It was noticed a few days ago that a contactor controlling a 75 HP motor load was - for lack of a better word - "clunking". While energized, the coil of the Square D contactor would momentarily de-energize and then re-energize. It would do this randomly once every 15 or 20 seconds about 60 seconds after the load is started. The de-energization is so short that the load contacts don't open. But the clunking is quite loud and we can see movement (though not breaking) of the auxiliary contacts through the transparent covering. There is no noise, interruption or distortion of the
120 VAC coil supply at the coil terminals, as determined with an oscilloscope. The phenomenon also occurs with the load disconnected and the coil forced on.We opened the contactor and examined the coil and the crimped winding interconnections within the contactor housing. Everything seemed solid there. There was no evidence of arcing or charring nor any burnt smell. I didn't want to unravel the tape binding the coil on the bobbin until a spare was available.
The 75 HP starting control uses an electronic soft start that's shunted by the contactor after the load accelerates to rated speed. For now, while we await a replacement, we're leaving the soft start energized after the contactor pulls in just in case the contactor's load contacts do open during a "clunk".
I suspect a broken wire in the coil winding that is vibrating and losing connection briefly and randomly after the coil warms up. I've never seen a coil fault quite like this. Usually, a broken coil winding will de-energize the coil long enough to drop out the load contacts. Anyone have experience with this type of fault?