STANDARD TYPES OF 3-PHASE SQUIRREL CAGE INDUCTION MOTOR.
These motors are available in a range of standard ratings up to 150 kW at various standard frequencies, voltages and speeds in order to meet the usual requirements of industry.
According to this is electrical characteristics, such motors are divided into six types as given below:
1. Class A Motors.
These motors have low resistance and low
reactance and have low slip and high efficiency at full load. These is high starting current (6 to 9 times full-load current at rated voltage) is the main drawback.
Such motors are used in small ratings for machine tools, centrifugal pumps, fans, blowers etc.
2. Class B Motors.
These motors have high reactance. there design is common in the 5-150 kW range.
Such motors have largely replaced class A motors for new installations as its running characteristics are quite similar to those of class A motor but have smaller starting current (around 5 times the full-load current at rated voltage).
3. Class C Motors.
These motors are double cage motors and provide high starting torque with low starting current.
Typical applications are in driving air compressors, conveyors, reciprocating pumps, crushers, mixers, large refrigerating machines etc.
4. Class D Motors.
Such motors are provided with a high resistance squirrel cage rotors and, therefore, give high starting torque with low starting current.
These motors have low operating efficiency and their use is limited to driving of intermittent loads involving high accelerating duty and to drive high-impact loads such as punch presses, shears, bulldozers, small hoists etc.
5. Class E Motors.
This is motors have a relatively low starting torque, normal starting current and have a relatively low slip at rated load.
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