Panel switch wiring

There are the following two methods for the Speed control of the Induction motors.

             The Induction motor speed can be controlled by changing the applied voltages the stator , Because in induction motor the out put torque is Directly Proportional to the Squire of the voltage and inversely proportional to the resistance of the rotor, while       S= Slip.

Thus the motor speed controlled with out changing the supply frequency, for example, if the supply voltage value is decreases to its half, the motor torque is decreases ¼ times; the torque is directly proportional to the speed of the motor.

In stator voltage control method, the stator voltage is controlled by a SCRs, these SCRs are connected with three phase supply (With each Phase) in anti parallel conduction, the out put voltage of the SCR is controlled by the firing angle of the SCR, Increasing the firing angle, decreases the out put voltage and this way the speed of the induction motor is decreases. By decreases the firing angle, increasing the out put voltages and the speed of the motor is increases.

In the give ckt an induction motor is shown, with each phase of the motor the SCRs are connected in anti parallel conduction.

In this ckt two SCR are conduct at the same time, this way a very large current flow through the Stator, therefore this method is used for small or medium size motors.

            The frequency method is very important through which the speed of the induction motor is controlled.

Form the above formula, it is clear that the speed of the induction motor directional proportional to the frequency. It means that the frequency of the supply voltage changes , the speed of the indication motor also changes , increases the in put frequency of the supply increases the speed of the induction   motor  also increase and vice versa.

In the frequency controlled method, the three phase supply is controlled through a controlled rectifier. The voltage is applied on the forced Commutated Bridge rectifier, which produce a variable frequency for control of the induction motor.