I have a DC drive motor that is showing 2x line frequency with running speed side bands.

It is also showing 21600. It was just showing 21600 before the 7200 showed up.

Where do I need to start looking?

Electrical_7200.doc (49 Kb, 25 downloads)

The 21600 is basically "normal" for a motor driven by rectified AC current. The 120 and the small 60 Hz (that appears to be in your spectrum) frequency might be the start of something causing LF to "leak" through. I have seen a bad SCR and/or a loose connection give LF and 2XLF problems. But I am no electrician.

I have a case with 6+ mm/s at 2xLF, 2 old drives identical one have it one have it not. Bad one remain shaking 30+ sec after shutdown. May or may not popped up after motor been to service sparkys find nothing wrong. Mystery. Olov