Effect of DC Motor Overspeed? 1

[macgen1]

The power station I work at has a separately excited shunt wound DC Drive Motor (682kW) for driving a diaphragm pumping system. Rated speed is 1244 rpm @ full field current. Some years ago the pump design was changed, resulting in a need to increase motor speed. It appears that field weakening has been used to obtain motor speed of 1438 rpm. Can anyone advise what effects this might have on commutation? We are wondering whether this could be the cause of some failures we have seen in the commutator/brush area.

 

[jbartos]

Suggestion: The increased speed will result in higher brush-commutator wear-out. Centrifugal force increase on the commutator and winding connections will be some; however, it may still be within the design margin of that design. Contact the manufacturer tech support for more accurate information.

 

[macgen1]

Addn info/Correction to original question. The motor has been operating for approx 4-5 years at 1538 rpm without falling apart, so it would seem that mechanically at least the increased speed of operation is within the motor design margin.
We have seen significant brush wear out (split brushes) and pitting damage on the comm. While part of the wear out of the brushes would be due to increased speed, we are also on occasion seeing incredibly rapid wear out (<300hrs operationas well as arcing from the brushes to the holders and melt down of the holders. To date no-one has been able to determine why this is occurring.
I have a few theories that maybe the 25% overspeed of the motor is
- altering the neutral plane, or
- reducing switching time when the brush changes segments, resulting in higher emf at the brushes, or
- causing circulating current around the split brush system,
but it's a long time since Uni and we have noone with experience of DC motors within the company who can advise. Most texts I've found do not consider motor overspeed situations.
Would really appreciate anyone who can confirm or eliminate any of these theories. The manufacturer tech support has only been able to say that they don't know what would happen at this speed as they only tested to overspeed of 1370rpm. Thanks.

 

[edison123]

As the speed of the motor increases beyond the designed maximum speed, the rate of voltage change (dv/dt) increases. This will result in severe sparking with resultant commutator and brush wear. Ofcourse, there is also mechanical wear due to increased speed.

I would suggest replacing the motor to suit your present speed requirement.

 

[jOmega]

A little more information would be appreciated.

1. What did the armature current do when you increased the speed by weakening the field ? Did it increase ?, Stay the same ? or Decrease ?

2. Did you observe increased arcing at the brushes in the field weakend range at 1538 rpm?

3. In going from 1244 RPM to 1538 RPM, did the pump mechanical torque load drop off 23.6% ?

 

[jOmega]

Further to previous post:

3a. Did mechanical load stay the same or did it increase as you went from 1244 rpm to 1538 rpm ?

4. Is the motor a straight shunt or is it compounded (with a series field) ?

 

[macgen1]

jomega
The motor drives a slurry pumping system via a crankshaft and three piston/diaphragm system. Changes made 5 years ago were to decrease piston bore, requiring increased speed for same capacity. We have no data on what occurred prior to the change, so amunable to comment on 1, 2 or 3. Mechanical load is basically the same (excluding speed considerations). The motor is a straight shunt (with interpoles).

 

[jOmega]

macgen1:

Thank you for the additional information.

Could you also please advise some nameplate (rating plate) data from the 682 kW dc motor; such as ..
rated volts ....
rated current ...

It would also be appreciated if you'd advise what the armature current drawn by the motor is at 1538 rpm.

Thank you.
jOmega

 

[jbartos]

Suggestion to macgen1 (Electrical) Jul 30, 2003 marked ///\\Addn info/Correction to original question. The motor has been operating for approx 4-5 years at 1538 rpm without falling apart, so it would seem that mechanically at least the increased speed of operation is within the motor design margin.
We have seen significant brush wear out (split brushes) and pitting damage on the comm. While part of the wear out of the brushes would be due to increased speed, we are also on occasion seeing incredibly rapid wear out (<300hrs operationas well as arcing from the brushes to the holders and melt down of the holders. To date no-one has been able to determine why this is occurring.
I have a few theories that maybe the 25% overspeed of the motor is
- altering the neutral plane,
///Yes, somewhat, since the counterelectromotive force will be higher.\\ or
- reducing switching time when the brush changes segments, resulting in higher emf at the brushes,
///Yes; however, the magnetic energy stored in the winding and being switched tends to have a shorter time available to change with the reduced switching time.\\ or
- causing circulating current around the split brush system,
but it's a long time since Uni and we have noone with experience of DC motors within the company who can advise.
///Please, would you elaborate on this one.\\ Most texts I've found do not consider motor overspeed situations.
///When it comes to DC motors with series winding, the overspeed situations are addressed.\\Would really appreciate anyone who can confirm or eliminate any of these theories. The manufacturer tech support has only been able to say that they don't know what would happen at this speed as they only tested to overspeed of 1370rpm.
///Try a different manufacturer tech support. Some tech supports &quot;play things safe.&quot;\\ Thanks.

