Motor Overload Relay Resetting Opinions

[SSendra]

I typically use motor AC drive to control a AC motor. I have an application that I do not need a AC drive, but I am using an overload relay. My question is how do people in the industrial or similar industry typically deal with the motor overload resetting. Do you require the overload to be reset and the machine to be restarted to restart the motor. According to NEC 430 section you can allow a motor to overload and reset automatically if there will be no hazard that will exist. On the overload relays that I was looking at it has a reset button that must be manually reset, but the motor will start immediately when the reset is pressed (is this automatically starting not acceptable?) unless the monitor the extra auxiliary contact is used to stop the motor control (relay, contactor) by control the relay or monitoring through PLC.

According to NFPA 79 (Industrial machinery) section 7.3.2 it states "Resetting of the overload device shall not restart the motor." I do not understand how this can be met when some motors have a thermal overload built into the motor that reset automatically.

So my question is do you typically use a motor overload with all motors and monitor a auxiliary contact of the motor overload. So when the motor overload trips it will stop the motor control (relay, contactor). I am trying to look to see what is the industry standard?

Any help will be great appreciated.

 

[itsmoked]

I would require human intervention to reset the protection. Obviously overloading to a trip point is an abnormal event (or certainly should be). A human should be notified of the trip. Forcing a human to reset it means someone has been informed of the abnormal event.

Having to open a live panel to perform the reset... is unacceptable. It either needs an external reset button or the entire panel should be brought down (powered off), opened, reset, panel closed, powered-up.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Look up the term '3 wire control scheme" to see how you use the Thermal Overload Relay to shut down the motor starter and require a manual Start command to re-initiate it.

"Will work for (the memory of) salami"

 

[hooverdale]

Just connect the NC contact of your overload relay at the very start of your control circuit. Once the overload trips and your NC opens it will break the circuit thus unlatching the contact that holds the contactor. That way resetting the trip button won't start the motor unless you manually press start.

 

[powerjunx]

i agree to itsmoked.


"..the more, the merrier" Genghis Khan

 

[squeeky]

Standard circuits exist for this and have been described above but the system has to be implimented. The machine should be able to be locked out in the safe state. When the isolator is turned off, the motor stops. You can now reset the overload safely. But you may need to test the circuit so there is a method where a functional test button can be fitted to the circuit to bridge out the N/O auxillary on the isolator that open circuits the control voltage when off. This is an emegency stay put push button that must be unlatched to activate the circuit. This is a conciuos decission. When complete you push it in again. If you forget, as you close the door it will reset closed by the door as it is fitted on the inside near the hinge.This for fills the NFPA requirement for arc flashes etc.

You then need to look at the application. Emergency pumps to empty tanks where there is a danger of engulfment or boiler feed water pumps may require an auto restart due to the fact that if they don't the risk of it not running exceeds the fact that it has to be manually reset. Signs stating that it may automatically restsrt need to be in place just like air compressors that turn on and off due to air pressure.

Train the staff. Do a small test for competancy. Get them to sign Annual reinforcement, again with signatures.

 

[automatic2]

yes it is industry standards to monitor the aux contacts, and break your latch logic accordingly.