Since few days , In our paper machineries, at some of the D.C. motors, we are getting vibrations at line frequency = 50 HZ (3000 rpm). At one of the D.C. motor (210 KW), the amplitude at this freq. is very high@ 8 -10 mm/sec. and is irrespective of speed of D.C. motor. Vibrations at 50 HZ are also observed while the motor is in decoupled condition, but with lesser amplitude i.e @0.7 mm/sec. What does it indicate? / what may be the cause? / How to find out the cause?
It might be coming from possibly a loose connection or bad rectifier in your AC to DC rectifier system, which is allowing the Line Frequency to "leak" passed the rectifiers and into your motor. Unless your motor is running on "real" DC current from a DC generator, but this is highly unlikely since there should be no AC LF current coming from a DC generator. Just my opinion and I could be wrong, since I am not an electrical guru.
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