Double DO Motor Control


Thread Starter



Normally i am using single output to start and stop a motor from PLC. When the relay is energized, the motor starts, when the relay deenergized, the motor stops.

But my customer requested 2 DO (start and stop) to control motors. And it should be momentary. I used NO contact of relay for Start, and NC contact of relay to stop. So the 1-2 second pulses can start and stop the motor.

The problem is, if the PLC or the relays fail, the motor can not be stopped and keeps running.

I am thinking to use NO contact for Stop relay too, but then it will not be momentary.

Any comments?

Thank you,
though you have not mentioned but it seems that, the motor is of high KW (>75Kw), and it is breaker fed not a contactor fed.

In this case use the anti-pumping method to start stop. for details google it. you can provide us some more details for better response.

Normally Double DO Motor Control requirement is from the type of breaker selected by Electrical Engineering team. any how to your query STOP relay to be kept normally energized through logic and NO to be wired to electrical breaker. This will help in case of PLC failure protection. For START relay, NO to be wired and by logic pulse (to be tuned as per the site condition) to be issued for proper operation.