 Myself, this is Veena Sunil Patki, Assistant Professor, Department of Electronics Engineering, Walchand Institute of Technology, Solaapur. Welcome you for this session. At the end of this session, students can analyze speed control methods from rotor side of three phase induction motor. So, these are the methods. Armature resistance control, cascade operation and by injecting EMF in rotor circuit. So, these are the three speed control methods from rotor side. So, first let us discuss about the rotor rheostat control. This method is similar to that of armature rheostat control of DC shunt motor, but this method is only applicable to slip ring motors as addition of external resistance in rotor of squirrel cage motor is not possible. Because here for squirrel cage induction motor, we cannot add the external resistance, only the conductor bars are used for that rotor. And for slip ring rotors, we can add the external resistance because the winding is placed on the rotor and slip rings are used to connect the external resistance for this method. External resistance is added on rotor side. So, torque equation is given as t equal to 3 by 2 pi n s into s e 2 square r 2 divided by under root r 2 square plus in bracket s x 2 bracket square. The three phase induction motor operates in low slip region and in low slip region s x 2 square becomes very very small as compared to r 2. If we see that equation s x 2 square is very small because the s is very less. So, we can neglect it and here e 2 is also constant. So, the equation of torque becomes torque is proportional to slip and inversely proportional to r 2. If we increase rotor resistance r 2, torque decreases and to supply the same load torque must remain constant. So, we increase slip which will further result in decrease in rotor speed and thus by adding additional resistance in rotor circuit, we can decrease the speed of three phase induction motor. The main advantage of this method is that with an addition of external resistance starting torque increases, but motor also suffers from some disadvantages. The speed above the normal value is not possible and large speed change requires a large value in large speed change requires a large value of resistance and it will cause large copper loss and hence reduction in efficiency. So, when the intermittent speed control is required, this method is used. Heat has some drawbacks. The rheostat per phase causes unbalancing in motor. The resistances generate huge losses and generate heat. As the resistance increases, heat also increases and it requires more maintenance, hence the cost is more and this method cannot be used for industrial automation purpose. Now, pause the video and think about this question. High starting torque can be obtained in three phase induction motor by a. Increasing rotor resistance b. Decreasing rotor resistance c. Increasing rotor reactance and d none of the above. So, what is the answer? Increasing rotor resistance. So, by changing rotor resistance, we can change the torque also. So, speed control by cascade operation. In this method, the speed control of three phase induction motor of two three phase induction motors are connected. The two three phase induction motors are connected on a common shaft and hence called cascade motor, cascaded motor. So, cascaded motor means two motors are connected with same shaft. So, one motor is called main motor and another motor is called auxiliary motor. You can see here in this diagram motor a is the main motor and motor b is the auxiliary motor. So, two motors are connected with same shaft and with slip rings, the rheostat is connected variable resistance is provided for motor b and the supply is given from the slip ring to motor b. Motor a is called the main motor, motor b is called the auxiliary motor, NS1 and NS2 are the synchronous speed of motor a and b, P1 number of poles of stator of motor a, P2 is the number of stator poles of motor b and N is the speed of set and same for both motors. So, with this method, four different speeds can be obtained when only motor a works NS1 equal to 120 F by P1 is the speed of the motor and when only motor b works NS2 equal to 120 F by P2 is the speed of the set and if cumulative cascading is done, then the speed is given as 120 F by P1 plus P2 and if differential cascading is done, then the speed is given by N equal to 120 F by P1 minus P2. So, let us see about the by injecting EMF in rotor circuit, speed control of induction motor. When the speed control is done by adding resistance in rotor circuit, some part of power called the slip power is lost as I square R losses and therefore, the efficiency is reduced and this slip power loss can be recovered and supplied back to improve the overall efficiency and this scheme recovering the power is called slip power recovery scheme. So, in this method, speed of induction motor is controlled by injecting a voltage of slip frequency in rotor circuit and if we inject EMF, which is in opposite phase with rotor induced EMF, rotor resistance will be increased and speed will decrease and thus by changing the phase of injected EMF, speed can be controlled. The main advantage of this method is a wide range of speed control above normal as well as below normal can be achieved. So, here this is the block diagram by injecting EMF in rotor circuit. You can see the converter is used and one inverter is used and by using that inverter where injecting the rotor EMF into rotor circuit so that we can control the rotor speed below or above normal speed. You can refer the book, Electrical Machines by B. L. Thareja, Principles of Electrical Machines, VK Mehta and Rohit Mehta. Thank you.