VFD (powerflex 525) Detection of an open circuit on it's load side

[ellcap]

Less concerned about the specifics of the powerflex, more curious if a circuit had a disconnect on a motor that was separate of the VFD, is there a method that a VFD would use to detect that without trying to run it and not seeing any load?

I recognize I'd see it in the current/torque data if I was trying to run the drive w/o the motor circuit connected but what about if it's just sitting there and Greg from accounting opens a disconnect on a pump that may need to run later and I want to know now.

 

[controlconcepts]

Any Disco that is after the VFD should have an aux contact in it to send a signal that it is open and not prevent a run with it open.

 

[Ken Roach]

PLC output modules can do open-wire detection by sending very short pulses that the load device conducts to the voltage common, and monitoring that very small current flow.

To do so on a VFD would require not just another very sensitive circuit that would need to be isolated during power output, but wire-to-wire detection. It's not *impossible* to do, but the number of users who would need it is very small.

The conventional solution for detecting an open load-side disconnect is an auxiliary contact in the disconnect assembly, wired to a general-purpose discrete Input on the drive or a PLC.

 

[Saffa]

but what about if it's just sitting there and Greg from accounting opens a disconnect on a pump that may need to run later and I want to know now.

Bloody Accountants. Also be careful of Karen from HR.

As Ken says, the auxiliary in the local isolator is the way to go. We buy VSD specific ones with an early-break auxiliary and wire that to either the Safe-Torque Off input or a standard input configured for free-wheel stop. That way the drive turns off the IGBTs before the main contacts open, which should result in a longer drive life if some numpty is opening the isolator under load.

As all our drives are on comms now I just read the status of the STO or that fast stop input and use that accordingly to display "Isolated" on SCADA.

 

[ellcap]

Incredibly helpful feedback. Thank you to all.

 

[jraef]

Any Disco that is after the VFD should have an aux contact in it to send a signal that it is open and not prevent a run with it open.

+1 on this.

But also, IF a 525 drive is in “SVC” or any other Vector Control mode, the drive will look for the feedback from the motor and fault on load loss when it doesn’t get that. But if it is in V/Hz (Scalar) control mode, it will not.

 

[Jeev]

The PF525 actually has this built in. Check out A490 and A491. It just monitors the current for a period of time. For example if the current is less than 0.1A for 1s then it will consider it a Load Loss Fault.