AB PowerFlex 750 series AC drive normal duty mode vs heavy duty mode 1

[fordn]

I am planning to upgrade a few AB 1395 DC drives and I am planning to go completely AC with both the new drives and motors. Reading the Rockwell publications I see where you can select Normal Duty Mode or Heavy Duty Mode (par 306 - Duty Rating). In normal duty mode, for example, a 40 hp drive will power a 40 hp motor. To run this same motor with a drive that is programmed in the heavy duty mode, you would have to use a 50 hp drive. My question is, do I need to apply this "Duty Rating" or just use the size drive = size motor in normal duty mode? Thanks

 

[itsmoked]

You should tell us what your motors are being used to do. That would be the deciding factor as to which mode you'd need.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Whether the drive is rated as Heavy a Duty or Normal Duty is a decision made at purchase. You BUY the drive as Normal or Heavy duty based on the application, as Keith suggested, then enable the mode you intend on using. So if you have a 40HP motor and you BUY a 40HP ND rated drive, putting it into "Heavy Duty Mode" with parameter 306 will only allow you to run a 30HP motor. If you bought a 40HP HD rated drive, you can run a 40HP motor, or if you change 306 to "Normal Duty Mode", you can use a 50HP motor.

Normal Duty is defined as a centrifugal load like a pump or fan. 70% of all AC induction motors are used for centrifugal pumps or fans, hence the term "normal". Other mfrs use the term "Variable Torque" or "Pump and Fan Duty", this is the same concept. ND/VT rating for the VFD means it is rated for 110% overload for 30 sec, and will trip to protect itself if the current attempts to go above 150% for more than 2 sec. This is because on a centrifugal load, you cannot overload a properly selected motor unless something else is wrong, meaning it needs to be shut down anyway. So since you can't overload it, the drive does not need the added capacity to handle that.

Any load that is not a centrifugal pump or fan will require a Heavy Duty rated drive. A HD/CT (Constant Torque) rated VFD will be rated for 150% overload for 1 minute, and over 200% for 3 seconds. This allows the VFD to develop full Break Down Torque in the motor, briefly, to re-accelerate a load after a large step change.

Parameter 306 enables the higher overload capacity in software, but cannot override the limitations of the hardware itself, which have separate firmware protection schemes. So if you enable HD Mode on a drive purchased as ND, you run the risk of having it shut itself down when trying to accelerate or re-accelerate the load.

As a gross generalization, loads typically operated by DC drives and motors will fall into the Heavy Duty category, so if you need a 40HP motor, buy a drive rated for 40HP HD, which will be the same as a 50HP ND drive.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[LionelHutz]

You always start at the load torque requirements. Pick a motor that is capable of supplying the torque required by the load under all operating conditions. Then, you pick a drive that can supply enough current to the motor that it can produce that torque. You will require a drive that can produce more than rated motor current when you require the motor to produce more than rated output torque.

Remember that a motor on a VFD always operates above the breakdown torque peak. Controlling the frequency basically shifts the motor curve so that the synchronous speed point of the curve is the drive output frequency.

 

[waross]

Controlling the frequency basically shifts the motor curve so that the synchronous speed point of the curve is the drive output frequency.

Well said, Lionel. A simple and accurate explanation of the action of a VFD at varying speeds.
I wish that I could have said this this well.

Bill
--------------------
"Why not the best?"
Jimmy Carter