Page 112 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

Source: ELECTRICAL EQUIPMENT HANDBOOK

CHAPTER 6
INDUCTION MOTORS

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.

BASIC INDUCTION MOTOR CONCEPTS

Figure 6.6 illustrates a squirrel-cage induction motor. A set of three-phase currents is flow-
ing in the stator. A magnetic field B is produced. It rotates in a counterclockwise direction.
S
The rotational speed of the magnetic field is given by
120f
n e
sync P

where f is the electrical frequency in hertz and P is the number of poles in the machine.
e
The rotating magnetic field B crosses the rotor bars and induces a voltage in them.
S

6.1
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