Do feeders going to a VFD have to go through a motor contactor?

[bdn2004]

We are installing 3 small vfds to three small metering pumps. I noticed in the info that comes with the vfd they are showing the line side of the vfd fed through a motor contactor.
I can't find that requirement in the NEC...430-128. Something I'm missing here?

 

[dpc]

Contactors are not required by the NEC on the line side of the VFD. You will need overcurrent protection and a disconnecting means within sight of the VFDs.

 

[bdn2004]

I’ll answer my own question. Finding later in the literature the Vendor says this is a recommendation.

For the reason:
“It is recommended to add a magnetic contactor in the power input wiring to cut off the power quickly and reduce malfunction when activating the protection function of the AC motor drives. Both ends of the magnetic contactor should have an R-C surge absorber. “

It also says “Please do not use a Magnetic contactor as the I/O switch of the AC motor drive, as it will reduce the operating life cycle of the AC drive.”

 

[jraef]

The way they word that is no accident by the way. It's NOT a good idea to put them on the line side either because every time you energize the drive, there is a surge in current to charge the capacitors that could trip the breaker or blow fuses. To prevent that, there is what is called a "pre-charge" circuit that puts a resistor in series with the capacitors to limit current. But that resistor cannot be left in the circuit, so something has to be done about it. Large drives use a contactor to short it out. Small drive used to, but everyone wanted them smaller and cheaper. So now they use an NTC thermistor. it's a type of resistor that looses resistance as it heats up. So when cold, it has high resistance for a second or two, then drops to nearly nothing when it gets hot. If you turn the VFD on and off constantly with the line contactor, you stress that pre-charge circuit and cause it to fail sooner than it should. That's why they said "when activating the protection function of the AC motor drives", they mean for you to leave it on ALL the time, excpet when the drive malfunctions.

You will not see this recommendation in a good quality drive by the way... it's a CYA statement.

"Will work for (the memory of) salami"

 

[bdn2004]

jraef,

I'm not understanding your comment...not a good idea to put what on the line side? the RC surge absorbers?

Looking at their recommended circuit they show the starter coil of the magnetic motor contactor connected to an alarm contact on the VFD. When the VFD trips it removes the 480V source power. The RC absorber is around the starter coil. What's wrong with this setup?

Reading further it says that the high voltage will stay on the output of the drive for 10 minutes due to the internal capacitor. I don't recall ever seeing warning stickers on VFDs (maybe they are there, I just don't recall) but that doesn't seem too safe without them. It also says that it's not a good idea to shut off the drive, it would be better to take it down to zero speed. Is this what you are explaining in your post?

 

[jraef]

...not a good idea to put what on the line side? the RC surge absorbers?

Contactors. Putting contactors ahead of a VFD and using them to turn the VFD off when not in use will shorten the life of the VFD.

"Will work for (the memory of) salami"