Torque-Speed Characteristics of 3-Phase Induction Motors

Because the torque-slip characteristic is one of themost important aspects of the inductionmotor, we will now develop an expression for torque as a function of slip and other equivalent circuit parameters. Recalling Equation and the equivalent circuit of Figure, we will obtain an expression for I'₂. To that end, let us redraw the equivalent circuit in Figure. By applying Thévenin's theorem, we have the following Figure:

Neglecting the stator resistance in Equation results in negligible error for most induction motors. If Xm of the equivalent circuit shown in Figure is sufficiently large that the shunt branch need not be considered, calculations become much simpler; R''₁ and X''₁ are then equal to R₁ and X_l1, respectively; also, V₁a is then equal to V₁.

The general shape of the torque-speed or torque-slip characteristic is shown in Figure, in which the motor region (0 < S ≤ 1), the generator region (S < 0), and the breaking region (S > 1) are included for completeness. The performance of an induction motor can be characterized by such factors as efficiency, power factor, starting torque, starting current, pull-out (maximum) torque, and maximum internal power developed.