High Power Dc Motor 4

[Hesab]

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 ?

 

[rasevskii]

Sounds like the commutator on the new armature needs turning/undercutting or is just plain dirty or oily from storage conditions. Check that all the brushes are free in their holders, have correct spring pressure and sre bedded in properly. It is usual that brushes may be noisy in one direction and not the other. There may be a skin of corrosion on the comm surface that needs cleaning off first.

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

 

[Skogsgurra]

Even more likely that you have to adjust brush position to better suit the operation.

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

That's worth a star, Gunnar.

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

 

[dArsonval]

A polite reminder of the correct terminology of electric motor nomenclature. (And not expressing this in a snooty fashion, just being authentic where I'm able to.)

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

 

[waross]

Those who work with a variety of motors in many unique configurations always call the part that turns, "The rotor."
Armature is not incorrect in this context, but neither is rotor incorrect in a lot of workplaces.

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

 

[waross]

And it is not hard to design an inside out DC motor for fan or wheel motor application where the field is stationary and the armature rotates.


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

 

[Skogsgurra]

I appreciate you striving for correct terminology, d'Arsonval. But I am also a person that tries to use le mot juste as often as my Swedish origin and citizenship doesn't play tricks.

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[dArsonval]

Appreciate the passionate replies regarding "terminology".

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

 

[Hesab]

Thank you for your kind ideas.yesterday we've checked the output waveform of dc drive and it was ok so we decide to indicate the correct neutral position of brushes ( it seems that original mark is not useful for spare Armature ) . as you know there are several method to do this . we decide to apply ac voltage to field windings and measure the induce voltage on armature . the number of segment of collector is 702 and main poles are 12 . the question is the value of ac voltage and how can we calculate the starting current to prevent any damages ?
i attached the photo of this motor .

 

[Skogsgurra]

My experience is that you can always apply nearest available outlet voltage and get good results.

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

As Gunnar says, just apply 120V AC to the field and rock the brush gear back and forth until you find the null point or point of minimum voltage on the brushes. The first time I did this I was working alone and had much less explanation than you have had. I was nervous starting but I found this to be an easy test with unambiguous results.
"Just do it!"

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

 

[rasevskii]

Skogs has it absolutely correct. Try using a 220VAC (max)from a relay test set connected onto the field. A supply isolated from ground preferably. The DC excitation supply must be disconnected (wires lifted). The AC current will be so small that you have no risk to anything on this motor. The armature circuit should be open. If there is a DC breaker in that circuit open that. Probably you will only have to shift the brushgear a very small amount (shall I guess: a cm or not more..)

Good luck on it. let us know the result..

rasevskii

 

[electricpete]

Bill – you said "And it is not hard to design an inside out DC motor for fan or wheel motor application where the field is stationary and the armature rotates."
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)' ?

 

[rbulsara]

Very interesting and useful Gunnar! Although I haven't dealt with a DC motor in many years.



Rafiq Bulsara

http://www.srengineersct.com

 

[Skogsgurra]

Pete,

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[electricpete]

I agree fully we expect to have to change the neutral plane position after changing the rotor. I also understand that uncorrected effects of armature reaction shift the neutral plane depending on load and direction of rotation. The interpoles should eliminate this, but I imagine your assumption is they don't do this perfectly? I can buy that.

I don't put much weight in current measurements until the conditions of those current measurements have been identified.


=====================================
(2B)+(2B)' ?

 

[Skogsgurra]

I am not so sure that the interpoles can always do their job 100 %. They have, after all, a limited and rather small number of windings. And since that is always an integral number, they may do their job 'almost' but not fully.

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[stardelta]

As a side note on the rotor/armature debate. I once attended a seminar held by an individual who was once an electrical designer engineer for Brush electrical machines He held a phD and had left the industry to become a lecturer at a very highly regarded University here in the UK that specialises in electrical engineering.

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.

 

[waross]

Although almost all small power generators (About 2 KVA to around 25KVA)are now made with a rotating field and are brushless, that was not always the case. When we were encountering a mix of generators, some with stationary fields and some with rotating fields, we avoided confusion by always using the terms rotor and stator. The terms carried over to our motor work.

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