REACTANCE

The exact equation for maximum torque is given in eq. (5.107) and the approximate equation for maximum torque is given in eq. (6.42). It can be seen from both of these equations that maximum torque is approximately proportional to the reciprocal of 2. 0 times the sum of the leakage reactance of the stator winding and of the rotor winding, or with quantities in per unit, Tm = 1/2(X1 + X2). Application requirements have established a minimum ratio of maximum torque to rated torque in the region 2.0 to 2.5, and this requirement has been incorporated into industry standards. To achieve this required value of maximum torque, the total leakage reactance must not exceed, say, 0.20 per unit. Then Tm?; 1/(2.0)(0.2)?; 2. 50. Even a value of

Reactance 481

Power factor is the ratio of the in-phase to the total stator current or stator volt-amperes. Thus the power-factor angle (} is a function of the out-of-phase or reactive volt-amperes [i. e., sin-l (} = (reactive volt-amperes)/ (total volt-amperes)). From the vector diagram shown in Fig. 7.2 it is apparent that the voltage vectors IlXl and 12X2 and the current vector 1m are essentially out of phase with the voltage vector VI. Accordingly, the

13.2 SEPARATION OF TOTAL WINDING REACTANCE INTO LEAKAGE REACTANCE AND MAGNETIZING REACTANCE

e,

Chapter 13

FIGURE 13.1 Actual circuit of one phase of the physical model of a polyphase induction motor.