Motor control circuit wired through NC contacts of overload relay.

[Dsg319]

What is the reason behind this? I know it has to be obvious and shooting right over my head.

Thanks.

 

[tom baker]

so the coil drops out on an overload

 

[Dsg319]

so the coil drops out on an overload

Even if the coil on the contactor didn’t drop out and remained energized (as in not wired through the NC on overload relay) wouldn’t the overloads still be in the open position in the even of an overload?

 

[don_resqcapt19]

Even if the coil on the contactor didn’t drop out and remained energized (as in not wired through the NC on overload relay) wouldn’t the overloads still be in the open position in the even of an overload?

The typical overload relay does not open the power circuit to the motor. It opens the control circuit to the contactor coil and that opens the power circuit.

 

[hillbilly1]

Even if the coil on the contactor didn’t drop out and remained energized (as in not wired through the NC on overload relay) wouldn’t the overloads still be in the open position in the even of an overload?

The overloads don’t work that way, they are thermal in the old fashioned ones, they heat a solder pot, which allows the cog to turn, releasing the contact. The magnetic ones are electronic, and signals the relay to open. Both types do not physically interrupt the line voltage to the motor, only through the starter coil which drops out the motor contacts.

 

[Dsg319]

The typical overload relay does not open the power circuit to the motor. It opens the control circuit to the contactor coil and that opens the power circuit.

Gotcha! Another good reason I just thought of is if you had a status light or input back to a PLC through the aux contacts on the contactor. It would show as motor still in operation even if it wasn’t.

 

[ptonsparky]

The contacts open. How you wire them will determine what happens next. Typically it opens the coil circuit to the motor starter.

 

[ptonsparky]

We use starter aux contacts as inputs. They open whenever the coil drops out. Be it coil failure or overload operation.

 

[Dsg319]

We use starter aux contacts as inputs. They open whenever the coil drops out. Be it coil failure or overload operation.

I had just done that for the first time this summer on a job with some pump operations.