Safey issue with motors & VFDs

[jstolaruk]

Another usefull document when using AB PowerFlex VFDs.

Interfacing AC Drives with Safety Relays.

 

[Jasonc24]

rdrast is correct in his assessment of the problem with the instantaneous and time delayed safety outputs.

What is being described here is referred to as a Category 1 stop as per NFPA 79 (section 9.2.2, 2002 edition). This is a controlled stop, where power is being used to control the stopping action, then having power removed after the safe stopping of the system.

I also agree with rdrast that I never put equipment in the field that does not have an isolation contactor between the drive and the motor. The only way that I would not do that is if the drive manufacturer can supply documentation indicating the drive will operate at a level of safety without the isolation contactor that satisfies your risk assessment. (Do you have one of those?)

Of course, with a 10 second wind down, I do not see a way around a solenoid locking safety switch (power to open, prefferably). The time delay output on the safety circuit is good, but a zero speed detector is better. This will be more reliable over the long haul.

Using a brake motor for this application is not the best idea from my expereince. I typically try to use brake motors for "holding" loads, not stopping them. Using them to stop the load will significantly reduce the longevity of the motor (also braking time will vary, see note above). If you are anticipating this happening only ocassionally, then it probably isn't a problem. If this is something that is going to happen more frequently, then you may need to consider the longevity of the system. Additionally, in some applications you want the motor to spin freely with the power off (like spindle applications, so that you can tram stuff). With a brake motor, this is more difficult.