Electrical Engineering ⇒ Topic : Torqueslip Characteristics

William
 
TORQUESLIP AND TORQUESPEED CURVES: The expression for torque is as follows
From the above expression it evident that : Torque is zero when slip 's' = 0 (i.e., speed is synchronous). When slip `,s' is very low the value of the term sX_{2} is very small and is negligible in comparison with R_{2}, therefore torque T is approximately proportional to slip 's' if rotor resistance R_{2} is constant. This means that at speeds near to synchronous speed the torquespeed and torqueslip curves are approximately straight lines When the slip 's' increases (i.e., as, the speed decreases with increase in load) torque increases and reaches its maximum value when The maximum torque is also known as 'pullout' or 'breakdown' torque. Torquespeed curves Torqueslip curves When the slip is further increased the torque decreases. The result is that motor slows down and eventually stops. The motor operates for the value of slip between zero and that corresponding to maximum torque. With higher slip, R_{2} becomes negligible as compared to sX_{2} and torque varies as follows : This means that speedtorque or sliptorque curves are rectangular hyperbola with the speed or slip beyond, that corresponding to maximum torque. Fig. show that the torquespeed and torqueslip curves for different values of rotor resistance. It is observed that although maximum torque is independent of rotor resistance R_{2}, yet the exact location of T_{max} is dependent on it. Greater the R_{9}, greater is the value of slip at which maximum torque occurs.  
 
Samual
 
TORQUESLIP CHARACTERISTICS Figure (a) reduces to Figure (b) by using Thevenin's theorem
Figure (a) Equivalent circuit of a threephase induction motor without core loss .............. (1) ............... (2)
figure (b) Thevenin's equivalent circuit X_{1} + X_{Φ}, is much greater than r_{1} for most of the induction motors. So r_{1} can be neglected. Thus, Eqs. (1) and (2) become where X_{1 }= x_{1} + X_{Φ}, is the stator self reactance per phase. From Figure (b), the rotor current is given by .............. (3) Using Eq. (3), the torque is given by ............. (4) Figure (c) shows the variation of torque with slip or speed of an induction motor. Figure (c) Torqueslip characteristic curve of an induction motor. The following are the three different modes of operation of an induction motor:
 
 
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