Question on NO/NC contact for motor application 4

[budhiman]

In a motor protection application using relays, I have seen the trip circuit having a normally closed contact in the run circuit. What is the reasons to have normally closed contact in this application where is in normal feeder protection the relay contact is programmed as normally open? Is the reason to have normally closed in the motor to break the run circuit or is there a different reason as well? Also, when will it be good to use fail-safe contact ?

 

[cdill21]

Loads of great information in here. My uneducated guess is that OP was confusing the purpose of a control relay and an overload. They did say "programmed as normally open." Perhaps they were a PLC programmer getting their first look at an overload on paper.

 

[SilverfoxUK]

Need to give us a clue on the size of the motor.
The NO or NC action is determined when the switch is the unenergised state as if it was in your pocket.

For the smaller rated drives a contactor is energized that in turns directs the power to the motor. The emg stop, stop and overload devices would be NC and would open to break the supply to the contactor and hence motor. The contactor is latched in with a holding contact. This circuit is broken by an overload or stop button being pushed. To start the motor again ideally you should push a start button.

As things get larger you can't rely on springs and whatnot and you need to open and close the breaker, usually an HV device such as a VCB or OCB with a 110Vdc tripping / closing supply. This supply has to be very robust and well maintained. In this instance you would close a contact to trip the breaker.

 

[waross]

Another point that hasn't been mentioned is that 100% of magnetic motor starters use Normally Closed contacts as part of the overload relay.
Every normal circuit using an off the shelf, magnetic motor starter will have normally closed contacts to stop the motor when the overload relay operates.
When a motor starter is held in by a magnetic coil, the current to the coil must be interrupted to stop the motor.
Hence N/C contacts.

There are exceptions.
When motors reach the size that they are controlled by power circuit breakers, the power contacts are not held in by a magnetic coil but are latched mechanically.
Comparing a motor to a feeder circuit, you don't want the feeder breaker opening on a power failure.
Depending on the application you may want the motor breaker to open on a power failure.
Safety and staggered start are two possible reasons.

Bill
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"Why not the best?"
Jimmy Carter