quick motor controls question

[drummin89]

I did get the motor to stay running by holding in the contactor then telling the PLC to start it. I could hold the contactor in all day with out it tripping. There is an amp readout on the PanelView that read 16amps when running with the contactor manually held in. It only gets this reading off of 1 of the legs though.

 

[milldrone]

If you slowly push in the contactor manually the motor starts up and will stay running, overload relay does not trip.At first the overload relay fixed it but when I came back in today it was back to tripping everytime the plc pulls in the contactor.

That is strange... Here are some stupid things to try:
1. Maybe the problem is mechanical. Is the contactor pulling in so hard that it is "shaking" the overload and causing it to trip?

I had a "peculiar problem" with some similar symptoms (my problem would only trip the overloads when it was shut off from a remote pushbutton. If you stopped at the local push button on the control cabinet it would not trip. Later testing proved that if you placed your hand on the control cabinet and had someone else operate the remote stop there was no trip.).

I think it's time for the patented "tap from a rubber insulated Klein screwdriver test" to see if you can trip the overloads. Try it with the power off first. If the overloads trip under the power off condition, you will have your answer.

 

[drummin89]

Still no luck with this. I suggested to management and maintenance to have someone megger the motor while turning it. They already ohm'd it out and between phases and to ground was fine. They also megged the motor and didn't find anything. Swapped overloads from another simular unit, still trips. I suggested wiring the contactor to a constant 120vac source with a switch wired in between to verify its not being caused by the plc output. Also suggested tapping around the overload to see if that causes it to trip.

 

[drummin89]

So far it seems the issue is resolved.

The machine is a tempering unit that tempers white chocolate before depositing in on a belt to run through a cooling tunnel. Anyways it was about 3-4 months since this line ran and it seems they shut the line down with out clearing out the lines. The chocolate setup in the lines before and after the tempering unit and also in the tempering unit. They were able to turn the unit by hand but it was very tight, not sure if they were even able to get 1 revolution.

All the tests on the motor checked out, continuity test and megger tests all came out fine but they were seeing very high amps with a clamp on meter when the contactor was pushed in. Burned up a few contactors in the process.

They ran hot oil through the pipes to try to melt the product that setup in the pipes and in the tempering unit. After doing this they got the tempering unit to start through the PLC without tripping, at least for an hour or so then it tripped due to a blocked pipe after the tempering unit.

Thanks for the help guys. I'm going to keep this thread in mind in case we run into a simular issue in the future.