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 1.1 illustrates a typical two-pole stator for an induction motor. The two main types of rotors are squirrel-cage and wound rotors. Figures 1.2 and 1.3 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. 1.4 and 1.5) 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 torquespeed 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 flowing in the stator. A magnetic field BS is produced. It rotates in a counterclockwise direction. The rotational speed of the magnetic field is given by

 

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

FIGURE 6.1 The stator of a typical induction motor, showing the stator
windings. (Courtesy of MagneTek, Inc.)

The induced voltage in a given rotor bar is given by: 

where : v = velocity of rotor bars relative to magnetic field
            B = magnetic stator flux density
             l = length of rotor bar

The voltage in a rotor bar is induced by the relative motion of the rotor compared to the magnetic field. The velocity of the upper rotor bars relative to the magnetic field is to the right. Therefore, the induced voltage in the upper bars is out of the page, and the induced voltage in the lower bars is into the page.

The current is flowing out of the upper bars and into the lower bars. However, the peak rotor current lags behind the peak rotor voltage due to the inductive nature of the rotor assembly. A rotor magnetic field BR is produced by the current flowing in the rotor. Since the induced torque is given by

 

the resulting torque is counterclockwise. The rotor accelerates in this direction.

Articles Source from :

ELECTRICAL EQUIPMENT HANDBOOK