As one of Innovative-IDM's Houston-area field service technicians, I was called out to look at a lathe.
The customer complained that the lube pump wasn't working. It actually was. But we started the machine and ramped it up, it sounded like the gears were going to grind out of the machine and the DB resistors started smoking. The customer indicated it was probably normal, due to low load and gear selection...I took the opposite view (there is something seriously wrong here) and convinced him to pull the belts. The motor continued to cog (and the DB started to smoke again) at about 1/4 speed even uncoupled.
The motor megged and ohmed normal.
The Yaskawa F7 4045 drive was running in Open loop vector.
I disconnected the DB Unit (it statically checked fine). The cogging continued and the drive OV tripped.
I saved the parameters and switched the drive to V/Hz. The cogging decreased dramatically (however the machine still started to vibrate. No OV trip.) The output waveform looked fairly normal phase to phase, and normal phase to+/- busses.
At about 15hz the DC buss started to get a 100VAC sign wave riding on top of it at ~ 11Hz and linearly increased to 12Hz as the drive was speed up to 20hz, above that the ripple disappeared. The machine vibration also followed starting at 15Hz and disappearing at 20 Hz.
I broke out my Fluke Scope current clamp on, and found the output current waveform to be a normal sine wave below 15hz and above 20Hz.
Between 15 and 20Hz however the Current waveform (on all 3 legs) appears to be the summation of 2 similarly sized ac waveforms.
I discussing the situation with one of our engineers, Steve Lyons, and he suggested that the issue could be the result of the motors rotor wobbling, changing the air gap.
I informed the customer that I'm leaning towards a motor issue ~ 85/15 and that the only way to be certain is to hook up the drive to a test motor.
We will do so at a later date as the customer will bring out his spare motor while he fixes the gearing (he found several issues during a visual inspection preformed while I was dinking with the drive). The machine will be down for a month or so.
I have included a pdf of the "Normal" current waveform VS the "Distorted" current waveform.
I've never seen this before ... if any one has any ideas I'm open for suggestions, experiences, anecdotes...what ever you've got. - DF