ECN Forum Forums Code Related Discussion NEC & other Code issues Motor OL's in series

I have two 1/2 hp motors that run seperate augers. Each motor has it's own motor starter & O.L. relay. Each motor starter is controlled by a dedicated output from a PLC. I need to change the current arrangemnt so that if one motor trips out on O.L. the other motor shuts off as well. Can I connect the neutrals for the two motor starters in series through the O.L. relay contacts?

No

Thanks for the reply Yoopersup. Can I ask the reason why?

You"d be using the Grounded conductor as a control switching
conductor. Now if the Overload on one motor trips out a relay that trips out the other motor yer fine.

I agree with you Ernie (Youpersup) and if both augers need to run at the same time, then he could put a relay in each of the two circuits in parallel with each motor with NO contacts that would stop the opposite motor when the first motor trips out on OL.

Hey Ernie- You going to Crystal Mountain?

I'm a biy confused here .... maybe it's just the vocabulary ... but here's what it comes down to:

Is a motor overloads a load or a switch?

If they're switches, well, we put switches in series all the time - and there's no neutral wire. Just 'switched legs.'

If they're loads themselves, then putting them in series is a bad idea; code issues aside, Ohm's law tells us the voltage across each load will only be half of what it needs to be.

Putting the overloads in series seems to accomplish exactly the same thing you would accomplish by using an additional auxiliary contact on one of the starters - with the difference that now the two coils could be on different circuits, or different voltages.

Putting overloads in series doesn't sound like a very good idea, as they are designed to fail via load if you combine they load one will fail. The above solution of using the control circuit is a much wiser choice.

Ob

Reno- A motor overload is a device in the circuit that is sensitive to current flow. To be sensitive to the current of the individual motors they would have to be in the feed to each motor. If you are talking about combining the two motors and installing one overload sized to match the total load of both motors- well I guess that would be okay but only if both motors were running at the same time. That would be a design issue. I do think that each motor should have it's own independent motor overload based on 430.32 if these are field wired motors. If an engineer has designed this "machine" to use only one overload then that's another story.

Reno- Hope I've not insulted your knowledge in any way because I get the impression you have a good education and I respect your comments on all of your post.

Having say two overloads in series makes no sense to me. As to your question about whether they are switches or loads, I'd say neither and possibly identify them as "fuseable links" of a sort.

I'm sticking with my previous post about using interlocking relays.

Thanks for the response guys! It seems that I didn't explain myself well enough so I'll try again. I'm working with two seperate motor starter/OL relay combinations. Each seperate starter has it's own OL relay that the three phase motor current runs through. When the motor current rises above the OL trip setting the OL relay trips and opens a set of contacts on that OL relay that are connected in series with the neutral conductor for that motor starter's coil. This is factory wiring and is typical of all factory MCC and combination motor starter wiring that I've ever seen so I assume that this is code compliant. What I'm asking is if anyone sees any problem (code or otherwise)with connecting the neutral conductor for two of these motor OL relays in series with each other (M.S. coil neutrals only not motor phase conductors).

Helectric- I guess I'm seeing a problem when you start putting relays in series when I don't see the diagram and understand the consequences of fusing and actual load in that circuit. As for the "neutral" switching, it's done all the time in the motor control circuit world. I used to do a lot of motor control work and have a rather good command of trouble shooting control circuits and ladder diagrams. I would not feel good telling anyone to put things in series without seeing then entire circuit and analyzing the effect. Sorry.