Design Considerations for Motors Used in Adjustable-Speed Drives

In adjustable-speed drives the constraints imposed on the motor at its electrical and mechanical port are not fixed but subject to change. It is found that the counterelectromotive force developed by the motor is uniquely defined by the specified load power, efficiency of the mechanical transmission, source voltage, and resistance of the energized circuit. In this respect ac motors fed by inverters behave like dc motors, because the dc source does not "see" the motor reactances. Relations are derived for the various machines and inverter types. With the EMF determined, the main design problem becomes the selection of its constitutive factors. The involved trade-offs are examined for the extreme cases of drives requring maximum torque at top speed and at stalling.