High Power Dc Motor
High Power Dc Motor
(OP)
Hi
In our factory we are using high torque reversing dc motor.the specification of this motor is : Westinghouse Electric - 1250 Hp - 40/120 RPM 700 Vdc - 1540 Amp . Recently because of some malfunctions we've replace the rotor and mount spare one ( 22 ton !! ) after that we start the motor and realize that in Forward direction the brushes have sparks in some holders and in reverse direction it has not any sparks but operator said that the amps is higher than forward .
What's the reason and how can we achieve best performance of machine ?
In our factory we are using high torque reversing dc motor.the specification of this motor is : Westinghouse Electric - 1250 Hp - 40/120 RPM 700 Vdc - 1540 Amp . Recently because of some malfunctions we've replace the rotor and mount spare one ( 22 ton !! ) after that we start the motor and realize that in Forward direction the brushes have sparks in some holders and in reverse direction it has not any sparks but operator said that the amps is higher than forward .
What's the reason and how can we achieve best performance of machine ?





RE: High Power Dc Motor
Better get a service dept. rep. on site ASAP for such a large and expensive machine. I think Westinghouse is now Siemens in the USA if that is where you are. Obviously you are a rolling mill client..
rasevskii
RE: High Power Dc Motor
You have probably observed a mark on the end shield and a corresponding mark on the brush holder ring. These marks shall coincide for normal operation, which usually is in one direction that is indicated on the machine plate. If both CW and CCW are indicated, then the brush position was probably OK with the older rotor but since rotors are individuals, it is possible that you need to identify the new optimum brush position.
There are several ways of doing that. The most convenient is to use an AC voltage on the field winding and measure voltage induced in the rotor winding. Adjust brushes for minimum voltage.
You can use ordinary outlet voltage. It is not critical in any respect, and measure voltage across nearby brushes with an AC voltmeter. You have to disconnect at least the excitation supply, of course, and it is usually also better to disconnect armature supply.
This adjustment will give you a new neutral position for the new rotor. Make a new mark on both brush holder and endshield (use paint with other colour than the original mark) and make sure the rotor gets a mark with the same colour.
The best method, but somewhat more elaborated, is to run the machine with as little load as possible, best with coupling opened, at a certain speed and measure armature voltage. Then run in reverse direction at exactly the same speed and measure armature voltage again. Calculate the mean value and adjust brush position to get this new armature voltage while running at same speed.
The latter method will adjust brush position so that your voltage constant (and also torque constant) is identical regardless of CW or CCW rotation.
The operator's observation, that current draw is different depending on direction is typical for a machine which needs a brush adjustment. This is something we used to do everytime a DC machine had been repaired and we still do it occasionally. DC is not quite dead. Not yet.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: High Power Dc Motor
Bill
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"Why not the best?"
Jimmy Carter
RE: High Power Dc Motor
The mechanical rotating element of a Direct Current Motor is an Armature. (Not Rotor)
Alternating Current Motors have Rotors and "Wound Rotors".
Segueing to a curious question:
Is there additional info the author of the original post can offer regarding, "...Recently because of some malfunctions we've replace the rotor..."?
And Gunnar is on the right path regarding the setting of the brush neutral position.
This can be accomplished in the field by competent electric motor people. Ideally with the motor disconnected from its "load". (Not absolutely necessary though.)
Another factor that comes to mind. Is the armature that was utilized to replace the problematic one absolutely identical to the original?
John
RE: High Power Dc Motor
Armature is not incorrect in this context, but neither is rotor incorrect in a lot of workplaces.
Bill
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"Why not the best?"
Jimmy Carter
RE: High Power Dc Motor
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: High Power Dc Motor
I therefore used 'rotor' to describe the rotating part of the machine and then, three times, the word 'armature' to describe armature connection, armature voltage etcetera.
I do not want to be 'snooty' either. But this time, I think that you may have been hitting your cap instead of the intended bird (a French expression derived from Alphonse Daudet's books on Tartarin from Tarascon).
I see that this was your first post here. I look forward to many more. Your handle is surely interesting.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: High Power Dc Motor
Having been working on Electric Motors for a long time now, I can safely say unequivocally,
that anyone within the industry in the United States represented by the world wide group known
as the Electrical Apparatus Service Association (EASA) would not refer to an Armature, as a "Rotor".
It would be similar to calling a car, a truck. Or a rocket as a plane. Or a Jeep, as a Land Rover.
I do fully appreciate the differences in terms expressed.
I'll hazard a guess it has something to do with the ocean that separates us.
All the above aside, and not trying to HiJack the tread here with technical "muck muck",
I'll be curious to learn why the original Armature was replaced due to "malfunctions"
Enjoying the forum,
John
RE: High Power Dc Motor
i attached the photo of this motor .
RE: High Power Dc Motor
I know, it sounds like a sloppy practice, but it is really so. No need for any calculation and no need for any fear.
The reasons are that:
A) The inductance of a DC motor main field is always very high. The time constant of a field winding is measured in seconds.
B) The insulation is (must be) many megohms.
C) The excitation current is at least a few amperes. In this case probably tens of amperes.
D) DMMs have AC voltage ranges with millivolt resolution, or better.
That means that the insulation supports any AC voltage available from a standard outlet. No fear there.
It also means that the reactive impedance at 50 or 60 Hz is quite high. At least several hundred ohms. Which, in its turn means that current will never be more than an amp. Probably a lot less. Put a fuse with field rated current (or somewhere close) in series with the supply voltage if you feel that you need to protect things.
