Speed Control
The theoretical unloaded speed (with slip approaching zero) of the induction motor is controlled by the number of pole pairs and the frequency of the supply voltage.
When driven from a fixed line frequency, loading the motor reduces the rotation speed. When used in this way, induction motors are usually run so that in operation the shaft rotation speed is kept above the peak torque point; then the motor will tend to run at reasonably constant speed. Below this point, the speed tends to be unstable and the motor may stall or run at reduced shaft speed, depending on the nature of the mechanical load.
Before the development of semiconductor power electronics, it was difficult to vary the frequency, and squirrel-cage induction motors were mainly used in fixed speed applications. Applications such as electric overhead cranes used wound rotor motors with slip rings to allow an external variable resistance to be inserted in the rotor circuit, allowing considerable range of speed control. Some very large slip-ring motor drives recovered energy from the rotor circuit, rectified it, and returned it to the system using an inverter. Many DC motor variable-speed applications can now be served by induction motors and accompanying inverters in industrial applications.The most common and efficient way to control the speed of asynchronous motors is using power inverters and that, in fact, is the only significant disadvantage of this kind of motors, because inverters are rather expensive and usually less reliable than motors themselves.
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