In Chapter 5, we saw how amortisseur windings on a synchronous motor could develop a starting torque without the necessity of supplying an external field current to them. In fact, amortisseur windings work so well that a motor could be built without the synchronous motor’s main dc field circuit at all. A machine with only a continuous set of amortisseur windings is called an induction machine. Such machines are called induction machines because the rotor voltage (which produces the rotor current and the rotor magnetic field) is induced in the rotor windings rather than being physically connected by wires. The distinguishing feature of an induction motor is that no dc field current is required to run the machine.
Although it is possible to use an induction machine as either a motor or a generator, it has many disadvantages as a generator and so is only used as a generator in special applications. For this reason, induction machines are usually referred to as induction motors.