VFD start current

[Mila15]

I just realized that the vendor is using a thermal-magnetic breaker to feed a VFD/Motor application rated at slightly above 125% FLA. Looking at motor starting curve Vs breaker curve, it seems pretty tight. My question is, what would be the maximum starting current drawn by a 753 Powelflex? Could that ever be as large as the motor starting current? I'm at a position where I can use a larger size breaker/cable, but wanted to know if that's even a concern. Thanks in advance for your help!

 

[jraef]

The maximum that drive can put out, assuming a Heavy Duty rating, is going to be 150% current for 60 seconds, 200% for 3 seconds. That breaker should not trip on that.

Motor current, when frequency is controlled WITH voltage, can be very significantly lower than if started Across-the-line (DOL), in fact the VFD is able to make the motor put out 100% FLT at 100% FLC, no other starting method can accomplish that. VFDs are quite frequently used as glorified soft starters for that very reason.

"Will work for (the memory of) salami"

 

[ScottyUK]

You don't state the drive size, but if it is a small one make sure that it either has a soft pre-charge for the DC link or that your breaker can withstand the very short inrush as the link capacitors charge up. Medium and large drives almost always have a soft pre-charge.

 

[Mila15]

Thanks all! below are drive/motor/breaker info:
VFD: Powerflex 753 rated for 40 HP motor (52 A)
Motor: 40 HP, 46 FLA, 306 LRA
Breaker: Type HFD 3070, 70A

The AB representative confirmed jraef's statement: max of 200% of FLA during motor start, which would not trip breaker even if motor start lasted for more than 10 seconds. Are there any good documentation I can read regarding this operation?

 

[jraef]

Yes there is a soft pre-charge on that drive.

Lots of learning resources on VFDs out there, try searching on "VFD tutorial".

"Will work for (the memory of) salami"

 

[Mila15]

Thank you guys for the help!