VFD Torque Control

Question: How can I control through the VFD Torque for a motor as I have a pump motor of 110 KW of rated current 200 A controlled by Soft Starter and during the actual operation it consumed only 65 KW with 98 A, so I want to change this motor with another one lower in KW with VFD to control at the new motor Torque, so could you help me how can I do that?

Answer: If the new motor is large enough to provide the load requirements then reducing the motor size will not affect performance. It is the load that dictates torque required at any given speed. So, if for example, your load is a fan, torque required by the fan increases approximately in proportion to speed squared so power increases in proportion to speed cubed. If the VFD has a power limit set at the rating of the motor, then if the speed request results in a power greater than the power limit, the VFD will limit the speed to keep the motor at the speed that achieves the power limit. As air density increases, speed will decrease or if air density decreases speed will increase. If the speed request results in a power less than the power limit, the VFD will follow the speed request. The same thing can be accomplished with a torque limit and some real time math in a PLC.

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