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  1. Septentrion

    DC motor // Field current oscillations

    John, As you wrote, it is very probable that the phenomenon was not seen before not because it did not exist but because... we didn't look through "a microscope". I am sure that the different values that could be seen on a vu-meter were "heavily" filtered before we put new drives in 2007...
  2. Septentrion

    DC motor // Field current oscillations

    John, I'm a little bit puzzled : before grinding a commutator, the amplitude of oscillations is big and just after, the amplitude is low. I don't make the connection between that and a power supply issue ? Have you got an idea ? Below, oscillations before grinding, then after grinding (May 5...
  3. Septentrion

    DC motor // Field current oscillations

    Waross, no Silicone or other aggressive chemical is used "around" the commutator. Edison, Interchange filed supply is not really possible. I will answer your second question later today.
  4. Septentrion

    DC motor // Field current oscillations

    You can see hereafter a commutator during the setting of neutral. I will make some photos of the motors tomorow.
  5. Septentrion

    DC motor // Field current oscillations

    Waross, You're right, the brush grade must be well choosen. I think it's good for our motors. Many test have been done few years ago with Carbon Lorraine and Schunk.
  6. Septentrion

    DC motor // Field current oscillations

    Thank you all for your participation! :)
  7. Septentrion

    DC motor // Field current oscillations

    Waross, I can try to change brush grade but what could be the link between brush grade and current field oscillations ?
  8. Septentrion

    DC motor // Field current oscillations

    Hi Panter, The two motors (on same shaft) have not exactly the same FEM for a given speed. the nominal FEM is set to 770V (nominal speed 280RPM) in the drive so, one field is weakening before the other. The commutator is grinding when it's out of round. The oscillations have big amplitude at...
  9. Septentrion

    DC motor // Field current oscillations

    Yes, the motors turn above their nominal speed by field weakening. There is a dedicated transformer for the field power supply. I had a look on the voltage curves, nothing to report.
  10. Septentrion

    DC motor // Field current oscillations

    What would you know about the power supply ? The drive is a Mentor MP75A4. It receive the firing angle by the main drive which is a PEC (from GE). All regulation loops are in the PEC. I made a test with a constant firing angle : the oscillations are still presents.
  11. Septentrion

    DC motor // Field current oscillations

    Hi itsmoked, I'm agree with you, it could (must) change the resistance between brushes and some bars . The commutation could be more difficult on these bars. But I could not evaluate if it is a "difficult commutation" that causes the oscillations.
  12. Septentrion

    DC motor // Field current oscillations

    Waross, As I wrote, that's what I'm thinking too. But I can't explain why a "out of round" commutator causes the amplitude of oscillations to increase ?
  13. Septentrion

    DC motor // Field current oscillations

    Waross, That’s what I’m thinking too but I'm not sure. Edisson, Except for a -very- few trips of fields drivers, there is no real issue with these oscillations. I "just" want to know what causes them. Finaly, these oscillations are good indicators for me : when the amplitude increase, I...
  14. Septentrion

    DC motor // Field current oscillations

    Hi, Edison123, I don't think that the 10 motors have turns shorts in their field coils. The voltage across each field windings is the same to obtain the nominal current.
  15. Septentrion

    DC motor // Field current oscillations

    Hi Darsonval, Hereafter the answers for your questions. Grinding a commutator on an electric motor stationed in place should be for purposes of restoring the surface of the copper bars so each segment can adequately mate with carbon brushes riding on their respective surfaces. Grinding should...
  16. Septentrion

    DC motor // Field current oscillations

    I'm not sure to understand what you mean by "number of bars between adjacent brushes" ? There are 10 motors with the same phenomenon. The neutral is set for each motor. Some motor's neutral have been reset after armature replacement. That doesn't have an effect on phenomenon.
  17. Septentrion

    DC motor // Field current oscillations

    Some other informations about stands 2 to 5 : 468 bars Lap windings 12 poles 234 slots Coil span = 20 slots
  18. Septentrion

    DC motor // Field current oscillations

    Hi, I'm technician in a steel plant. I'm French and my knowledge of the English language is lacking (Sorry !) It's my first post here but I often read the topics of this forum. The context of my subject : - a cold rolling mill with 5 stands. - Each stand is driven by 2 motors, separated...

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