Question

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Question 7 An induction motor operates in stable region at which of the following slip values? s=0 o high slip region low slip region S

Answer 1

Answer: Low slip region

At low values of slip, torque is approximately proportional to slips and the torque slip characteristics of induction motor is a straight line as shown in the figure. The region (from s= 0 to s=sm) is called the stable region of operation and operating point of the motor should be in this region. Torque Stable region- ------- Unstable region - - ---- Low slip- region Tas High slip region Ta 1/8 Slip S=0 S=Sm S=1 Torque Slip characteristics of Induction motor As the slip increases torque increases and attains its maximum value when s= R/X2. This maximum value of torque is also known as break down or pull out torque. When a further increase in slip occurs due to increase in load beyond the point maximum torque i.e. when slip is high, the value of term (sX2) is very large in comparison to Rz? Therefore, R2 is neglected as compare to (sX2) and torque is given by the expression: T=ksE, Rz/(sX2) or T=kER/(sX22) In the above expression, all quantities are constant excepts Therefore, T a 1/s Thus at higher values of slip (i.e. the slip beyond that corresponding to maximum torque) torque is approximately inversely proportional to slip, s and the torque slip characteristics of induction motor is rectangular hyperbola as shown in the figure. The region (extending from s = Sm to s = 1) is called unstable region. In this region with the increase in load, slip increases but torque decreases. The result is that the motor could not pick up the load and slows down and eventually stops.