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.
14.2 TYPICAL LOAD SHAFT TORQUE/SPEED