Torque slip and Torque speed curve obtain from the torque equation.
The slip() defines as the ratio of the difference of synchronous speed () and rotor speed () to the synchronous speed. The synchronous speed defines the speed at which the magnetic field rotates. Rotor speed or actual speed is the speed at which rotor rotates.
The range of slip is between zero to one and the range of rotor speed is between zero (standstill) and (synchronous speed).
Torque slip characteristics
Torque slip characteristic is a curve plotted between the torque and the slip of induction motor. It also gives the relationship between torque and slip.
From the above equation, we can conclude below statements and draw the curves.
1) When the rotor speed is equal to the synchronous speed, the slip is zero. In this condition, the torque is zero from the above equation. Hence, the torque-slip curve stats from zero (origin).
2) When the rotor speed is not equal to the synchronous speed but near to the synchronous speed, the slip is very low. In this condition, the is very small and negligible compared to the . If the rotor resistance consider as constant, the torque is proportional to the slip. Hence, torque slip characteristic is an approximately straight line as shown in the above figure.
3) If slip increases with the increase in load, the torque reaches the maximum value. At this time, slip is equal to the rotor resistance and rotor reactance. The maximum torque is also known as the pull-out torque or break down torque.
4) If further increase in the slip with an increase in load beyond the maximum torque, the torque start decreases. In this condition, the motor slows down and maybe stop running. With a higher value of slip, is negligible as compared to the . Torque is inversely proportional to the slip.
Motor operates in the range of slip between zero and that corresponding to the maximum torque.
The graph of Torque slip characteristic is as shown in below figure.
Torque speed characteristic
1) Starting torque is slightly larger than the full load torque. Hence, the induction motor can start with a load that it can supply at full power.
2) Torque speed curve is linear between no load and full load. In this condition, the is negligible compared to the . The maximum torque is 2 to 3 time of rated full load torque. At full load, speed is not equal to the synchronous speed but the actual speed of the rotor is near to the synchronous speed.
3) If the rotor speed is equal to the synchronous speed, induced torque is zero.
Effect of Rotor Resistance
Torque slip characteristics is depends on the rotor resistance. As the rotor resistance varies, the value of maximum torque remains as it is. It means that, the value of maximum torque is independent from the rotor resistance.
In squirrel cage induction motor, we cannot change the rotor resistance. So, for this experiment, we have to use slip ring induction motor. As the rotor resistance changes, the change in slip at which this maximum torque takes place.
The below graph shows the effect of change in rotor resistance.
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