 Myself, Mrs. Veena Sunil Patki, Assistant Professor, Department of Electronics Engineering, Vulturen Institute of Technology, Solapur, welcome you for this session. At the end of this session, students can compare different starters of three-phase induction motors. So, first let us see about the different methods for starting of three-phase induction motor. This method depends upon the size and type of the motor, direct on-line starting, stator resistance starting, auto transformer starting, star delta starting and rotor resistance starting. So, already we have discussed about the ABC in last session. So, today we are going to discuss about the star delta starting and rotor resistance starting. So, star delta stator is the type of reduced voltage stator, we use it to reduce the starting current of the motor without using any external device or apparatus. This is a big advantage of star delta stator as it typically has around one-third of the in-rush current compared to DOL stator. So, direct on-line stator generally we are going to use that for a very small machines less than 5 HP and we cannot decrease the current for that stator, but here we are going to decrease the voltage which is connected to the motor so that we are going to decrease the in-rush current of the motor. So, this is the diagram of the star delta stator. So, at the starting the reduced voltage is connected by connecting the winding to the star connection and then we are converting that winding to delta winding so that the full voltage is given to the motor and motor run with full speed. So, this is about the diagram of star delta stator. Now, let us see how that reduced voltage is given to the motor. The stator phase windings are first connected in star and full voltage is connected across its free terminals. As the motor picks up the speed the windings are connected through a switch and they are reconnected in delta across the terminal voltage. The current drawn by the motor from the line is reduced to 1 by root 3 as compared to the current it would have drawn if connected in delta. Now, we shall see how the star delta stator reduces the starting current of three phase induction motor. Let us consider the VL is the line voltage, ILS is the supply line current, IPS is the winding current per phase and Z is the impedance per phase winding at stand still condition. So, if the winding is connected in star the total voltage VL line voltage is given and the voltage per phase is given by VL by root 3 and line current equal to phase current. For star connection the relation is phase voltage is given as VL by root 3 or we can say that line voltage is given by root 3 into phase voltage ok. So, line current and phase current both are equal. So, we can calculate that ILS equal to VL by root 3 Z. Now, if we consider the delta here you can see in this diagram the line voltage and phase voltage both are same, but line current and phase current has some relation line current is given by root 3 times phase current. So, we can calculate the phase current equal to VL by Z and if we calculate the ratio of delta connected and star connected here we will get the relation that is ILD divided by ILS equal to root 3 VL by Z divided by VL by root 3 Z that is nothing but 3. In delta connected system the current flows 3 times greater than the star connected system means we can also say that in star connected line system the current flows one third of delta current that is the relation between the line current of star and delta. The advantages of star delta starters include this starter is inexpensive, no heat is produced or tap changing device needs to be used, hence efficiency increases, starting current reduced to one third of direct online starting current, so produce high torque per ampere of line current. Now pause the video and think about this question and auto transformer starter is suitable for A star connected induction motor, delta connected induction motor then both A and B and none of these. Yes, what is the answer? Answer is C both A and B because the auto transformer starter is suitable for star and delta connected induction motor both. So, let us discuss about the rotor resistance starter. The name itself indicates that the resistance is inserted in rotor circuit, so generally we cannot use these starters for squirrel cage induction motor as we cannot add the external resistance for that rotor. The variable resistance is connected with the rotor circuit to limit the rotor current, the external resistance is inserted in each phase of the rotor circuit through slip rings and brush at starting the resistance is kept to its maximum value as the motor picks up the speed the resistance is gradually reduces. You can see in this diagram the rotor and the slip rings are given with that slip rings we are going to insert one rheostat external rheostat and that rheostat actually is kept at the maximum resistance at the starting, so that we are going to reduce the starting current which is flowing through the rotor circuit. When the motor attains the normal speed the rotor windings are short circuited through the slip rings and brushes as the external resistance been removed and this operation may be manual or automatic and this method not only limits the starting current but also increases the starting torque due to added rotor resistance because the torque is directly proportional to rotor resistance that is R2. We can also increase the starting torque by using this starter. Now let us see the comparison between the three starters direct online starter, auto transformer starting and star delta starting. So, full voltage is applied to the motor at the time of starting requirement. The starting voltage can be adjusted according to the starting and for star delta starting each winding gets 58 percent that is nothing but 1 by root 3 of the rated line voltage at the time of starting. Then for DOL the starting current is 5 to 6 times of the full load current. For auto transformer the starting current can be reduced as desired and for star delta starting the starting current is reduced to one third that of the direct online starting. Then direct online starting only three wires are to be brought out from the motor. For auto transformer only three wires are to be brought out from the motor but for star delta starting six wires are to be brought out from the motor. So, direct online starting the cost is low auto transformer cost is high again for star delta starter low cost is there. Direct online starting is very easy operation but auto transformer starting not so easy to operate skilled operators is needed and for star delta starting not so easy to operate since the connections are first to be made in star and then in delta either manually or automatically. For DOL starter used for motors up to 5 HP auto transformer starting used for large motor ratings and star delta starting is used up to 10 HP motors. So, this is about the comparison between DOL auto transformer starting and star delta starting. You can refer the book Electrical Machines by B.L. Thareja, Principles of Electrical Machines by V.K. Mehta and Rohit Mehta. Thank you.