DC motor's spark grade have relationship with power lead? 1

[boydli]

Hi,
We meet a problem on a DC motor. We adjust the brush position OK by same speed at CW&CCW direction with non_load. But only one direction is OK if judged by spark and torque between brush and commutator bar with load on(15N.m/1000rpm). In aother direction the spark is very violent, also the motor cann't output 15N.m torque(the motor should output 15N.m/1000rpm at CCW&CW). The armature current is about 170A(4 poles,4 brush carbons). But if I exchange the power lead "+" & "-" which connected to the brush lead wire, the spark is changed also. That's to say, if brush 1,3 to '+' and brush 2,4 to '_", the CCW is OK and CW is very bad(violent spark and no 15N.m), when the brush 1,3 to '_' and brush 2,4 to '+', then CCW is very bad and CW is OK. We also adjust the brush position in CCW or CW the phenomenon still appears. We are not sure why this happens. Any advice about it will be very appreciate.
Thank you.

Boyd

 

[DickDV]

Sounds like this is a compound wound motor. In other words, the motor has a shunt field and a second series field that is in series with the armature.

If the motor is compound wound, it is designed to operate in one direction only. That direction is the one which makes F1 and S1 both positive when running. For reverse rotation, only the A1 and A2 leads are reversed.

Running with the series field bucking the shunt field can cause the arcing problem you mention and certainly takes away a good share of the torque output.

 

[boydli]

Hi Dick,
Thank you. Sorry I donn't describe the motor clearly. This motor is separate field motor. Two shunt-wound of two series-wound of field pole windings are here. And pls see the enclosed file. I just exchange the A1 and A2 lead connected to power source.

 

[sreid]

From the basics of the problem description (more sparking in one direction, less torque in one direction), there is an asymmetry in the motor fields. I'd have the armature checked out especially on a growler to check for internally shorted windings.

 

[burnt2x]

Agree with sreid.

Rocking the brush arms should correct most of the sparking, but limited the allowed range you can shift the arms. Shorts on the winding relocates the motor flux fields around and is usually the culprit.

Additional question: Did you change brush grades or changed the brush tensions? Basically, higher contact resistance on one/ two brushes of a four-brush arm will load up the other brushes resulting to sparking and lesser torque too.

Just trying to cover other gray areas.

 

[boydli]

Sreid,
Thank you. In fact I have try two rotors. I'll check the armature.

Burnt2x,
Yes. I have moved the four brush about 1Min-6Max degrees in CCW or CW direction but no effect. I'll check the tension again, also the brush.

Thank you.

 

[roydm]

The reversing motors I worked on as an apprentice had Interpoles in series with the armature to correct for field imbalance. If the interpoles are connected backwards it's possible to get what you describe.
Are the brushes a good fit into the holders i.e. not tilting in reverse.
Roy

 

[waross]

I think that RoyDMatson has your answer. Most industrial DC motors have commutating poles (sometimes called interpoles). They are never shown on the drawings. You must interchange the actual A1, A2 leads. If you reverse the connections at the brushes the interpoles will be causing sparking instead of preventing it. If the motor does not have commutating poles, you will have to set the brush position for the direction of rotation. Roy's observation concerning the brush holders is also valid. If the brush holders are not perpendicular to the commutator you will not get good performance in reverse.

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

 

[ankervik]

Maybe a little late to chip in but you can verify Interpole polarity by using low volt DC & a compass. Alternatively you could apply 20v AC on two adjacent brush holders & measure the output at A1 - A2. If output voltage is lower than input then the interpole polarity is correct. If it is higher the interpole polarity is wrong & you'll have to switch leads at the brush holders to correct it.
You can also check the armature for shorts with the motor fully assembled. Lift all the brushes & apply shunt field excitation only. Rotate the armature slowly by hand, if there is a short the armature will have a tendency to lock in one position.

 

[roydm]

If I remember corectly the polarity of the interpole in a motor is the same as the pole it proceeds. In a generator its the reverse. My simple explanation is the field in a motor is streached out of alignment like an elastic band as the motor loads up thus causing the neutral point for the brushes to shift. The interpoles pull it back into alignment (more load, more pull). If you have the interpole polarity wrong you can still eliminate the sparking in one direction by rotating the brushes but when you reverse the field alignment (sparking) will be much worse. Yes a compas is the recommended way of checking but just switching the leads is OK too.
You also see the interpoles in generators that never reverse.

 

[boydli]

All,
Thanks very much to all of you. The cause have been found out finally and no problem in motor, but a unbelievable mistake in the test dynamometer. Much time on motor but finally not it, a lesson to me!

 

[burnt2x]

boydli,

Thanks for the update and closure.

 

[roydm]

Now I,m curious. I can understand how the torque might be wrong but the sparking problem has me wondering what would cause that?
Roy

 

[burnt2x]

My after thoughts:
1. Dynamometer registered a lower torque when the motor was actually loaded beyond its rating. Overloaded DC motor gets more amps on each of the brushes which was indicated by the 'sparking'.
2. Degree at which the corrective action of interpoles (if any) wasn't enough to relocate the flux center.