Torque-Slip Characteristics Assignment Help

Assignment Help: >> Induction Motors - Torque-Slip Characteristics

Torque-Slip (T-S) Characteristics:

For the better expression of the torque make thevenin's equivalent circuit for the left side of the terminal 1-2.

The thevenin impedance.

Z¯T  = (R1 + j X1 ) | | j X m  = RT  + j XT

Thevenin voltage

T  = V1 ( j X m / ( R1 + j X1 + j X m ))

2139_Torque-Slip Characteristics.png

Substitute the value of I2′ in Eq.

T =  3/ ωs  .  (V T) 2 (R2′ /S) /(RT +( R2′/S)2 +(XT + X2 ′)2

Eq. (57) expresses the torque as a function of voltage and slip. This is seen that for a fixed value of slip, torque is equal to proportional to the square of voltage. Figure illustrates the torque slip characteristics of an induction motor for the rated voltage.

531_Torque-Slip Characteristics3.jpeg

Figure: Torque-Slip Characteristics

The torque is zero While slip is zero; (Nr = Ns). We have already discussed that induction motor torque is zero at synchronous speed. At slip Sm, T; the motor gives the maximum torque that is also called as break down torque. While the rotor is stand still (S = 1), the torque corresponds to beginning torque Ts. In normally designed motor Ts is much less than Tmax (Break down torque). The torque slip characteristics from no-load to rather beyond full-load are almost linear.

The condition for maximum torque

                    2125_Torque-Slip Characteristics1.png

where SM, T is the value of slip at which maximum torque occurs, and the maximum torque is then

                      435_Torque-Slip Characteristics2.png

From the Eq. (59), we came to the conclusion that the maximum value of the torque does not based upon rotor resistance ( R2′ ) .

The slip at which maximum torque occurs indirectly proportional to the rotor resistance ( R2′ ) . At S = 1, we have the beginning torque which increases by adding resistance in the rotor circuit.

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