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Joined: May 2005
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G
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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?


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Joined: Dec 2000
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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.





Joined: Jan 2005
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Cat Servant
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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.

Joined: Mar 2003
Posts: 849
Y
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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.

Joined: Jul 2004
Posts: 787
L
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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

Joined: Mar 2003
Posts: 849
Y
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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.)


Last edited by Yoopersup; 04/13/09 03:31 PM.
Joined: Jan 2004
Posts: 1,507
G
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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.


George Little
Joined: Dec 2000
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Thanks for the Code reference, Yoopersup.

I would never have found it!



Joined: Nov 2001
Posts: 75
B
Member
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.

Joined: Jun 2007
Posts: 301
J
Member
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.

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