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Dc Motor's Current Variation Fault


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Dear All,


I want to tell you my experience in removing a load variation fault in an 85kw dc motor. Sorry for poor English.

We have a corrugator machine which has two cutters (top and bottom) for cutting corrugated sheets. Each has an 85kw, 500rpm, and dc compound motor. Both cutters don’t work continuously in line with machine. They cut the sheet and wait for next reference pulse from cutter control desk which has 13 PCBs for each cutter for calculating sheet length and sending pulse to dc drive. When cutter cuts the sheet, it draws 280A peak current from dc drive (for armature).

Once, the top cutter motor started taking the 320-430A peak current from dc drive when it cut the sheet. After half an hour, drive also tripped. I took following steps for removing the fault:

1. Checked cutter’s mechanical parts for any jamming, all ok.

2. Checked the dc motor, ok.

3. Checked the cable from drive to motor, ok.

4. Checked the armature dc drive 12 thyristers (6 for fwd and 6 for rev), ok.

5. Checked the reference voltage from control desk to drive, ok. No ripple found.

6. Checked the source voltage for any variation, found ok.

7. Interchanged both cutter dc motors. Fault wasn’t removed.

8. Interchanged the control desk PCBs. Fault wasn’t removed.

9. Interchanged the series reactors. Fault wasn’t removed.

10. Interchanged the cables from dc drive to motors. Fault wasn’t removed.

11. Interchanged the feed back Encoders. Fault wasn’t removed.

12. Interchanged the dc motors field supply dc drives (each drive provides 25A at 90VDC). Fault wasn’t removed.

13.Interchanged the drives out put components (MCCB, contactors, diodes and shunt). Fault wasn’t removed.

14.Replaced all 12 thyristers of top cutter armature drive. Fault wasn’t removed.

15.Finally I interchanged the Armature dc drives. Now fault shifted from top cutter to bottom cutter motor. I checked the 24vdc power supply of faulted drive and found a little bit ripples. Adding a 100uf 50vdc capacitor in the parallel of 24dc power supply terminal block removed the fault. Now motor started taking 265-280A.


That was a very interesting experience of me ever before that only a little bit ripples caused a very large current variation of dc motor armature. After that experience, I bound myself to check the power supply of any device first. I have removed 80% faults in electronics devices in their power supplies. (I also work in electronics field.)


So you should all learn from this experience for protecting yourself any trouble as I had experienced.


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