In induction machines, the rotor voltage (which produces the rotor current and the rotor magnetic field) is not physically connected by wires to the rotor windings—it is induced in the rotor. The main advantage of induction motors is that there is no need for dc field current to run the machine. An induction machine can be used as a motor or a generator. However, it has many disadvantages as a generator.
INDUCTION MOTOR CONSTRUCTION
Figure 6.1 illustrates a typical two-pole stator for an induction motor. The two main types of rotors are squirrel-cage and wound rotors. Figures 6.2 and 6.3 illustrate squirrel-cage induction motor rotors.
The rotor consists of a series of conducting bars installed into slots carved in the face in the rotor. These bars are shorted at both ends by shorting rings. This design is known as a squirrel-cage rotor. The second type is known as a wound rotor. A wound rotor (Figs. 6.4 and 6.5) has three phase windings that are mirror images to the stator windings.
The three rotor phases are usually Y-connected. Slip rings on the rotor shaft tie the ends of the three rotor wires. Brushes riding on the slip rings short the rotor windings.
The rotor currents are accessible. They can be examined, and extra resistance can be added to the rotor circuit. This is a significant advantage of this design because the torque- speed characteristic of the motor can be modified.