ECN Forum
Posted By: Helectric Motor OL's in series - 04/07/09 08:36 PM
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?
Posted By: Yoopersup Re: Motor OL's in series - 04/07/09 09:06 PM
No
Posted By: Helectric Re: Motor OL's in series - 04/08/09 03:27 PM
Thanks for the reply Yoopersup. Can I ask the reason why?
Posted By: Yoopersup Re: Motor OL's in series - 04/11/09 07:06 PM
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.
Posted By: George Little Re: Motor OL's in series - 04/11/09 08:47 PM
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?
Posted By: renosteinke Re: Motor OL's in series - 04/12/09 12:31 AM
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.
Posted By: Obsaleet Re: Motor OL's in series - 04/12/09 02:02 PM
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
Posted By: George Little Re: Motor OL's in series - 04/12/09 06:55 PM
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.
Posted By: Helectric Re: Motor OL's in series - 04/13/09 06:31 AM
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).
Posted By: George Little Re: Motor OL's in series - 04/13/09 12:19 PM
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.
Posted By: ghost307 Re: Motor OL's in series - 04/13/09 02:46 PM
It sounds like somewhere in this whole lashup you'd be connecting the neutrals for the 2 motors' separate circuits. This would not be a good idea if they are on the same phase, and probably not even if they are on different phases.

I'd just tie an auxiliary contact for each starter into the control circuit on the other one, so if either motor shuts down its partner would shut down as well. Wouldn't that work for your installation?
Posted By: electure Re: Motor OL's in series - 04/13/09 03:32 PM
I've never liked the fact that the factory wiring on motor starters breaks the other (grounded neutral in this case) side of the control circuit.

A ground fault in the wiring between the o/l contacts to the coils could render the Overloads inoperative in this case.

A very simple fix would be to place the o/l contacts on the line side of the control circuits via the jumpers, and run the neutral straight to the coils.




Posted By: renosteinke Re: Motor OL's in series - 04/13/09 04:39 PM
Helectric ... remember what I said about loads? The coils are loads. For a pair of (for example) 120v coils to operate properly in series, you would need to supply the circuit with 240v.

You avoid this by running the feed to the coils through an auxilliary contact on each of the starters. You can usually mount another contact next to the N.O. that comes with the starter. Contacts are, of course, switches - so there's no problem placing them in series. Contacts in series, coils in parallel.
Posted By: Yoopersup Re: Motor OL's in series - 04/13/09 05:07 PM
If you change the coil thru ungrounded conductor you"d void the listing & labeling of the starter, BUT this has been discussed for years. You Can easily program the PLC that when mtr drops out on overload other one does same.
Posted By: LarryC Re: Motor OL's in series - 04/13/09 06:29 PM
Quote
You Can easily program the PLC that when mtr drops out on overload other one does same.


How will the PLC know that the motor drops out on overload?

Larry C
Posted By: Yoopersup Re: Motor OL's in series - 04/13/09 08:26 PM
2005 N.E.C. 430.73
Where one side of the motor control circuit is grounded, the
motor control circuit SHALL be arrainged so that a Accidental ground in the control circuit remote from the motor controller will
(1) not start the motor and ,
(2) Not bypass manually operated shutdown device or automatic safety shutdown devices.

If you use the neutral on a control circuit an unintentional grounding can start the equipment.

Neutral CANNOT be used for control circuits.

430.31 (Specifies intended uses of Overload devices.)

Posted By: George Little Re: Motor OL's in series - 04/13/09 08:56 PM
Good code references Ernie, I think we are back to some form of interlocks with pneumatic timers that will allow a person to start a single motor and keep it running until the timer runs out or until the second motor is running. As it is described, if both motors are off, and they are on separate starting circuits, you would not be able to start either one because the alternate motor interlock would not allow it to start. Yep, pneumatic timers would work. The PLC thing would/could work but I know they want to break the neutral improperly.
Posted By: electure Re: Motor OL's in series - 04/13/09 10:53 PM

Thanks for the Code reference, Yoopersup.

I would never have found it!


Posted By: Bill39 Re: Motor OL's in series - 04/14/09 02:29 PM
Originally Posted by LarryC
Quote
You Can easily program the PLC that when mtr drops out on overload other one does same.


How will the PLC know that the motor drops out on overload?

Larry C


Most IEC-style motor starters have a built OL contact to use as feedback for alarms or to a PLC. Most NEMA-style starters can have one added for the same purpose. NOTE: I am NOT referring to the NC OL contacts usually factory wired in series with the coil.

Wire the OL contact from each starter as PLC inputs and change the program to drop out the coils.
Posted By: JValdes Re: Motor OL's in series - 04/23/09 07:13 PM
You can wire both OLR's in series if you want. Neutral or hot. The overload contacts (switches) are not loads and will not effect voltage by any means. It is common everyday procedure for your application. One motor trips, it kills the other motor. (both contactor coils drop out).

Since you have a PLC, I would write this into the logic too. This is called a fail safe. Should either device fail to stop both motors you are backed up with the hard wiring and the PLC logic. Most OLR's have more than one set of contacts. If not, you can get contacts for them. Use the NO or NC contact on the overload as a PLC input. Should the OLR trip, the input will turn on and kill both motors. You can use both of these methods or use either or.
Posted By: Yoopersup Re: Motor OL's in series - 04/23/09 11:03 PM
Pushbuttons, Limit switches, ect are Not loads either But still CANNOT be in the neutral circuit.
Writeing it in the PLC circuit would be the way to go.
Posted By: renosteinke Re: Motor OL's in series - 04/24/09 01:05 AM
I think I see the problem a bit more clearly ... after taking apart a motor starter.

The key thing to understand is that mpotor overloads, while 'switches,' are actually on the neutral side of the coil. They are the only item positioned on the neutral. In this instance, we most certainly 'switch the neutral.'

Mind you, they are also located right next to the coil. Indeed, they are usually physically attached to the contactor assembly.

With that in mind, I have to conclude: ANY connection involving the contactor coils MUST be on the line side of the coil. Therefore, you'd have to use that auxiliary contact to link the starters together.
© ECN Electrical Forums