 I, Mrs. Veena Sunil Patki, Assistant Professor, Department of Electronics Engineering, Walchand Institute of Technology, Solapur, welcome you for this session. At the end of this session, students can analyze speed control methods of three-phase induction motor. So, first let us discuss about the requirement of speed control. For some application, it is required to change the drive speed as desired for energy saving and speed control is different than speed regulation when there is a natural change in speed due to change on load on the shaft. Speed control either done is manually or by means of some automatic control device and some application require low speed starting. An induction motor is particularly a constant speed motor. In case of induction motors, speed reduction is achieved by a corresponding loss of efficiency and poor power factor. As induction motors are widely being used, their speed control may be required in many applications. So, let us see about the different types of speed control methods. Speed control from stator side, stator voltage control, stator frequency control, constant V by F control and changing number of stator poles and speed control from rotor side. Rotor resistance control and some another methods are also there. So, speed control from stator side, let us discuss about this first by changing applied voltage. So, first let us see how the torque depends upon the applied voltage. So, torque equation is given as t equal to 3 by 2 pi n s into s e to square r 2 divided by under root r 2 square plus in bracket s x 2 bracket square. So, here r 2 is constant and if slip s is small, then s x 2 square can be neglected. Therefore, torque is directly proportional to s e to square and e to is directly proportional to applied voltage, thus torque is proportional to s and applied voltage. Hence, for providing the same load torque, the slip increases with decrease in voltage and consequently the speed decreases. So, here this is the relation of the speed and applied voltage. So, let us discuss about this method. So, this method is easiest and cheapest still rarely used because large change in supply voltage is required for relatively small change in speed and large change in supply voltage will result in a large change in flux density. Hence, this will disturb the magnetic condition of the motor and if the motor goes into saturation, then that will affect the working of the motor. The graph indicates the torque and the voltage relation. So, as we change the voltage, the maximum torque also changes. So, you can see here v 1 is greater than v 2 is greater than v 3 and the maximum torque also increases with increase in voltage. So, these are the salient features of this method. For low slip motor, the speed range is very low. This method is not suitable for constant torque load. As the voltage changes, torque also changes. So, this method is not suitable for constant torque load and power factor is also poor. So, this method is used mainly in low power applications such as fans, blowers, centrifugal pumps, etc. So, this method is suitable for the applications where the load torque decreases with the speed and where the intermittent operation of the drive is required. For example, in fan and pump load and this method gives a speed control only below the normal rated speed. The variable voltage for speed control of small size motors can be obtained by the following methods given below by connecting an external resistance in stator circuit of motor by using an auto transformer, by using a thyristor voltage control and by using track controller. Now, pause the video and think about this question. The method which can be used for the speed control of induction motor from stator side A, V by F control, B controlling number of stator poles to control synchronous speed, C adding rheostat in stator circuit and D all of these. So, what is the answer? Yes, answer is D all of these. So, second method we are going to discuss about this by changing the applied frequency. So, as the synchronous speed of magnetic rotating magnetic field is given by NS equal to 120 F by P where F is the frequency and P is the number of stator poles. So, synchronous speed is directly proportional to frequency. So, actual speed of an induction motor is given as N equal to NS in bracket 1 minus S. So, the rotor speed is given in terms of synchronous speed. So, we can change the synchronous speed by changing the frequency. At lower frequency the motor current may become too high due to decreased reactance and if frequency is increased beyond the rated value the maximum torque developed falls while the speed rises because here at lower speed the inductive reactance is small. So, the current flowing through the winding is very high and motor goes into saturation and that will affect the flux in the motor. And if we increase the frequency beyond rated value that will affect the maximum torque. So, let us see this by using this graph see here as the frequency increases the maximum torque decreases, but we can see the speed also increases. So, that is why we are not going to use this method in industries because here if the flux changes that will disturb the working of motor. So, here we are going to use the constant V by F control of induction motor. If the supply frequency is reduced keeping the rated supply voltage the air gap flux will tend to saturate and this will cause excessive stator current and distortion of the stator flux wave. Therefore, the stator voltage should also be reduced in proportional to frequency so as to maintain the air gap flux constant because the magnitude of stator flux is proportional to ratio of stator voltage and frequency. So, these are the mathematical equations V equal to 4.445 k T F. So, from this equation we can write down the phi equal to V by 4.44 k T F. So, the flux is proportional to V by F where k is the winding constant and T is the number of turns per phase and F is the frequency. Also by keeping V by F constant the developed torque remains approximately constant. So, we can see here from this graph so here you can see the maximum torque remains constant as the frequency changes. So, this method has higher runtime efficiency majority of AC speed drives employ this method wide range of speed control is there and here soft start capability is also there for this method. So, generally in industries V by F drives are used to control the speed of induction motors. So, the third method that is changing the number of stator poles synchronous speed of rotating magnetic field is given by N S equal to 120 F by P and synchronous speed N S is inversely proportional to P number of poles. And actual speed of induction motor is given as N equal to N S in bracket 1 minus S. So, by changing the synchronous speed we are going to change the rotor speed. For squirrel cage induction motors change in stator poles is achieved by two or more stator windings wound for different number of poles in same slots. For example, a stator is wound with two three phase windings and one for four poles and other for six poles for supply frequency of 50 hertz. One synchronous speed when four pole winding is connected. We can calculate the synchronous speed as 120 F by P is the formula for synchronous speed. So, N S equal to 120 into 50 by 4 that is equal to 1500 rpm. For second case synchronous speed when six pole winding is connected here the synchronous speed we can calculate as 120 into 50 by 6 that is equal to 1000 rpm means if we increase the number of poles we are going to decrease the synchronous speed and then the rotor speed because the rotor speed is given as N S in bracket 1 minus S. So, you can refer the book Electrical Machines by B. L. Thareja, Principles of Electrical Machines, V. K. Mehta and Rohit Mehta. Thank you.