Q10Electrical Machines and Drives

Fan and pump loads have a torque that varies approximately as the square of speed (T proportional to N^2), so their power requirement varies as the cube of speed. Stator voltage control reduces motor torque approximately as the square of the applied voltage (T proportional to V^2), and because the intersection of this reduced-voltage torque-speed curve with the load's own N^2 torque curve provides a stable operating point at reduced speed, and because the required torque at reduced speed is naturally low for such loads, the associated increase in rotor copper loss (heating) at reduced speed remains within acceptable limits, making the simple, low-cost stator voltage control method well suited specifically to fan and pump-type loads, unlike constant-torque loads where it would cause excessive rotor heating.