Contactor before or after the VFD, or ?

[JesperMP]

Hi,

I would like to hear some of you experts on which method of disengaging a VFD is the best, in particular with respect to E-stop.

1. A contactor in front of the VFD.
Problem is that communication to the VFD will be lost when the contactor is opened (when the VFD control is powered from the mains supply - most are these days).
Also, I have heard that a long waiting time is required before reapplying power (is that really true, does it apply to some or all VFDs ?).

2. A contactor after the VFD.
Problem is that the VFD will experience high voltages on the output terminals if the contactor is opened while the VFD powers the motor.
Some counter this by sending an "inhibit" command to the VFD (from an aux contact on the same contactor), the "inhibit" input is then setup to let the VFD enter cost-to-stop mode thereby avoiding overvoltage alarms. Will this work with any VFD ?

3. No contactor at all.
No problem if no E-stop or safety stop is required (a completely enclosed pump or fan for example).
Some use the VFD to bring the motor to a controlled stop at E-stop.
Problem is that this approach is not allowed in the EU (is it really OK to do so in North America ?).

So, what are you guys doing ?
Are there regulations that has to be followed ?
Thanks for any info

 

[snoogie]

With respect to e-stop do this and you will be safe. Position the contactor on the line side of the drive, and screw the communications, if its e-stop its e-stop and after an e-stop you can expect to follow certain procedures to get the machine going again, and normally if the drive is the fault it will coast itself to a stop and give an alarm output that you use to "normal stop the line".
Take a contact to the drive that tells it to start stopping when the e-stop is activated, however do not pull the contactor out. The contactor is connected to the e-stop circuit via a timed e-stop contact that is set for "x" seconds (keep it relatively short, based on the drive decel ramp would be good) So the sequence is; the e-stop is triggered the drive gets told to start stopping, then after "x" seconds (which allows the drive to utilise regen stopping etc..) the timed e-stop contact opens the drive contactor and the drive coasts the rest of the way if not stopped already and is ultimately positivley disconnected from the supply. This has been the "standard" for the years i have been invovled in this type of application they make safety e-stop relays with timmed contacts to do this very thing.
I understand the frustration with this as the guidance from our suppliers, and regulations seems to be unsure with advent of AC drives but the above works in my field (metal processing.)

 

[rbulsara]

How about shunt tripping the breaker feeding the VFD via E-stop?

Contactor is like installing a starter..sounds cumbersome, takes up additional space and costly..

I would not worry too much about damaging drive etc. in case of a real emeergency....