Effect of DC Motor Overspeed?
Effect of DC Motor Overspeed?
(OP)
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.





RE: Effect of DC Motor Overspeed?
RE: Effect of DC Motor Overspeed?
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.
RE: Effect of DC Motor Overspeed?
I would suggest replacing the motor to suit your present speed requirement.
RE: Effect of DC Motor Overspeed?
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% ?
RE: Effect of DC Motor Overspeed?
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) ?
RE: Effect of DC Motor Overspeed?
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).
RE: Effect of DC Motor Overspeed?
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
RE: Effect of DC Motor Overspeed?
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 "play things safe."\\\
Thanks.
RE: Effect of DC Motor Overspeed?
You may talk to the brush manufacturer they may suggest a rgraded brush which will give better brush life.
RE: Effect of DC Motor Overspeed?
RE: Effect of DC Motor Overspeed?
RE: Effect of DC Motor Overspeed?
reversing shunt field polarity will only reverse the direction of rotation. that will not cause any sparking.
RE: Effect of DC Motor Overspeed?
Could... if the brush neutral isn't set correctly.
RE: Effect of DC Motor Overspeed?
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.
RE: Effect of DC Motor Overspeed?
RE: Effect of DC Motor Overspeed?
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.
RE: Effect of DC Motor Overspeed?
"By reversing the shunt field polarity, both the polarity of shunt field coils and the direction of rotation change"
I Agree
"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.
"in a dc motor, the interpole coil should have the same polarity of shunt field coil that is behind it"
I Agree, i thought that was what i had wrote, though i can clearly see it isn't.
RE: Effect of DC Motor Overspeed?
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.
RE: Effect of DC Motor Overspeed?
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.
RE: Effect of DC Motor Overspeed?
I remember a case where the dc motor on a dyne didn't produce the same torque in both directions.... Brush neutral was off. There wasn't any sparking in one direction... but there was in the opposite direction.
Could be that the brushes chattered in one direction as opposed to the other ... as this was a new installation and the brushes may not have had time to wear in.
RE: Effect of DC Motor Overspeed?
But possibly the commutator is suffering mechanically with the higher speed, 24% is a big increase for a large machine. Bars can move, you can check with a clock gauge. What about the comm-end bearing, is that giving excessive run-out? One clue is to look at the brush sides, if they are worn, it indicates they have been running on a poor profile. Another clue is that the outer brushes suffer more, the bar deflection is usually maximum at the outer end.
If you can clock the commutator, maybe do it regularly so that you can watch for any deterioration in the profile. It may be useful to measure both on and off the brush track (on-track only variations implies electrical problems e.g. commuation, any off-track variation implies a mechanical problem).
RE: Effect of DC Motor Overspeed?
Are you convinced now about direction reversal with shunt field reversal without affecting interpole-shunt field relation ?
RE: Effect of DC Motor Overspeed?
Im trying to figure out why the motor manufacturer ( I think it was GEC The motor was an MDX frame i think ) advised me to swap the field connections back and then swap the armature connections for motor reversal when i had the problem i described earlier. It worked.
The application was different and i have been trying to remember the connection configuration, It was actually on a crane hoist were a series motor was used for the main hoist connected 'series' for hoisting and 'shunt' lower, with dynamic braking. At the time i couldn't get my head around what the guy was saying, so i went to see him - he drew the motor pole configuration and he had me convinced. He also supplied me with a recommended motor reversal connection sheet for different types of dc motor, all of which showed armature reversal. I have always stuck to this and have never had a problem. I have even had unexplained motor torque / sparking problems on different applications - swapped both field and armature connections and hay presto, cured. I have always remebered what this guy from GEC told me....but now you have me questioning why it has worked.
This could be the start of a new tread.
Ive got lots of strange DC motor stories which ive cured by the above, thinking i new why it worked, but now doubt has crept in to my reasoning. IE I would still do the same but im not sure why ?
RE: Effect of DC Motor Overspeed?
Crane DC motors have special connections. May be, if you can post the connection details (preferably in another thread), then we could pick them apart.
RE: Effect of DC Motor Overspeed?
Your crane motor probably had a series field winding cumulatively compounded with the shunt field winding. Cumulative compounding is used where high torque is required (e.g. cranes), since the dc motor torque is proportional to the product of armature amperes and field flux. In cumulative compounding, the series field (called series stability winding) is connected so that series field mmf assists the shunt field mmf (i.e. adds) thus increasing the field flux and torque with increasing armature amperes. In differential compounding, the series field mmf opposes the shunt field mmf. Due to safety reasons (like over speeding due to less flux, reversed rotation at starting due high starting current etc.), differential compounding is seldom used.
Now, there are two ways to reverse the direction of rotation of a cumulative-compound dc motor.
1. You can reverse the shunt field to get a reversed rotation. But the same time, the series field, connected in series with the armature, must also be reversed to get back the cumulative compound connection. If you just leave the series field as connected before, then the motor will become differentially compounded with the above attendant safety problems. So, this was probably why your GEC man asked you to reverse both the field and “armature” connections. But the so called “armature connection” was actually the series field connection.
2. As you can see, the reversal of shunt field to change direction invites its own hassles. So, it will be simpler just to reverse the (armature + interpole) connection alone (not the series field connection) to change direction of rotation. By this method of (armature + interpole) reversal, all the requirement of cumulative compounding and the relationship between the shunt field and interpole (as discussed in our earlier posts)are maintained.
I hope my clarifications helped.
RE: Effect of DC Motor Overspeed?
http://www.ckit.co.za/Secure/Conveyor/Troughed/electric...
for:
6. How are interpole windings connected?
In series with the armature windings and with a polarity of interpole opposite to that of the main pole following the interpole in the direction of rotation (i.e. the interpole has the same polarity as the preceding main pale against the direction of relation).
RE: Effect of DC Motor Overspeed?
http://www.reliance.com/prodserv/motgen/c7090/
for: DC Motor Brush Life, and References
RE: Effect of DC Motor Overspeed?
RE: Effect of DC Motor Overspeed?
I don't see how a series field with very low resistance can be connected in "shunt" without taking excess current. I have never heard of it.
And, yes, compund motors are often used for high torque applications like hoist, mills etc.