Motor Design Classes
Since different applications need different characteristics from induction motors, the National Electical Manufacturers Association (NEMA), a US organization, has specified different classes of induction motor, with different characteristics. Typical torque speed-curves for classes A-D are sketched below.
Motor Classes A and D are obtained by designing the rotor resistance to be either low (class A) or high (class D). Classes B and C are obtained by exploiting the effect of skin depth to obtain a variable resistance rotor circuit. Example rotor conductor designs for classes A through D are shown below. Comparing classes A and B, class B has a deeper bar to exploit skin depth effects. The bar width may vary in order to enhance this effect. Class C rotors are typically fabricated with two seperate cages. Only the outer cage will conduct at starting. There is air between the cages and at the top of the slot (to reduce leakage flux). Note that cast rotor designs must have a closed slot design to prevent molten aluminum from escaping. Class D has a small conductor, giving a high resistance at all slips. The class D example shown has an open slot, for a fabricated rotor design.
A summary of the performance of the motor classes is provided in the table below
|Class A||Class B||Class C||Class D|
|Type||General Purpose||General Purpose||High Starting Torque||Very High Starting Torque|
|Start Torque||100% rated for larger motors, 200% rated, smaller motors||100% rated for larger motors, 200% rated, smaller motors||Approx 250% rated||> 275% rated|
|Start Current||˜800% rated||500%-600% rated|
|Pullout Torque||200%-300% rated||≥200% rated||Slightly lower than class A|
|Pullout Slip||<0.2||<0.2||High, can be as much as 1.0|
|Rated Slip||<0.05, lower than similar sized class B||must be <0.05, usually <0.03||<0.05, higher than class B||High, typically 0.07 to 0.11, can be up to 0.17|
|Applications||Fans, Blowers, Pumps, Machine Tools||As for Class A||Compressors, pumps, conveyors||High inertia applications, e.g. mechanical punches|
|Notes||High starting inrush current causes power system problems, it can cause the supply voltage to sag and requires special starting techniques. More efficient than same sized class B||Replacemesnt for Class A due to lower start current. The standard off-the shelf commodity motor.||Applications that require high start torques. Note that the pullup and pull-out torque can both be lower than the start torque. Less efficienct than class B||Very high inertia applications. e.g. in a punch or reciprocal pump where the slip may vary between 0 and 0.50 Much less efficient than other designs|