Eye twitcher

[ptonsparky]

Started out with a simple motor change out. The guys had just labeled & located all the disconnects for motors at a small industrial. Leg motor needed changed so their maintenance disconnected the old & mounted the new. My guys connected it noticing a tingle from the wires as they did. After reminding them why the hell I provide nice testers I went up to find the problem. Haven't found the problem with the leg motor because I need to find out why it takes 7 different disconnects to remove power from the 480v control circuit first. Somewhere, somehow after the control is turned off, we still have 277 on one side, turn next off, it drops another 20v or so, then again and again until power is gone. By noon yesterday my eye was twitching. We initially will have to use the T&P method of locating wires so this may take awhile. Dang. Not a ghost problem.

 

[George Stolz]

Sounds like as big a headache as I had when I tried to troubleshoot a 208v control circuit without a schematic, not understanding why things were changing phases inexplicably, and why coils weren't closing when they were energized to 120v.

 

[LarryFine]

. . . I need to find out why it takes 7 different disconnects to remove power from the 480v control circuit first. Somewhere, somehow after the control is turned off, we still have 277 on one side, turn next off, it drops another 20v or so, then again and again until power is gone. . .

This sounds like another example of the ever-elusive phantom voltage. Have you tried this with a solenoid or other low-impedance tester?

 

[iwire]

This sounds like another example of the ever-elusive phantom voltage. Have you tried this with a solenoid or other low-impedance tester?

I agree, no doubt that is at least part of the problem.

 

[Rick Christopherson]

If it is dropping in small increments as you cut out various sources/devices, have you considered that this is a back-feed problem from one or more devices?

 

[ptonsparky]

All of the above are part of the equation. The guys should have made at least a partial schematic by last night. We do know that each remote pilot light does come on very dim if the MCP for that motor is off. Handy, but we don't know if that was by design or accident.

 

[ptonsparky]

If it is dropping in small increments as you cut out various sources/devices, have you considered that this is a back-feed problem from one or more devices?

Back feed is from what results in a seriesed pilot light & coil from each disconnect. Control circuit uses B & C phases. They also pick up B phase @ each starter for the coil via overload contact. Turning the control disconnect off allows the still hot B phase from each starter to feed back thru the coil & pilot light and raise the voltage @ B phase control fuse. Each starter adds a little voltage.

Amazing what a guy can figure out with a schematic. It also revealed a SP full of other problems.

 

[IMM_Doctor]

480v control circuit

480v control circuit

480 volt coil motor starters are typically only used when the motor is a stand-alone application, and the control circuit does not leave the motor starter enclosure.

If you have remote control (start / stop / and pilot lights), you may want to look at NFPA-79 9.1 Control Circuits

Sounds to me that the initial installation was of poor design, or unqualified persons added something after-the-fact.

Lucky, only your eye was twitching.

 

[ptonsparky]

You might say it is poor design or creative control work. More than just control is wrong.

Looks like we may to get to update the whole thing, from POCO transformer bank in. Dang. Like brand new. That means some PLC and HMI work. Double dang.