"Pump Duty " just means that the transistors are sized differently because in a centrifugal pump (or fan), the load drops off at the CUBE of the change in speed. So for example at half speed, the load on the motor and thereby the VFD is .5 x .5 x .5 or .125 (1/8) of what it would be at full speed. This is called a "Variable Torque" application, meaning the torque requirement of the load varies with speed, where the only time you need full torque is at full speed.
When using transistors to recreate a voltage and frequency pseudo-sine wave to vary the motor speed, as you slow down the speed, the switching losses inside of the transistors increase. In a "regular" drive, also called a "Constant Torque" rated drive, it is expected that the torque will be the same at lower speeds as it is at higher speeds, and current roughly equates to torque. So at lower speeds on a CT rated drive, the transistors have to put put the same current, but the switching losses inside of the transistor increase, so they have to be sized for that worst case scenario of total heat dissipation. In addition, the transistors will be sized to handle short term overloading conditions, typically 150% for 60 seconds, 180-200% for 3 seconds, to handle sudden changes in load.
In a Variable Torque (VT) application, the torque requirement, and thus the current seen by the transistors, DROPS as the speed decreases, so even though the switching losses become a higher percentage at lower speeds, it is a higher percentage of a drastically lower value. So the transistors are sized based on the switching losses at full speed only, resulting in SMALLER transistors on a VT rated drive. Those smaller transistors then can also not handle as much OVERLOAD capability either, but the VDFD mfrs know that the NATURE of a centrifugal machine is such that is it not possible to overload them in a normal operation; you cannot "force" a correctly sized pump to pump more than it is capable of unless there is an accident, like a broken pipe ("open channel flow"), in which case you will WANT to shut it down immediately. So the over Load capability is unnecessary and is typically limited to 115% for 30 seconds, no instantaneous OL capacity at all. This results in a smaller and lower cost VFD. It should also be noted however that under 20A (most) VFD mfrs do not make a VT rated drive because there is no real significance in the sizing of the smaller transistors.
So bottom line, you can use ANY CT rated drive for a VT load, but you should not use a VT rated drive for anything other than a centrifugal pump or fan.
