Contactors in Series, Solved but wanna see how other minds would look at it.

[TwoBlocked]

Troubleshooting the problem took longer than I think it should and am interested in how others might have gone about it in order to try a different mindset next time. I fully expect that 19 out 23 replies will be in the category of Monday morning quarterbacking or design changes. Hopefully I can glean something useful from that 5 out of 23 more useful replies. (And what do 17, 23, and 5 have in common?)

The situation was a 5 HP, 480V, 3-ph induction motor running a pump that would run to fill a 500 gallon (?) tanks with some liquid (doesn't matter what). There was a remote panel fed from a distribution panel, and a local panel fed from the remote panel. (Actually they were only a few feet away from each other.) In each panel there was a contactor. The one in the remote panel had a 24VDC coil and overload relay that would be energized either by hardwired level sensors or by a "bypass" (hand) switch. The contactor in the local panel also had an overload relay, but a 480VAC coil. And there was an on/off selector switch on the local panel wired in series with the OL relay. The power for the coil came from the line power provided by the remote contactor.

To sum up, the two contactors were in series. The first was controlled by a 24VDC circuit with two modes; level control and "bypass" (hand). The second contactor was controlled by receiving line voltage from the first contactor that could be interrupted with a selector switch.

So what I am asking is for you fine folks to read the following, word for word, brief, service report and at each step consider what your NEXT step would have been and WHY. I am looking for different mindsets so I might have additional tools to work with. No fair saying what you would have done AFTER finding out what was wrong. (I can do that myself...)

"Motor/pump not running in either auto or bypass mode. Checked incoming voltage and control voltage - ok. Checked motor for open, shorts, and grounds - ok. Manually pushed local contactor in, resulting in loud hum and no rotation. Suspected a jammed pump. Dismantled pump, rotor turned free. Again manually pushed local contactor in, resulting in loud hum and no rotation. Checked motor connections in jbox - ok. Inspected and bench tested local and remote contactors with meter - ok. Re-installed remote contactor and tested while energized. Determined A-phase contacts not closing when coil was energized, but would close when operated manually. This was the cause of the local contactor not pulling in electrically and the motor single phasing. Provided customer with ordering info for remote contactor."

 

[ptonsparky]

Remove motor leads. Ohm or continuity check. Meg if suspicious. Leave disconnected.
Activate 480v starter, check load side voltage.
Check line side voltage at contactor. Activate. Check voltage.
Humming motor should have been first clue for single phasing.

 

[TwoBlocked]

Remove motor leads. Ohm or continuity check. Meg if suspicious. Leave disconnected.
Activate 480v starter, check load side voltage.
Check line side voltage at contactor. Activate. Check voltage.
Humming motor should have been first clue for single phasing.

Thanks, why remove motor leads for meter check?

 

[ptonsparky]

Thanks, why remove motor leads for meter check?

You could leave them on the contactor for continuity check but if you know it’s humming already, disconnect it prior to applying voltage and lessen the risk of smoke escaping.

 

[TwoBlocked]

Humming can also mean a stalled motor.

 

[LarryFine]

Humming can also mean a stalled motor.

Or a poor memory.

Best way to check fuses, switches, contactors, etc., is to measure across them (i.e., line-to-load).

You're looking for voltage between points where there shouldn't be any, a.k.a. fall-of-potential.

 

[ptonsparky]

Humming can also mean a stalled motor.

It was stalled, but removing it prior to checking for phase loss was wasted effort.
IEC stuff excluded, most starters and contactors have contacts that are fairly easily accessible.

 

[ptonsparky]

You do not want to do a FOP test on a motor that is locked rotor or single phasing.
You need a meter that reads mV.

 

[TwoBlocked]

What do you folks think of the "mindset" of non-voltage testing, like securing power and using an ohm meter to test contacts by manually pushing the starter in? It is "mindsets" that I'm really interested in here.

 

[ptonsparky]

Manually pushing it in did not work for you, if I read it correctly.

