UL508 Motor Disconnect SCCR After VFD

[herding_cats]

Adding MOCPD on the LINE side is fine. Fuses, disconnects, breaker, whatever. Putting a fuse or disconnect on the LOAD side of the drive creates the potential to destroy the bus on the drive. There is no NEC or UL regulation or code requirement that I know of. Been a YASKAWA dealer for 10+ years. Never heard of it.

 

[EC Dan]

With all of this said, I'm still a bit hazy on how I can put forth an argument to my UL inspector where I can state that I do not need to install the recommended ampere rated Class J fuses ahead of the motor disconnect if it is used after a VFD.

SB4.2.1 Exception No. 5: Components installed on the load side of a variable-speed drive where the variable speed drive has built-in electronic short circuit protection, are not required to have a short circuit current rating.

 

[Jraef]

SB4.2.1 Exception No. 5: Components installed on the load side of a variable-speed drive where the variable speed drive has built-in electronic short circuit protection, are not required to have a short circuit current rating.

Boom… there you go!

 

[petersonra]

Look closely at the supplement in UL508a that tells you how to calculate the SCCR of the control panel. There is a note or exception in there that covers equipment downstream of a vfd.

 

[JovialBulge]

Thank you all for the advice and answers here!

 

[Knightryder12]

Absolutely. Disconnecting the load side when energized will kill the bus in the drive. NEVER should you put any disconnect on the LOAD side of a drive.

The newer drives now have current sensors in them and will shut off if no load is detected. At least that is how it was explained to me. On the older drives, if we wanted a disconnect ahead of the VFD we had to put a set of contacts in the disco that sent a signal to the VFD to drop out.
There are many scenarios where you have to put a disco ahead of a drive so its not that big of a deal.

 

[Jraef]

The newer drives now have current sensors in them and will shut off if no load is detected. At least that is how it was explained to me. On the older drives, if we wanted a disconnect ahead of the VFD we had to put a set of contacts in the disco that sent a signal to the VFD to drop out.
There are many scenarios where you have to put a disco ahead of a drive so its not that big of a deal.

How it works is that drives mostly now use SVC (Sensorless Vector Control) as the method of controlling the motor. That uses current sensing to approximate the relative rotor shaft position so that it can calculate the correct values of current to allow to the motor (oversimplified). So when the motor is not connected, it sees no feedback and shuts down. But that takes WAY too long to protect the transistors of the VFD from the DV/Dt that happens when you open a disconnect or contactor on the load side. So if someone told you that, they didn’t know what they were talking about.

There has never been an issue with opening a disconnect on the line side. There is an admonition against using a CONTACTOR on the line side, but that’s for a totally different reason.