Classification of Squirrel Cage Induction Motor
NEMA (National Electrical Manufacturer’s Association) in United States and IEC in Europe has classified the design of the squirrel cage induction motors based on their speed-torque characteristics into some classes. These classes are Class A, Class B, Class C, Class D, Class E and Class F.
In Class A Design
- A normal starting torque.
- A normal starting current.
- Low slip.
- In this Class, pullout torque is always of 200 to 300 percent of the full-load torque and it occurs at a low slip (it is less than 20 percent).
- For this Class, the starting torque is equal to rated torque for larger motors and is about 200 percent or more of the rated torque for the smaller motors.
In Class B Design
- Normal starting torque,
- Lower starting current,
- Low slip.
- Induction Motor of this Class produces about the same starting torque as the class A induction motor.
- Pullout torque is always greater than or equal to 200 percent of the rated load torque. But it is less than that of the class A design because it has increased rotor reactance.
- Again Rotor slip is still relatively low (less than 5 percent) at full load.
- Applications of Class B design are similar to those for design A. But design B is preferred more because of its lower starting-current requirements.
In Class C Design
- High starting torque.
- Low starting currents.
- Low slip at the full load (less than 5 %).
- Up to 250 percent of the full-load torque, the starting torque is in this class of design.
- The pullout torque is lower than that for class A induction motors.
- In this design the motors are built from double-cage rotors. They are more expensive than motors of Class A and B classes.
- Class C Designs are used for high-starting-torque loads (loaded pumps, compressors, and conveyors).
In Class D Design
- In this Design of Class motors has very high starting torque (275 percent or more of the rated torque).
- A low starting current.
- A high slip at full load.
- Again in this class of design the high rotor resistance shifts the peak torque to a very low speed.
- It is even possible at zero speed (100 percent slip) for the highest torque to occur in this class of design.
- Full-load slip (It is typically 7 to 11 percent, but may go as high as 17 percent or more) in this class of design is quite high because of the high rotor resistance always.
In class E Design
- Very Low Starting Torque.
- Normal Starting Current.
- Low Slip.
- Compensator or resistance starter are used to control starting current.
In Class F Design
- Low Starting Torque, 1.25 times of full load torque when full voltage is applied.
- Low Starting Current.
- Normal Slip.