 

[petersonra]

Personally, having never come across a "jammed" pump in the wild I would be inclined to focus on the control side of things.

I cannot imagine what would possess someone to come up with such a screwy setup in the first place though.

 

[TwoBlocked]

Personally, having never come across a "jammed" pump in the wild I would be inclined to focus on the control side of things.

I cannot imagine what would possess someone to come up with such a screwy setup in the first place though.

Adding to the screwyness, The local control box was over an oil pit. The door, with the on/off SS, swung toward the middle of it. I was straddling the pit, with my boots on top of the vertical steel sides, while troubleshooting. There was no grating. Yeah, I opted for non-voltage testing as much as possible.

 

[ptonsparky]

What do you folks think of the "mindset" of non-voltage testing, like securing power and using an ohm meter to test contacts by manually pushing the starter in? It is "mindsets" that I'm really interested in here.

How good is your ohm meter?
VD across two contacts of 400 millivolt is considered excessive.. A 9 amp load with .4 volt drop across the contacts shows about .044 ohm resistance. The contact next to it that shows .1 volt drop with 9 amps has about .011 ohms of resistance. Is your meter still good enough?

 

[ptonsparky]

A pallet across the pit would be better than nothing.

 

[garbo]

How good is your ohm meter?
VD across two contacts of 400 millivolt is considered excessive.. A 9 amp load with .4 volt drop across the contacts shows about .044 ohm resistance. The contact next to it that shows .1 volt drop with 9 amps has about .011 ohms of resistance. Is your meter still good enough?

I have PM'ed countless contactors & motor starters. Never trusted using an ohm meter only for testing contacts. Found best way is to measure the voltage drop and compare them. On maybe under 10 amps found that under 150 millivolts to be a satisfactory reading and 300 millivolts on much higher ampere contacts. On very worn contacts as they heat up resistance will greatly increase.On some of the newer VFD'S with bypass they have two of the three contactors behind a plate that you have to remove two screws then hinge plate back then lift it up off slots and hold it with one hand while trying to take measurements. One a 100 HP fire pump that I PM'ed had the cheapest contactors where you could not remove cover to inspect contacts. On yearly fire pump PM after inspection of contacts liked to give them a spray of contact cleaner. As far as using an ohm meter my $700 Fluke combination VOM / megger like a lot of digital ohm meters do not have anything to zero it out. Always touch the leads together and try to remember the resistance. As test leads get used the resistance would more the double. Used my meter to take hundreds of reading a month and never lasted more then two years.my

 

[TwoBlocked]

A pallet across the pit would be better than nothing.

Later, they did bring out a piece of grating. It was still an awkward place and I did not feel comfortable running all possible voltage tests.

 

[TwoBlocked]

How good is your ohm meter?
VD across two contacts of 400 millivolt is considered excessive.. A 9 amp load with .4 volt drop across the contacts shows about .044 ohm resistance. The contact next to it that shows .1 volt drop with 9 amps has about .011 ohms of resistance. Is your meter still good enough?

I have an industrial Fluke. Don't feel like going out to get the model. But does it matter? The problem was that when energized, the A-phase did not close, but did when operated manually.

But let me repeat the question that you replied to:

What do you folks think of the "mindset" of non-voltage testing, like securing power and using an ohm meter to test contacts by manually pushing the starter in? It is "mindsets" that I'm really interested in here.

 

[ptonsparky]

What caused the difference? Obviously pushing did not solve the problem.

 

[TwoBlocked]

What caused the difference? Obviously pushing did not solve the problem.

The contactor mechanism must have been sluggish. Energized electrically, only two contacts would close. But when operated manually they all would. It was an Eaton, where the operating lever moves sideways. When I realized what was going on, I could see that it did not travel all the way electrically.

 

[TwoBlocked]

That is, the remote contactor would not close completely electrically. The local contactor did not close electrically because the missing phase went to the coil. Closing the local contactor manually closed all three contacts, but there were only 2 phases.