well-learned-from-the-past.

If vishram is reading; "shotgun-answers" can, on occasion, manage to hit the nail on the head (eventually).

There are lots of good DC Motors training videos online, <here as well as youtube.

Wishing you "Good Luck" ~

["Edit_PS"] reading-down, I see that Andy made inference to the topic, referenced herein. "Nice-going!"

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#2

Re: DC Motor - Sparking

08/06/2017 10:41 AM

This is the reason why AC motors last longer than DC motors.

If someone chose a DC machine for this application, it must be better in overall, but lifetime and maintenance will always be a drawback. You can either implement a better solution or stay with what you have. There is not much to do about the sparking.

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#3

Re: DC Motor - Sparking

08/06/2017 11:35 AM

Have you checked that you have the correct brushes installed? Although they appear physically the same, they do vary in composition, hardness, resistance, etc., all of which can affect the brush-commutator interface. Also check the brush holders and springs, both of which can affect the pressure that the brushes exert on the commutator.

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#4

Re: DC Motor - Sparking

08/06/2017 12:11 PM

What is the nameplate full-load current?

What is the operating armature current & voltage when you get sparking?

Low operating currents can cause more problems than overloads, the interpoles are designed to do their job well at rated current.

Brush choice is very important, the right brushes for the operating load can give negligible wear.

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#8
In reply to #4

Re: DC Motor - Sparking

08/07/2017 5:01 AM

Good point - a brush supplier assisted me on a similar problem many aeons ago and we found that the motor was running well below nameplate capacity. A different brush helped (cannot recall exactly how we solved it).

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#5

Re: DC Motor - Sparking

08/06/2017 11:06 PM

Check for freedom of movement of the brushes and brush pressure. Sometimes brushes can get stuck or semi-stuck in their holders. You might have to remove the brushes and clean the holders. Also sometimes when that happens, it creates excessive heat (as in all high-resistance connections) and makes the springs lose some of their effectiveness (depending on the design). Also you might have to re-seat the brushes if the contact surface got contaminated or hardened somehow.

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#6

Re: DC Motor - Sparking

08/07/2017 12:54 AM

When you say sparking, is this somthing that is getting worse or has it allways been the same? Some degree of sparking can be apparent depending on the design of the machine.

If it is due to long term usage and all the other considerations regarding brushes that have been suggested, then you should consider that wear & tear could have caused the commutator to be out of round causing brush bounce, the armature needs to be re skimmed a big job it has to be done otherwise the problem will only get worse.

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#7
In reply to #6

Re: DC Motor - Sparking

08/07/2017 2:53 AM

I like your comment, it makes sense.

I personally have never seen brushes that do not spark to some degree, the question is "How much is normal on this machine?"

Only an expert on this type of machine can say if the amount in question is normal or excessive......

...and as someone else here said, if thats the best type of machine, DC not AC, then more maintenance must simply be accepted.....assuming that the exactly correct type of brush is already in use.....

But if they were recently replaced, going by size and connection type only, now that might be part of the problem!!!

Also, a brush with the wrong hardness may wear the commutator out far faster than the correct ones will.....now that will certainly push up the cost of maintenance dramatically!

Thats why the OEM brushes are often more expensive than other after "NoNames"! But work out cheaper over time.

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#11
In reply to #6

Re: DC Motor - Sparking

08/07/2017 12:20 PM

6 & 7 make good points.

Has the motor always sparked or is it just starting to spark? Are the windings starting to fail?

I note you say there is some sparking under the brush.

Are there streamers? What grade of sparking?

DC seldom has totally black commutation. If the sparking is "under" the brush there usually is no problem.

What condition is the commutator in? Do the brushes click or bounce?

Is there banding or threading or grooving of the com?

Is there a good film developed on the commutator? If sparking is severe it will erode the film and damage the commutator.

If the brushes are too abrasive they will remove the film. If they are too soft there will be carbon all over the place.

If the air supply is too dry or too damp sparking will be aggravated.

What temperature are you operating at?

If there are chemicals in the air the film can also be destroyed.

What duty cycle are you using? If it is controlled rectified power - have you checked the waveform - are all the pulses there and even? Are there intermittent sudden increases in current? If the current rate of rise is too high it will spark during those periods.

In a 60kw machine I would expect brush changes every 6 months to a year under 24/7 operation. I have had some go several years - but that is the exception.

A couple pictures of the commutator and brushes showing the sparking in the operating state would help.

(AC machines don't spark. But DC is easy and inexpensive to have 4 quadrant operation with power returned to the grid.)

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#12
In reply to #11

Re: DC Motor - Sparking

08/07/2017 12:28 PM

Really good comments.

I also just remembered, that many DC applications of yesteryear, are now VFD. Though I have no idea if VFDs are made as large as the OP needs.

If yes, then maybe a VFD is the way to go? Then no sparking and no brushes....

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#13
In reply to #12

Re: DC Motor - Sparking

08/07/2017 12:43 PM

60 K watt 440V VFD machines are in a "sweet spot" if 4 quadrant operation is not required. It would require new motor, mountings, couplings, cables, etc and that can drive the cost up also. If it is part of a system with HMI and data collection the cost also gets exponentially expensive to change. The peripherals($) can kill you.

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#9

Re: DC Motor - Sparking

08/07/2017 10:05 AM

Do you mean sparks that are not coming from the brushes? Brushes can throw down carbon that is hot and appear to be from something else. In addition to the other suggestions made, check the bearings. Worn out bearings can allow the armature to contact the field and scrape off metal, but I would think the excess noise and vibration would be obvious. New brushes will spark more until they wear the normal curve (until they seat).

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#14

Re: DC Motor - Sparking

09/07/2017 2:01 AM

Vishram ,

if you missed the polarity check of main and inter poles there may be a problem here. If is not good the auxiliary poles intend to reduce the spark they reinforce it .

This is easiest to check on a disassembled DC motor by using low volt DC & a compass, although there is a way to now .

You could apply 20v AC on two adjacent brush holders & measure the output at A1 - A2. If output voltage is lower than input then the interpole polarity is correct. If it is higher the interpole polarity is wrong & you'll have to switch leads at the brush holders to correct it.
You can also check the armature for shorts with the motor fully assembled. Lift all the brushes & apply shunt field excitation only. Rotate the armature slowly by hand, if there is a short the armature will have a tendency to lock in one position.

Good luck

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#15
In reply to #14

Re: DC Motor - Sparking

09/07/2017 2:50 AM

GA

I like your post, beautifully simple!!

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