Partial rewinding of Large DC Motor 3900KW

I'm having a little trouble following your descriptions of this exact motor, but here are some ideas.

You mention damage to the equalizer windings. Ideally, these windings have very little current in them and shouldn't be overheating (of course, you did have a flashover). If one or more magnetic poles on the *stator* are weak, then the mismatch in magnetic field as the armature rotates will induce high currents in the equalizer windings and can burn them out. You don't mention if this is a compound wound machine, but such a large machine, I'm guessing it is. So a weak pole can be caused by several things and each should be checked. 1)incorrect series winding connections on poles (especially if they've been worked/repaired in the past) 2)shorts/opens in shunt winding

Flashover of commutators is often caused by either excessive voltage or buildup of carbon/soot. Since this is tyrister controlled, I'm guessing the field is supposed to be constant and the armature voltage/polarity is controlled. In this case, the higher the speed, the higher the armature voltage (especially bad if reversing from high speeds). High-enough voltage with dirt/soot buildup in commutator and brush-rigging and flash-over is almost inevitable. Clean it, clean it, clean it. This is the one down-side of DC machinery, it has to be cleaned every time you change brushes. If that isn't practical, you will have to find some schedule that allows for periodic cleaning. In industrial environments with oil vapors, a black film can build up on commutator bars, causing a lot of brush wear (had to rebrush it often??). All that brush carbon has to end up somewhere and if it builds up all around the risers and bars, look out. Don't just clean the bars where the brushes ride, clean the whole interior. Brush rigging/wiring, risers, end-turns of windings. A lot of carbon buildup on the armature/risers/commutator won't necessarily show as a low meggar reading to ground. But remember the commutator bars one, pole-pitch apart have to withstand line voltage. Even though the winding resistance between them is very low, the insulation between bars/risers must be high to avoid flashover.

daestrom

Daestrom has said that he thinks that the field would be constant in this motor due to control by thyristor. The 800 hp motors we have in our bar mill have constant field up to base speed ( Approx 500 RPM ) and after that the speed is controlled by field weakening. A_R snipped-for-privacy@hotmail.com

thanks for response. i think i agree to most of your observations. however you have not commented on my other 3 questions:

1. Whether the method of partial repair followed in this situation is technically correct? Which other methods of partial repair prevail?
2. Is the failure on 20/09/2003 independent of the failure on

1. Is the failure on 02/10/2003 result of bad workmanship of repairs carried out due to failure on 14/09/2003?

can you pl suggest something on this?

regards,

arun

It is difficult without actually seeing the thing and being able to 'poke around'. You mention "commutator side hang over bandage got peeled off from outer skin." This should not have occured if the banding had been properly installed.

From my armchair in my den, I feel that although the partial re-winding repairs may have been adequate, the banding was poorly done and the root cause of the flashover and burned equalizer windings was not addressed. Repairing the damaged windings without addressing what caused it is just asking for a repeat occurance. The repeated tripping and another flashover when brought up to full load in such a short time bears this out.

Partial re-winding as you call it (the selective replacement of just some damaged coils) is common. The basic idea is sound. The exact implementation is important. If banding 'peeled off' afterwards, then at least the banding repair was not done properly. The broken riser may have been as a result of the forces the winding exerted on the brazed connection to the riser because the banding was not tensioned properly. Ideally, the banding is wrapped on with considerable tension so that the winding connection to the riser does *not* have any centrifugal force exerted on it when running at rated speed. But putting banding over the older sections of winding can cause damage to the insulation on the older sections, so care must be taken.

As far as who was responsible for finding the root cause and correcting it, I can't say. In our shop we would clean the machine and check a lot of the more obvious things, but in the field it depends on who is supervising. Whoever reassembles it should be the one to check brush rigging clearances, brush seating and pole air-gap evenness.

Considering you had a failure the previous year, it really sounds like this machine isn't being maintained properly. A quality repair should last many years. But the original machine should have lasted that long as well. If there is an underlying maintenance or environmental issue, no repair can stand up for very long.

What do the commutator slots look like? Is there metal drag off the bars in the direction of rotation? Does the 'mica' need undercutting? Are the slots filled with carbon debris? Are the windings coated with carbon dust? Does the dust have oil film that contributes to the carbon buildup? How fast is this machine using brushes? Check the brush tension? What is the general color/appearance of the bars? Are the brush holders spaced the correct gap above the bar surface? Is there a lot of vibration in the brushes? Is the rotor balanced? So many questions, so little time.... ;-)

I'm not very familiar with rolling mills, these are just large DC machinery issues. But can you compare this machine to others that don't fail? Even something as simple as an oil leak on this machine and not others could cause a build up of an oily film inside that attracts carbon dust.

That's about all the help I can give you, I know it may not be enough, but there are limits to the internet (as my father used to say, "Son, my screwdriver ain't quite long enough to reach from three states away.")

daestrom