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.

 

[jOmega]

Edison123,

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.

 

[UKpete]

Maybe it's commutation - it's a bit of a black art; black-bands tests were the true measure of commutation quality and adjustment but these are really factory tests, not easy to do on-site.

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).

 

[edison123]

castera,

Are you convinced now about direction reversal with shunt field reversal without affecting interpole-shunt field relation ?

 

[castera]

Yes and no !!
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 ?

 

[edison123]

castera,

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.

 

[edison123]

castera,

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.

 

[jbartos]

Suggestion: Visit

http://www.ckit.co.za/Secure/Conveyor/Troughed/electric_motors/Alstom/Alstom - DirectCurrent.htm

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).

 

[jbartos]

Suggestion: Visit

http://www.reliance.com/prodserv/motgen/c7090/

for: DC Motor Brush Life, and References

 

[castera]

No it was a series motor not a compound, Connected series for hoisting - Shunt for lowering. Ive never seen a compound motor used on a crane hoist.

 

[edison123]

castera,

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.