VFD's - Variable or Constant Torque

[mjcandlec]

I have 25hp fans running 50% of the time at 60 Hz, near FLA and RPM for the motor, given the fixed prop pitch and static pressure, and 50% of the time at about 30 Hz. I am using ABB vfd's rated @ 25Hp VT. Should this be a constant torque application?

Thanks for any input to clear this up.

 

[dpc]

Fans are a prototypical variable torque load.

But to determine the proper VFD sizing, you really need to compare the VFD output amps with the motor full load amps.

The horsepower ratings on VFDs are great for salesmen, but not so great for engineers. They will get you in the ballpark, but the amperes are what matter.

Are you having a problem with VFDs?

 

[mjcandlec]

Yes I am having problems with motor failure causing the drive to fail. The +UDC bus melts down and causes other damage when a motor fails due to insulation breakdown. The drive sees the motor as a dead short and melts the UDC bus instead of protecting itself via output ground fault enabled.

I haved proposed output filter (distance < 100 ft) and ultafast input fuses. Maybe I am missing something crucial for proper protection?

Thanks for your help.

 

[dpc]

Are the motors inverter-duty?

How many failures have you had? The drive should be able to protect itself - even working into a short circuit. What does ABB tell you?

 

[mjcandlec]

ABB has only one answer....send it in for analysis at my cost.(one year off warranty)and maybe find something.

I have been out of the loop on motor replacement, so I will have to say they are not inverter-duty motors. That needs to change, but I still don't see how that would cause a melt down instead of a nuisance fault?

My nameplate FLA is 34 and the drive is rated at 34A output. My actual normal is about 26 max.

I think most drives should be bumped up one size vs. motor size (would make them sized for CT) but I am a little late for that.

Thanks for your reply.

 

[dpc]

Sounds like the answer I usually get from ABB. I'm not sure a larger drive would have been any more robust in terms dealing with an output fault.

Sizing is a little marginal, maybe - but it should work OK.

At 25 hp, the most cost-effective plan is probably to put in a new drive and see what happens. If you lose another drive, then you may want to investigate further.

You're assuming the motor failed first and the short circuit caused the drive failure, which is logical, but maybe it was more complicated than that.

Good luck.

 

[ozmosis]

mjcandlec
What's your AC supply feeding your VFD's? If >460Vac on non-inverter rated motors, this could cause quick insulation breakdown if there is no filtering between VFD and motor.

 

[mjcandlec]

You are correct. It is a very high 480-490 VAC and I am ordering filters right now. Should have had them before but wasn't my call. Thanks for the reply.

I have just been told by ABB that my motors could have been back-feeding after a failure and the drive faulted off. He thinks it back fed enough voltage from motors spinning backwards to blowout the DC bus? He reccomends output contactors that open when drive faults for any reason, to isolate the motor. Interesting.....

 

[ozmosis]

I would have thought the over-voltage pretection trip would shut the drive down before any serious damage is incurred by regenerative volts.
I cannot see that output contactors will have much protection in the event of OV. By the time the drive has registered an OV, it would shut down the VFD, cut the output voltage and thus the motors will simply spin producing no back emf. Motors will not regenerate after the VFD has tripped.

 

[dpc]

ozmosis,

So does higher ac input voltage translate directly to higher transient voltage out of the PWM inverter?

Thanks,

dpc

 

[DickDV]

Asan ABB rep, I hate to say it but it sounds like you are being fed a bunch of bunk! Putting a 34amp motor on a 34amp drive is not going to shorten the drive life or cause catestrophic failure.

Motor failures whether phase to phase or phase to ground should produce a drive fault but definitely not a drive failure.

UNLESSSSSSSSSSSSSSSSSSSS!!!!!!!!!

Your power to the drive is another of those blasted floating delta systems. In that case, grounding one phase on either the input or output side of the drive will burn up the CE noise suppression network unless the EM1 or F1/F2 screws are removed allowing the networks to float. You must also turn off the short circuit and ground fault protection in Group 30(sorry but that's the price for running these floating systems). If you read the Instruction Manual very carefully you will see specific instructions for dealing with floating networks.

Having gotten that off my chest, if you have a nice grounded wye power source and the drive is still destroying itself, I would suspect a drive defect. The fault circuitry is so fast that there is very little that can go wrong with the motor or motor leads that would hurt the drive if it is working properly.

And, at 25hp 460V, there is very little in the way of repairs that can be economically justified. Better to just replace it. And no, ABB in general has a very good reputation for reliable drives. This sounds like something out of the normal pattern.

 

[ozmosis]

dpc
yes, the higher the AC, the higher the DC link, the higher the peak-peak voltages. This will have an effect on the winding insulation. Inverter rated motors are typically ok for the higher levels of 400Volts but >500Vac they will need careful consideration. A non-inverter rated motor is a difficult call as you are not really sure of the insualtion quality but as a general rule, >460Vac is heading into danger zone.
mjcandlec
I would agree with DickDV. I'm surprised this advice has come from ABB, it sounds a little uncharacteristic.