Motor overload Contacts

[BPoindexter]

I had a question come up that I promised the guys I would try to find the answer. Why do motor starters interrupt on the neutral side of the contactor coil? Obviously this would be a cse of 120VAC control. Wondering if there is a specific reason or has it just always been this way? Anyone know?

Thanks!!

 

[Dennis Alwon]

That's a new one on me.. I would have thought they would switch the hot conductor but the manufacturers are not guided by the NEC. I am assuming you are talking a factory contactor with an On/Off built in.

 

[augie47]

The Overload contacts are often factory wired in the "neutral circuit" for a 120v coil.
No idea why.

 

[petersonra]

There is a specific exception written into the NEC and UL508a to allow it.

I suspect it goes back to NEMA contactor days. They came with the NO aux contact wired to one side of the coil and the NC overload contact wired to the other. maybe to save a terminal in the starter assembly.

 

[fmtjfw]

Tradition, with a reason.

Tradition, with a reason.

The area between the contactor coil and the neutral is called the "sacred" area. The use of overload relay contacts between the coil and the neutral is to reduce the possibility that some dufus will miswire the controls and somehow bypass the NC overload contacts.

By having the "complex" wiring between the hot control and the other end of the cable you segregate the two functions.

 

[jim dungar]

There are many reasons for the location of the OLR contacts on the 'grounded' side of the coil, among them:

It simplifies the wiring in situations where there are mechanically and electrically interlocked devices, such as reversing and multi-speed starters

The OLR contacts are not very robust (they have very little fault handling capability). By putting them after the coil, but still within the contactor, a control circuit fault in 'field' wiring, or the coil, would likely not destroy the contact.

Because this is the way that the automotives (JIC standard practice) did it many decades ago.

http://static.schneider-electric.us/docs/Motor Control/Overload Relays-NEMA Style/9065PD9301.pdf
page 33 of file, page 20 of bulletin