Variable frequency drive principles and practices (above NEMA) AC motors for variable frequency application

Since AC motors are increasingly used in applications traditionally served by DC motors, a comparison of some characteristics of AC and DC motors seems appropriate. Aspects of motor design and application is reviewed, including typical construction, torque production, equivalent circuits, power and torque density ratings, speed and load ranges. Understanding the effects of variable frequency on the motor's characteristics helps in the selection process of a motor and control unit matched to the application. Depending on the type of inverter, the input voltage and current waveform supplied to the motor will cause some degree of distortion from a pure sine wave. This distortion can be mathematically expressed in terms of harmonics. Harmonic currents create no useful torque at the motor shaft, just additional heating in the motor. The motor application must also be considered. Different load types have different effects on the motor. Constant torque, variable torque, and constant horsepower are the three different load requirements typically considered, and affect motor heating in different ways. While all three are discussed, the primary focus of this paper is on the "constant torque" applications with PWM inverters, as applied to process machines such as paper machines and winders.