AC Drive + Hard Wired Motor Starters

[mjkoc518]

Now I know from hearing through the grap vine that hooking up a hardwired Motor overload with a AC VFD is no good. I am not sure why you would do this being that pretty much all VFD are equiped with internal OLs, but that is a different story. Anyways, what is the reason to why this shouldn't be done? Also is their any documentation that would explain this?

Thanks

 

[Lancie1]

Mjkoc,

Many VFD manuals warn against putting any type of disconecting device downstream of the VFD output terminals, between the drive and the motor, UNLESS it is also interlocked to FIRST disable the "drive run" command. The reason is that if the output current is suddenly interrupted, it will have nowhere to go and may fry the drive electronics.

If you have got to do it and you can't figure out how to do the interlock, then it is better to put the overload or disconnect on the incomming power line to disconnect power to the drive.

 

[rdrast]

Are you asking about a disconnect device? Or simply an overload relay?

Overload relay's are fine, and necessary in the event that multiple motors are driven off of a single drive.

Contactors, or overloads that can trip and interrupt the motor circuit are generally not advised, unless they are interlocked to the drive to immediately kill the output (Generally by opening the power to the firing circuits).

That being said, on Common bus drives, well, any drive actually, I do usually insist on a contactor before the motor. Just so that when the drive is disabled, the motor leads are physically disconnected from the capacitive-coupled bus voltage... Not all electricians (or mechanics) will properly lock/tag out a drive, assuming that hitting ESTOP is enough :/

 

[Sheldn]

External OLs are not required with VFD capabilities. If you do put them in, wire the overload NC contact into the VFD "Enable" circuit. When the OL trips opening up the OL contact, the drive will shut down the output cleanly and as the manufacturer intended.

 

[Jim Dungar]

It is not unusual to have a single drive feed several smaller motors, in this case each of the motors needs its own OLR.

 

[DickDV]

All the above is good advice. Just for a little background, before 1996, UL required an overload block with the control contact wired back to an External Fault loop on the drive to keep the system in compliance. That is no longer the case on single motor on one drive systems.

Since the subject of multiple motors on one drive came up, you do need the OL blocks in each set of motor leads and that is all you must have until you get to four motors on a single drive. At that point, I always recommend adding a fuse set or fused disconnect to each motor leg since the ampacity of the drive can begin to exceed the capacity of each motor lead to handle short-circuit current.

 

[JesperMP]

Also remember that a regular OL relay cannot protect the motor correctly when not running on the nominal frequency. The OL may trip prematurely or too late to protect the motor.

So, if possible, it is a good idea to ditch the OL relay and use PTC sensors (thermistors) in the windings to protect the motor.
Many VFDs have a PTC input for just that purpose.
And when they have they are normally also certified to replace the normal OL relay.
Problem is if it is an existing motor without PTC sensors.

You still have to have shortcircuit protection of course. I would put it before the VFD so that the entire circuit is protected.

With multiple motors you would have to have an external PTC relay as well as shortcircuit protection for each motor.

 

[DickDV]

That's good advice, Jesper. PTC thermistors or, more often, thermostats like Klixons, are always preferred regardless of how many motors are involved.

But, as you said, you're having a good day when the motors come equipt that way!