Fact 'D' means that you will always have a useful indication on your meter. All you need is enough numbers to be able to find the minimum and you do not need more than a few ten millivolts for that. Anything above that is a bonus.
"Just do it!"
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: High Power Dc Motor
"Just do it!"
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: High Power Dc Motor
Good luck on it. let us know the result..
rasevskii
RE: High Power Dc Motor
Isn't that the normal configuration ? (doesn't seem inside out).
dArsonval – Thanks for your suggestions, including terminology. I remember overhearing a motor shop guy talking about "balancing an armature" once and it sounded weird to me at the time... now I know why he said it that way. On the other hand Stephen Chapman (author of "Electric Machinery Fundamentals") uses them interchangeably: "Rotor (armature) windings are further classified according to the 'plex' of heir windings...... simplex... duplex..triplex". Personally I'd say there is no one right answer, but I was interested to hear the comment anyway.
All - I am not much on dc motors, but wanted to throw out a thought - There were two possibile causes of the sparking symptoms mentioned: brushes not seated and brushes not adjusted to commutate at the neutral plane. I can see where both need to be adjusted upon replacing a rotor, but I'm not sure why the neutral plane is deemed more likely in terms of a cause for brush sparking that occurs in one direction only.... I would have guessed the opposite based on the fact that interpoles (and possibly compensating winding) are designed to create equal/opposite emf to the armature current in order to cancel effect of armature current upon flux distribution. Therefore if these interpoles are functioning as designed ideally there would no no change in the neutral plane location regardless of load and direction changes... right? (and if the neutral plane location is unexpectedly shifting as a result of load, then any adjustment performed by energizing field only will surely not recreate it). On the other hand it seems very credible that systematic brush seating error may show up in one direction and not the other. Am I missing something?
Hesab - Regarding the fact that different current was measured in two directions, some questions:
1 – what were the current values
2 - do we know if there is difference in loading for forward and reverse directions? A
3 – do we have speed measurement for forward and reverse directions?
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(2B)+(2B)' ?
RE: High Power Dc Motor
Rafiq Bulsara
http://www.srengineersct.com
RE: High Power Dc Motor
the demagnetizing effect of the armature reaction is such that the main field is weakened in one side of the main pole (the iron saturates)and that deplaces the neutral brush position. The risers fom commutator to rotor winding are not always identical, there may be a one segment difference between the two rotors. That is why it is almost always necessary to adjust brush position when the rotor is changed.
The displaced new neutral position is different depending on torque direction and direction of rotation and since the displacement causes sparking, the old brush position may be closer to the neutral position when rotating in one direction than when rotating in the other direction. It also influences torque and EMF constant and the fact that the operator says it takes more current in one direction than in the other direction also points to the need to adjust the brush position.
Re undercutting of commutator: that is always done in the factory or in the rewinding shop. It would be a very poor shop, indeed, that ships a rotor without undercutting it. It can also be easily seen if that is the case and these circumstances, taken together, are a clear indication that undercutting is not needed.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: High Power Dc Motor
I don't put much weight in current measurements until the conditions of those current measurements have been identified.
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(2B)+(2B)' ?
RE: High Power Dc Motor
The compensation windings are there to counteract the shift in neutral plane and associated armature reaction. But not many motors have compensation windings. Not sure about this motor.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: High Power Dc Motor
He too described what we know as an armature as a rotor several times during his lecture and before long was "corrected" by someone in the audience.
It was quite amusing to see the heckler cringe in his seat as our hero pointed out to him in front of 50 or so delegates that as far as he was concerned by definition, "rotor" is the term applied to the rotating element of any electrical machine be it an armature, a S/C rotor a wound rotor or more importantly any combination of the above but, would be happy to use any terminology or descriptions whatsoever if it would make his lecture easier too understand for certain individuals.
The poor guys face was still bright red an hour later when we were sat in the bar.
RE: High Power Dc Motor
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: High Power Dc Motor
We find the correct neutral position of brushes and adjust it.
and now the amp of CCW and CW of rotation are almost equal.
at the nameplate amp the brushes haven't any sparks . but when the motor operates at the 200% overload we can see little sparks under some brushes .
dArsonval: three month ago we realize that commutator make some noise(chatter ) when the armature rotate at high speeds(about 40 to 50 rpm ) . it was clear that the frequency of noise is proportional to commutator speed . so we call some repair shops to prevent more damages . after investigations they said to us one region of commutator segments needs to repair and the segments becomes up!! . they said that the commutator only needs to grinding . but we think that we must choose another services ( or tests ) to make sure that segments are in good condition .
is it realy enough only grinding the commutator ?
RE: High Power Dc Motor
Sparking at 200 % load is not unusual. Some motors spark at 150 %. So you did a good job with the brush position.
The reason that segments rise above the others is usually that there has been a high current flowing for some time. If you measure the difference (use an indicator clock) and find that the difference is more than 100 microns (0.1 mm), then you probably need to use a lathe to correct the difference. But if less than that, grinding in the motor is adequate.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: High Power Dc Motor
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(2B)+(2B)' ?
RE: High Power Dc Motor
Thanks
RE: High Power Dc Motor
1: why the segments rise and how we can avoid this condition in future?
2:how can we be sure that the segments are in good condition?
is there any posibility that some of them cracked?
RE: High Power Dc Motor
When the commutator diameter is worn out, then the segments can rise up due to centrifugal forces. Another reason could be that the motor went into sustained overspeed.
Muthu
www.edison.co.in