The induction motor is an alternating current motor fed from a single or a three-phase supply. We shall only consider the threephase machine in this chapter, the single-phase machine being essentially a small power device used generally in domestic appli ances and where a three-phase supply is not available. It has a drooping speed/torque characteristic very similar to that in a DC shunt motor starting from a no-load speed slightly less than the synchronous value. There are basically two types:
1 cage machines, which have short-circuited bars in slots on the rotor and need no external connections to the rotor. This is a relatively cheap method of construction which is very robust.
2 wound-rotor machines, which carry windings on the rotor and need slip rings to provide a connection to external resistance banks.