VFD on step-up transformer problem

[Sahib]

Again, you are ASSuming.
A VFD is like an entirely new power source for the motor. All it needs from the line side is raw energy to convert to DC, everything else from that point on going to the motor is fabricated by the VFD, and the VFD can fabricate it to be whatever is needed.

That is right but here the point of debate is the effect of input voltage variation on VFD. The VFD has over/under voltage alarm trips to prevent any fault.

But I picked 375V because that is below the threshold of OTHER control components working correctly, such as anything with a coil. So the SYSTEM might trip, but the VFD itself might not.

The system might not trip but the relay contacts would chatter due to low voltage because the drop out voltage for relays are usually below 20%.

 

[Jraef]

Ok. I give up.

 

[Sahib]

Sorry, just for the sake of clarification, I argued.

 

[tom kanzler]

This is a step UP transformer to feed ONE machine that apparently has a VFD on it, no future growth, no neutral loading issues, NONE OF THAT. He got a VOLTAGE DROP on the secondary (in this case the 480V side) that caused his control system to fault, and increasing his transformer size cured the problem, indicating that the nature of his load profile was such that it was overloading that 300kVA transformer and causing the voltage drop. A 300kVA transformer is rated for 360A max, his loading per the machine data was 354A peak, it was just too close to the bone, a slight voltage drop on the 208V side (which with a 354A load is a LIKELY outcome on a 208V service) and you don't even have 360A capacity on that transformer. 400kVA was the way to go, and what do you know? It solved the problem!!!

That pretty well sums it up. I'm still trying to get actual readings from the site, but I'm not getting a lot of cooperation, especially since it runs well now (it ain't broke any more). The 208V supply stiffness is questionable, but again, I'm waiting for some actual readings from the site. I've also asked for fault codes or readouts.

The machine will step itself to a low-power mode (same load, lower speed) if voltage is pulled down too far, if that helps. The customers don't want slow.

The manufacturer is very closed-mouth about their equipment, and they aren't going to give me any details about the drives. If I can get a good look, I may be able to get enough numbers off the drives to figure out what they are.

But I was just trying to understand if/why transformer size affects performance, and I think I have enough to make some sort of case (to the customers) for upsized transformers, as they have to supply it.

Thanks to everyone for considering and discussing this. It's much appreciated.

 

[Sahib]

The machine will step itself to a low-power mode (same load, lower speed) if voltage is pulled down too far, if that helps.

If the input voltage is too low so that the thermal capability of the machine is exceeded even in low power mode (same load, lower speed), the VFD will trip on low input voltage (under voltage alarm trip is provided in VFD for that purpose.)

Did that happen with earlier 300KVA transformer?

 

[Besoeker]

Ok. I give up.

Good move.........

 

[Sahib]

Still over size transformer solution does not meet user requirement fully:

.The machine will step itself to a low-power mode (same load, lower speed) if voltage is pulled down too far, if that helps. The customers don't want slow.,

A UPS at input instead would be more effective solution.

 

[Sahib]

Other solutions to the OP's problems are presented in

http://www.powerqualityanddrives.com/vfd_ride_through_sag/

Note that, instead of the over size transformer solution, a less expensive solution at (j) of sub-title 'Techniques for Power Ride-Through' is also presented in it.