ABSTRACT
Induction motors (IMs) are commonly understood as three-phase transformers with shortened, and freely rotating secondary winding. An IM can be considered as a special three-phase transformer. Slip is calculated as the difference between the rotational speed of the air gap field and the shaft speed. If a load increases at a fixed frequency operation, the slip increases, which in turn induces a larger secondary current, producing higher torque. This chapter discusses the torque-speed curves, and illustrates the speed control methods. A simple speed control method would be to change the voltage only. The starting method by connecting IM directly to the power grid is called line start. Due to the voltage drop associated with the leakage field, the IM has a lower power factor than the transformer. With the use of semiconductor switches such as IGBTs, it is possible to change the feeding voltage and frequency.