 

[castera]

Are you getting the same effects on all motors since the speed increase or are some motors failing differently. I ask this as i have had rapid motor failures after changing motors which run for a number of years without problem - it was eventally traced to the new motor direction being swapped by reversing the field rather than the armature connections - thus causing the interpoles to fight against the main pole as opposed to assisting in commutation. Needless to say this took a while to find the cause or these intermittant rapid failures.
You may talk to the brush manufacturer they may suggest a rgraded brush which will give better brush life.

 

[macgen1]

Thanks for the comments castera. These failures have occurred in a similar pattern on both motors. We have been in discussions with two brush manufacturers and have tried various options - none of which seems to have eliminated the problem. We are currently in the process of determining the best and most economic way to modify the pumps to reduce motor speed to design, either by a gearbox ratio change - or return of the pump pistons to original size (allowing field weakening to be removed). Hopefully this will see an end to our problems. If it does, then at least I will be able to report back and let people know what happened.

 

[castera]

I'm sorry if i am stateing the obvious but it is worth checking that the field is connected the correct way around ( with regards to the polarity ) as it is acceptable to swap the armature connections over to reverse the motor direction as this by arrangement also swaps the interpole polarity- but it is not acceptable to reverse the field polarity to reverse a motor as this swaps the main pole polarities but does not swap the interpole polarity to match the new motor direction.

 

[edison123]

castera,

reversing shunt field polarity will only reverse the direction of rotation. that will not cause any sparking.

 

[jOmega]

edison123:

Could... if the brush neutral isn't set correctly.

 

[castera]

Edison123
Reversing the shunt field polarity will reverse the direction i agree, but it will not reverse the polarity of the interpoles as these are in series with the armature. Interpoles are wound and placed so that each interpole has the same magnetic polarity as the main pole ahead of it, in the direction of rotation. Swap the polarity of the main poles and you reverse the motor direction - but now the effect of the neutral plane compensation has gone, this leads to sparking due to bad commutation. Motors with interpoles should only be reversed by swapping the current through the armature, this swaps the polarity of the interpoles, keeping them correct for the motor direction.
The point i was trying to make was that the motor may have been replaced at some time and after a direction check was found to be going the wrong way - The electrician has two choices..... he either swaps the big leads with a big spanner....or ?
Its happened to me, two motors later and a lot of checking / down time i realised what had happened.

 

[cbarn24050]

hi, the armature reactance moves with speed, the interpole is designed to move the field to re align with it. As you have changed the speed from the original design speed it is likely that it no longer lines up hence the arcing. try advancing your brush gear a little.

 

[edison123]

jOmega,

Incorrect brush neutral will result in sparking in both directions.

castera,

By reversing the shunt field polarity, both the polarity of shunt field coils and the direction of rotation change, thereby maaintaining the relationship between interpole polarity and the shunt field polarity.
BTW, in a dc motor, the interpole coil should have the same polarity of shunt field coil that is behind it, keeping the direction of rotation as reference. For dc generators, converse (your statement) is true.

 

[castera]

Edison123
&quot;By reversing the shunt field polarity, both the polarity of shunt field coils and the direction of rotation change&quot;
I Agree
&quot;thereby maintaining the relationship between interpole polarity and the shunt field polarity.
I Disagree - Draw a simple 2 pole (pairs) motor on paper showing interpoles correct for a given direction, then swap the polarity of the field poles leaving the interpole polarity as was (as the armature current is the same). You should see that it is incorrect.
&quot;in a dc motor, the interpole coil should have the same polarity of shunt field coil that is behind it&quot;
I Agree, i thought that was what i had wrote, though i can clearly see it isn't.

 

[edison123]

castera,

Ok, let us assume a 2 pole motor. Assume that for CW rotation as motor, one interpole (call it I1), which must have the same polarity of the main field (call it M1)behind it, has a polarity of (N). Call the other main field coil as M2, which has a polarity of (S)for CW rotation.

Now, if you reverse the main field connections, two things occur.
1. The motor rotation is ACW
2. The polarity of the main field coil M1 is now (S) and and M2 is now (N) because of connection reversal.

For the new direction of rotation of ACW, M2 which has now the polarity of (N) is the pole behind the interpole I1 (whose polarity has remained the same (N) in both the directions of rotation). Thus the motor relationship of interpole being of same polarity as the main pole behind holds good for the reversed rotation and reversed main field connections also.

 

[jbartos]

General Suggestion: Visit

http://www.reliance.com/prodserv/motgen/h7000ch3.htm

for: Problem M - Brushes sparking excessively; may be accompanied by brush chatter and/or excessive wear and chipping.