It's a foolish man who relys on a signal from a motor contactor to say a machine is running

It's not the motor you need to check but the actual machine, typically by the use of a rotation sensor on a non-motorised shaft. ■

dear sganesh, designer,

I would say that you need both signals.

The motor contactor and the speedsensor.

When the motor contactor is switch on, then the speed sensor must reply within x seconds.

If the speed sensor stays zero, then the contactor is switched of again.

In this case the motor contactor auxiliary contact is telling the controll system that there is something wrong.

Motor contactor and speedsensor belong together

Both signals high or low means OK

Motor signal high and speed sensor signal low after a time delay means NOT OK

A matter of logics

have a nice day ■

Hi sganesh,

There is really no right or wrong, it all depends on what you want to do:

A typical OEM wants to save money and would typically only go with main contactor feedback. That would be the minimum in installation cost but could provide you with troubleshooting and operation headaches in case the motor keeps running and the machine it is supposed to drive is not because the drive train broke.

The middle of the road scenario would be designers proposal. It takes just ust a tad more thought and money but it would make troubleshooting and especially operations more fun.

The expert way would be Teuis' version. Here you could provide the maximum information straight into the operator's room without having to russle up technicians for the subsequent troubleshooting event. Because in involves more wiring (unless you have devices connected to a bus system like Profibus, DeviceNet, Ethernet,Fieldbus, ControlNet, Modbus etc) this is the most expensive way to do it.

It is always a good idea to ask yourself how critical the operation is and that may provide the reasoing to with one system over another. ■