Standard types of squirrel cage motors

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Squirrel cage motors are standardized into different types according to their electrical characteristics. These standard types are called as class A, B, C, D, E and F respectively. Class A motors are referred as 'normal starting torque, normal starting-current, normal slip' and are used as reference motors.
Standard types of squirrel cage motors are as explained below:

(i) Class A  - (Normal starting torque, normal starting current, normal slip)

Class A is the most popular type of squirrel cage induction motor. Motors of this type employ squirrel cage having relatively low resistance and reactance. Its blocked-rotor current (with full voltage) is generally more than 6 times rated full load current. For smaller size and less number of poles, starting torque with full load voltage is nearly twice the full load torque.For larger size and more number of poles starting torque is only a bit more than full load torque. The full load slip is less than 5%.   The rotor bars are placed close the the rotor surface to reduce the rotor reactance.
Class A motors are used in fans, compressors, pumps, conveyors etc. which are having low inertia loads so that the motor can accelerate in less time.

(ii) Class B - (Normal starting torque, low starting current, normal slip)
Class B motors can be started at full load, developing normal starting torque with relatively low starting current. Their blocked-rotor current with full voltage is generally 5 times the full load current. Rotor bars are narrow and placed deeper to obtain high reactance at starting.
These motors are used where load is having high inertia, e.g large fans, machine tools applications, for driving electric generators, centrifugal pumps etc.

(iii) Class C
- (high starting torque, low starting current, normal slip)
Class C motors are generally double squirrel cage type. Their blocked-rotor current and slip with full voltage is nearly similar to that of class B motors. Their starting torque with full applied voltage is genrally about three times the full load torque.
These motors are used where sufficiently high starting torque with reduced starting voltage is reqired. They are used for crushers, compression pumps, large refrigerators, textile machinery, wood working eqipment etc.

(iv) Class D - (high starting torque, low starting current, high slip)
In class D motors rotor bars of high resistance are used so as to give high starting torque with low starting current. Their blocked rotor current with full applied voltage is similar to that of class B or class C motors. Full load slip may vary from 5% to 20% depending upon application. Thin rotor bars are used which reduces the leakage flux and increases the useful flux, thus giving high starting torque with low starting current.
These motors are used where extremely high starting torque is required. E.g. bulldozers, shearing machines, foundry equipment, punch presses, stamping machines, metal drawing equipment, laundry equipment etc.

(v) Class E - (low starting torque, normal starting current, low slip)
Class E motors are having relatively low slip at rated load. For motors above 5kW rating, starting current may be high, so they require compensator or resistance starter.

(vi) Class F - (low starting torque, low starting current, normal slip)
As these motors are having low starting torque with low starting current, they can be started at full voltage. The rotor is designed such that it gives high reactance at starting. The blocked rotor current and full load slip with full applied voltage is similar to that of class B or class C motors. The starting torque with full applied voltage is nearly 1.25 times the full load torque.

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