When it went bang it made me jump!

OK Guys, are you sitting comfortably?

I got asked by a good customer to attend his site last week, as his local controls specialist (knows his electronics).
Could not identify why at various run times, his number 2 of 3 screw Comps was tripping its 73a motor overload.
All the usual questions like run pressures and temps were answered correctly particularly the HP side.
Basically as was explained to me, the errant compressor would run for anything between 1 to 6 hours prior to it tripping its Delta contactor overload (set at 73 Amps).
Apparently no-one had been present when the plant tripped.

Off I trundle 150 miles one way.
On arrival the plant is shut down and at my disposal. So first things first, all the contacts and terminals from the Star Delta contactors through to the Motor terminals (40 kw motor).
Are metered for resistance, visually checked for corrosion etc and windings meggared.

Nothing un-toward came to light.

OK, the comp along with the other 2 was set to work. All three were pulling similar amperages across the 6 windings.
As the overload in question was on the Line contactor, I monitored the line phases with a clamp meter.
All 3 comps showed the middle white phase as the highest, with No 2 comp pulling the most.

As the load increased so did the white 2 phase. I actually watched this increase to 72 amps once momentarily. Although its average was 68 /69 amps, to high for my liking.
At which point it dawned on me that I had never witnessed the motor bearing ever having been greased.
A quick conversation with the site engineer confirmed that they had never been greased to his knowledge in their 21 years history.
Stal R50 mini screws for those that are interested, built to last is all I can say!

The Engineer returns with some suitable lithium grease which is used to grease the motors.
Whilst him and I are doing this there is a almighty clunk clunk from the comps electrical panel.

I shoot round to it's front staying there when all of a sudden the No2 comp delta contactor starts dropping in and out in unison
with the star delta timer.
These are 90amp contactors mind! It ain't pretty and more than a little scary when contacts
of that size start chattering.
I quickly turned the no2 comp run switch off and watched with the engineer who had by now joined me at the panel, the delta contactor continue to drop in and out.

So as there was a replacement star delta timer on site it was changed and the greasing of the motors was completed.
On start-up the average motor phase amperage had dropped by around 10 amps.

The plant has not (I believe!) had any further issues since.
Try finding that one in a manual, hence my post.

Oh! and the irony is my friend and colleague whilst discussing the issues earlier that morning has said " I wonder if there is an issue with the star delta timer".
I phoned him up later and accused him of being psychic.


I did take credit for the greasing of the bearings however!

Grizzly