MOTOR SPECIFICATIONS AND DESIGN PRINCIPLES
Induction motors are used to drive loads in various industries for powers from less than 100W to 10MW and more per unit. Speeds encountered go up to tens of thousands of rpm. There are two distinct ways to supply an induction motor to drive a load: • Constant voltage and frequency (constant V and f) – power grid connection; • Variable voltage and frequency – PWM static converter connection
The load is represented by its shaft torque-speed curve (envelope). There are a few basic types of loads. Some require only constant speed (constant V and f
supply) and others request variable speed (variable V and f supply). In principle, the design specifications of the induction motor for constant and variable speed, respectively, are different from each other. Also, an existing motor, that was designed for constant V and f supply may, at some point in time, be supplied from variable V and f supply for variable speed. It is thus necessary to lay out the specifications for constant and variable V and f supply and check if the existing motor is the right choice for variable speed. Selecting an induction motor for the two cases requires special care. Design principles are common to both constant and variable speed. However, for the latter case, because different specifications with machine special design constraints, or geometrical aspects (rotor slot geometry, for example) lead to different final configurations. That is, induction motors designed for PWM static converter supplies are different. It seems that in the near future more and more IMs will be designed and fabricated for variable speed applications.