DC Motor Performance

[ColinR]

I have a 2800 kW separately excited bidirectional DC motor which is drawing more armature current in one direction (when accelerating a given load) than in the other. Field current limits are identical in both directions. The armature converter is a series/parallel conencted device, which can source or sink current in one direction only (non-reversing armature circuit).

The load (required torque both directions) is identical.

The motor was recently replaced, which makes me wonder whether the brushgear has been set to the correct position with respect to the stator's neutral axis. Any thoughts?

 

[cbarn24050]

Hi, you don't say how much difference there is between the directions. generaly speaking torque is proportional to current, all other things being equal. Did you measure the torque in each direction or are you assuming that it's the same? If the brushgear is out of alignment you will see arcing.

 

[ColinR]

Hi, thanks for replying.

The armature current in each direction is approximatley 5900 and 6300 amps respectively. This is with an equal current limit on the field converter in both directions. The current limiting protection will trip the DC converter if the current exceeds 6200 A for more than about 5 seconds, so you can see our concern here.

The application is a mine winder, so direct measurement of torque is difficult, however, the payload in each skip is measured and is identical. I have no reason to believe that one skip + its payload is any different to the other. This was the case before the motor was changed out, and no current imbalance was apparent.

Yes, I agree we should see some sparking if the brushes are misaligned, but I am trying to find reasons why motoring in one direction should be different to the other.

 

[jbartos]

Suggestion: The motor magnetic hysteresis curve (B-H) is supposed to be followed symmetrically in both directions to achieve the same motor output parameters in both directions.