VFD's - Variable or Constant Torque
VFD's - Variable or Constant Torque
(OP)
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.
Thanks for any input to clear this up.





RE: VFD's - Variable or Constant Torque
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?
RE: VFD's - Variable or Constant Torque
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.
RE: VFD's - Variable or Constant Torque
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?
RE: VFD's - Variable or Constant Torque
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.
RE: VFD's - Variable or Constant Torque
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.
RE: VFD's - Variable or Constant Torque
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.
RE: VFD's - Variable or Constant Torque
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.....
RE: VFD's - Variable or Constant Torque
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.
RE: VFD's - Variable or Constant Torque
So does higher ac input voltage translate directly to higher transient voltage out of the PWM inverter?
Thanks,
dpc
RE: VFD's - Variable or Constant Torque
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.
RE: VFD's - Variable or Constant Torque
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.