Load Characteristic of AC Induction Motor
In real applications of AC Induction Motor, various kinds of loads exist with different torque-speed curves. These various kinds of loads include, for example, Constant Torque, Variable Speed Load (screw compressors, conveyors, feeders), Variable Torque, Variable Speed Load (fan, pump), Constant Power Load (traction drives), Constant Power, Constant Torque Load (coiler drive), and High Starting/Breakaway Torque followed by Constant Torque Load (extruders, screw pumps).
The ac motor load system is said to be stable when the developed motor torque is equal to the load torque requirement. The ac induction motor will operate in a steady state at a fixed speed. The response of the motor to any disturbance gives us an idea about the stability of the motor load system. This helps us in quickly evaluating the selection of a ac induction motor for driving a particular load.
And in most drives, the electrical time constant of ac motor is negligible as compared to its mechanical time constant. So during transient operation, ac inducion motor can be assumed to be in an electrical equilibrium, implying that the steady state torque-speed curve is also applicable to the transient operation.
As an example, the figure below shows torque-speed curves of the ac induction motor with two different loads. The system can be termed as stable, when the operation will be restored after a small departure from it, due to a disturbance in the motor or load.
For example, disturbance causes a reduction of △ωm in speed.
In the first case, at a new speed, the ac induction motor torque (T) is greater than the load torque (Tl). Consequently, ac motor will accelerate and the operation will be restored to X. Similarly, an increase of △ωm in the speed, caused by a disturbance, will make the load torque (Tl) greater than the ac motor torque (T), resulting in a deceleration and restoration of the point of operation to X. Hence, at point X, the system is stable.
And in the second case, a decrease in the speed causes the load torque (Tl) to become greater than the motor torque (T), the drive decelerates and the operating point moves away from Y. Similarly, an increase in the speed will make the ac induction motor torque (T) greater than the load torque (Tl), which will move the operating point further away from Y. Thus, at point Y, the system is unstable.
This example shows that, while in the first case, the ac induction motor selection for driving the given load is the right one; in the second case, the selected motor is not the right choice and requires changing for driving the given load.
