# Allen Bradley PF4 VFD Question



## JRaef (Mar 23, 2009)

The PF4s and 40s will operate on single phase 480V power, but must be de-rated to 35% of their ampacity, or kept at 25 degrees C (77F) if you de-rate to 50% ampacity.


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## MDShunk (Jan 7, 2007)

Not only derated because of single phase supply, but derated because they're a weak drive to begin with. :laughing: Misleading dataplate with something like 'normal rating' and 'heavy duty rating', which can be a couple horsepower apart. If you put a 2 horse motor on a 2 horse PF4, that's using the 'heavy duty' rating, which I think is marketing term. :whistling2:


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## Cow (Jan 16, 2008)

I love the PF4's. Yeah, they are simple little drives, but they're fairly cheap and a basic program takes less than 2 minutes. You can't beat it.


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## JRaef (Mar 23, 2009)

MDShunk said:


> Not only derated because of single phase supply, but derated because they're a weak drive to begin with. :laughing: Misleading dataplate with something like 'normal rating' and 'heavy duty rating', which can be a couple horsepower apart. If you put a 2 horse motor on a 2 horse PF4, that's using the 'heavy duty' rating, which I think is marketing term. :whistling2:


"Normal Duty" and "Heavy Duty" are indeed marketing terms, the same as is used by all VFD mfrs in one form or another. "Normal Duty" is the same as "Variable Torque" or "Pump and Fan" or "HVAC Duty" as used by other mfrs. "Heavy Duty" is the same as "Constant Torque". Rockwell uses "Normal Duty" because 90% of people wanting to use them have no idea what "Variable Torque" means, and "Pump" is a little too vague since there is a big difference in terms of what the VFD must do between a centrifugal pump and a positive displacement pump. But since 70% of all AC induction motors are used for running _*centrifugal *_pumps and fans, that would be the definition of "normal", and everything else needs a "heavy duty" drive. 

The entire concept only has only to do with the overload capability of the VFD. With a "Normal Duty / Variable Torque / Centrifugal Pump or Fan" type of load, you CANNOT overload the motor if the motor was properly selected for the machine and everything is working as it should, it is a law of physics. Plus, as the speed drops, the LOAD on the motor drops at the cube of the speed change, so the drive is expecting to do even LESS work at lower speeds. So for that drive, the overload capability of the components is limited to 110% of rated current for 60 seconds, 150% for 3 seconds. If you were to ask that drive to make the motor deliver Break Down Torque to re-accelerate a load, which would require 200-220% current, it will give you a middle finger and either go into current limit to protect itself, or trip on over current. But because it is INTENDED to go on centrifugal machines that CAN NEVER be overloaded like that, no problemo. A HD / CT rated drive on the other hand will be capable of 150% overload for 60 seconds, and more importantly, 200-250% overload for 3 seconds. So if you DO need BDT to re-accelerate a load, it can give it to you for about as long as the motor is capable of delivering it safely. That's the difference.

That said, the PF4 does not have two ratings, they are all sized based on the HD overload capacity. On all PowerFlex 4, 40, and 5 series drives, the ND/HD sizing differences only kick in at above 10HP. On all PF400 drives, they are expressly stated as being Pump / Fan duty drives, so they are all Normal Duty. It's only on the PF7 series drives where there is a difference at low HPs.


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