Induction_Motor.pdf

 NEMA Design Classes if a rotor is designed with high resistance, then the motor's starting torque is quite high, but the slip is also quite high at normal operating conditions. Recall that P cony = (1 - s) P AG, so the...

 NEMA Design Classes if a rotor is designed with high resistance, then the motor's starting torque is quite high, but the slip is also quite high at normal operating conditions. Recall that P cony = (1 - s) P AG, so the higher the slip, the smaller the fraction of air-gap power actually converted to mechanical form, and thus the lower the motor's efficiency. A motor with high rotor resistance has a good starting torque but poor efficiency at normal operating conditions. On the other hand, a motor with low rotor resistance has a low starting torque and high starting current, but its efficiency at normal operating conditions is quite high. An induction motor designer is forced to compromise between the conflicting requirements of high starting torque and good efficiency.  One possible solution to this difficulty was suggested by using a wound-rotor induction motor and insert extra resistance into the rotor during starting.  The extra resistance could be completely removed for better efficiency during normal operation.  Unfortunately, wound-rotor motors are more expensive, need more maintenance, and require a more complex automatic control circuit than cage rotor motors. Also, it is sometimes important to completely seal a motor when it is placed in a hazardous or expensive environment, and this is easier to do with a completely self-contained rotor.  It would be nice to figure out some way to add extra rotor resistance at starting and to remove it during normal running without slip rings and without operator or control circuit intervention.  This figure shows two wound-rotor motor characteristics, one with high resistance and one with low resistance. At high slips, the desired motor should behave like the high-resistance wound-rotor motor curve; at low slips, it should behave like the low- resistance wound-rotor motor curve. Fortunately, it is possible to accomplish just this effect by properly taking Advantage of leakage reactance in induction motor rotor design. Control of Motor Characteristics by Cage Rotor Design *The reactance X2 in an induction motor equivalent circuit represents the referred form of the rotor's leakage reactance. *Recall that leakage reactance is the reactance due to the rotor flux lines that do not also couple with the stator windings. *In general, the farther away from the stator a rotor bar or part of a bar is, the greater its leakage reactance, since a smaller percentage of the bar's flux will reach the stator. *Therefore, if the bars of a cage rotor are placed near the surface of the rotor, they will have only a small leakage flux and the reactance X2, will be small in the equivalent circuit. *On the other hand, if the rotor bars are placed deeper into the rotor surface, there will be more leakage and the rotor reactance X will be larger.  The rotor bars in the figure (a) are quite large and are placed near the surface of the rotor. Such a design will have a low resistance(due to its large cross section) and a low leakage reactance and X2 (due to the bar's location near the stator). Because of the low rotor resistance, the pullout torque will be quite near synchronous speed and the motor will be quite efficient. Remember that Pconv = (1 - s) PAG so very little of the air-gap power is lost in the rotor resistance. However, since R2 is small, the motor's starting torque will be small, and its starting current will be high. This type of design is called the National Electrical Manufacturers Association NEMA design class A. It is more or less a typical induction motor, and its characteristics are basically the same as those of a wound-rotor motor with no extra resistance inserted.  Figure d, shows the cross section of an induction motor rotor with small bars placed near the surface of the rotor. Since the cross-sectional area of the bars is small, the rotor resistance is relatively high. Since the bars are located near the stator, the rotor leakage reactance is still small. This motor is very much like a wound-rotor induction motor with extra resistance inserted into the rotor. Because of the large rotor resistance, this motor has a pullout torque occur ring at a high slip, and its starting torque is quite high. A cage motor with this type of rotor construction is called NEMA design class D. Both of the previous rotor designs are essentially similar to a wound-rotor motor with a set rotor resistance. How can a variable rotor resistance be produced to combine the high starting torque and low starting current of a class D design with the low normal operating slip and high efficiency of a class A design? ???  At low slip. the rotor's frequency is very small, and the reactances of all the parallel paths through the bar are small compared to their resistances. The impedances of all parts of the bar are approximately equal, so current flows through all parts of the bar equally. The resulting large cross-sectional area makes the rotor resistance quite small, resulting in good efficiency at low slips.  At high slip (starting conditions), the reactances are large compared to the resistances in the ( rotor bars, so all the current is forced to flow in the low-reactance part of the bar near the stator. Since the effective cross section is lower, the rotor resistance is higher than before. With a high rotor resistance at starting conditions, the start ing torque is relatively higher and the starting current is relatively lower than in a class A design.