The slip-ring or wound-rotor 3-phase induction motors are used in the following applications −
Slip ring induction motors are suitable for loads requiring high starting torque and for applications where the starting current is low.
Slip ring induction motors are used for loads having high inertia, which results in very high rotor energy losses during acceleration.
The slip ring induction motors are also used for loads which require a gradual build-up of load.
They are used for loads that requires speed control.
Typical applications of wound rotor or slip ring induction motors are crushers, plunger pumps, cranes & hoists, elevators, compressors and conveyors.
In order to meet the various starting and running requirements of variety of industrial applications, several standard designs of squirrel-cage induction motors are available in the market. The most significant design parameter in the squirrel cage motors is the effective resistance of the rotor cage circuit.
The applications of different classes of squirrel cage induction motors are given as follows −
The class-A squirrel cage induction motors have normal starting torque, high starting current and low operating slip (from 0.005 to 0.015). This motor design has a low-resistance single-cage rotor. The full-load efficiency of class-A motors is high. Therefore, these motors are suitable for loads such as −
Class-B motors have normal starting torque, low starting current and low operating slip. By increasing the leakage reactance, the starting current can be reduced and the starting torque is maintained by using double-cage or deep bar rotor. These motors are most popular and used for full-voltage starting. Therefore, these are also used for loads such as fans, blowers, machine tools and centrifugal pumps etc.
The class-C motors have high starting torques and low-starting current. These motors have a high-resistance double-cage or deep-bar rotor construction. The applications of class-C motors are for practically constant-speed loads that require fairly high torque with low starting current. Hence, these motors are used for driving the loads such as compressors, crushers, conveyors and reciprocating pumps.
The class-D motors have the highest starting torque of all squirrel cage induction motors. The rotor conductor bars of these motors are made up of high resistance material like brass instead of copper or aluminium. The class-D squirrel cage motors have low starting current and high operating slip (from 0.08 to 0.15), therefore, the running efficiency is low. Hence, these motors are used for driving intermittent loads requiring rapid acceleration and high impact such as punch presses, bulldozers, die-stamping machines and shears. To provide the kinetic energy during impact, the motor is to be coupled with a flywheel.