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from the Electrical department...
DC motor troubleshooting
Engineering and workplace issues. topic
Posted by Dmitriy Moshkovich on 2 April, 2002 - 9:19 am
How Troubleshoot DC motor with oscilloscope ?
Practically I am interested in how to troubleshoot the shunt Direct Current Motor with electronic drive?
And more specific, HOW I can determine?
- the ripple factor for commutator current (from drive)
- the ripple factor for field windings current (from shunt)
Is that measurement important or not?
What kind of criteria for each current?
If you can, please tell me name and author of book, where I can read and find answer.
I looking forward listen answer from you.
My E-mail: "mosh24pc@1stnetusa", mailto:mosh24pc@1stnetusa
Best regards
Dmitriy


Posted by Tom Gianni on 3 April, 2002 - 12:14 pm
Don't put scope probes on the motor circuits unless they are isolated from the ac line. Otherwise, float scope (pay attention to safety
hazard) or use isolated measurement like Hall current probe/amplifier set-up. The current waveshape can give clues if the power section is OK but you should also do circuit/device level checks. Regarding ripple factor, I assume you mean form factor (FF) = Irms/Iavg. This is important when sizing/derating motor for specific drive design, but once the application is done (after-sale), you shouldn't need to be concerned as long as drive is good. FF = 1.0 is the best attainable e.g. pure DC/battery. Any 6 pulse (3 phase) DC drive will also have good FF - almost 1. A single-phase half bridge (B2H) is poor FF (1.4 to 1.8) which is why external chokes can be used to improve the FF.

If I remember correctly, the book by Wildi - Electrical Machines gives good basic theory of modern drives etc. However, the best "troubleshooting" information may be available from the drive manufacturer directly.

Also, troubleshoot motor-drive with any chokes used in the installation. Some older four-quadrant SCR drive designs require chokes for proper operation e.g. 4Q circulating bridge chokes.


Posted by Dmitriy Moshkovich on 7 April, 2002 - 8:22 am
Dear Sir:
Thanks a lot for your answer.
I am sorry for your disturbance, but please make attention and help:
I work on the plant where we have 15 DC motors with drive "FLEXPAK 3000",but only one of
this motor work in very heavy environmental (a lot of sand, moisture, and some water). I am
interested about troubleshooting that motor only.
We installed that motor two years ago and don't have problems during a year, after that sometimes the equipment, which that motor drive, jumping and we can't find problems. We inspect mechanical parts but it okay, we inspect drive and armature with brushes, it look like good, and maybe the problem is the motor. That motor: 10 HP Direct current shunt motor with drive.
When I put scope probe on the motor circuit (FF) we saw not only smooth continuously
curve, but sometimes line falls down at approximately 70-80% of margin and return to continuous curve, that happened during very short period. That happened 2-3 times during the cycle.
I am interested:
Why do we have that curve not smooth and continuous?
What is the criterion, how many times during the cycle, and how many percentage
of margin ( or any parameter) that will be good enough for a motor?
What name of book where can I found information about that questions?
What do you recommend more for that critical motor troubleshooting?
I looking forward saw your message, my E-mail mosh24pc@1stnetusa.com
Best regards
Dmitriy.

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