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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.
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IMAGE 1: The relationship between the three phases of current
Three-phase alternating current (AC) induction motors are more common for commercial and industrial pumping applications than single-phase motors. Some of the reasons are:
Image 1 shows how the maximum current is passed from Phase A to Phase B to Phase C in a three-phase motor. After this sequence, the maximum current follows the same pattern but with reversed polarity.
Image 2 shows how the three-phase currents create a magnetic field that smoothly rotates inside the motor. The magnetic poles are color-coded to match the background color of the graphs of the current in each phase. The large letters, N and S, next to poles in Image 2 indicate the location of the peak magnetic field. The small letters, N and S, show the locations of weaker magnetic fields.
Notice how the magnetic field moves between poles and forces the rotor to spin in a specific direction. Because the AC is sinusoidal, the magnetic field of each phase is also sinusoidal. This results in a net magnetic field that rotates smoothly around the inside of the motor.
IMAGE 2: The motion of the magnetic field in a three-phase motor
For more information on electric motors, refer to the Hydraulic Institute&#;s &#;Drivers Application Guidebook: Electric Motors&#; at www.pumps.org/guidebooks.
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