 Myself, Mrs. Veena Sunilpathy, Assistant Professor, Department of Electronics Engineering, Valkan Institute of Technology, Sallapur, welcome you for this session. At the end of this session, students can compare different starters of three-phase induction motor. So, let us see what is the need of starters. A three-phase induction motor is self-starting. At the time of starting, the motor slip is unity and the starting current is very large. And this large starting current will produce large line voltage drop and this will adversely affect the operation of other electrical equipment connected to the same line. And therefore, it is desirable and necessary to reduce the magnitude of stator current at starting and several methods are available for this purpose. And the purpose of starter is not to just start the motor, but it performs the two main functions. They are as follows, to reduce the heavy starting current and to provide overload and under-voltage protection. So, here these are the methods. The method depend upon the size and the type of the motor. So, direct on line starting, stator resistance starting, auto transformer starting, star delta starting and rotor resistance starting. So, these are the methods which are used to start the three-phase induction motor. So, let us discuss about the direct on line starter. Small capacity motors approximately below 5 HP does not have very high starting currents. Such motors can withstand the high starting currents for very small time. And this starter does not reduce the starting voltage. It provides the protection to the motor against overloading, single phasing and low voltage. You can see here, this is the diagram of the direct on line starter. You can see here the different parts, fuse, thermal overload relay, then magnetizing coil is there, then contactor is there, contact to keep coil energized and NO, normal open start and NC, normal closed stop. So, two switches are there to switch on the motor and to switch off the motor and the rotor is there. So, when we press the normal open start, then the supply is connected to the motor through fuse and thermal overload relay. And the magnetizing coil is then magnetized due to supply and that magnetizing coil here you can see in this magnetizing coil, the magnetic field is created and due to that magnetic field, these contactors gets attracted towards this magnetizing coil and these contactor get connected to the supply and the supply is connected to the stator. But when we switch off this means normal closed switch is used to stop the supply. This contact then becomes off the supply from the stator, this magnetizing coil then demagnetizes and then the contactor move towards this side. So, stator is then disconnected from the supply, but if in the motor the overload or the heat is created, then the thermal overload relay activates and supply is disconnected from the stator side. So, this is the working of this DOL. Shortly we can say that direct on line as a DOL, it is the simplest, cheapest and most reliable method and hence widely used. This provides nearly full starting torque at starting because full voltage is connected to the stator and the main disadvantage of DOL stator is that the motor draws a very high current during start for the period of 5 to 8 times full load current for a very short period. This 5 to 8 times full load current flows through the motor. Now, we can directly write the expression for torque of induction motor as t equal to 1 by ns i square r2 by s and from the help of the above expression, we write the ratio of starting torque to full load torque as ts by tf equal to in bracket is by if bracket square into sf, where ts is the starting torque and tf is the full load torque. Like that is is the starting current and if is the full load current. So the torque is directly proportional to slip and proportional to square of the current. So in reduced voltage method, we have three different types of starting methods. Stator resistance starting method, auto transformer starting method and star delta starting method. A stator resistance is connected in each line series with each phase winding of the stator. Initially, their maximum resistance is connected in series with stator winding. So reduced voltage is connected to motor. As the motor accelerates, the stator resistance is then reduced. So here you can see in this diagram, the resistance is connected in series with the stator winding this. So at the starting, very large resistance is connected in series with the stator winding and then we are going to reduce that resistance for run time. So that the full torque will be generated on the rotor. Thus by adding resistor, we can control the supply voltage. Similarly a fraction of voltage X of supply voltage is applied at the time of starting of the induction motor. The value of X is always less than 1 and due to the drop in voltage, the starting torque also decreases and the ratio of starting torque to full load torque is given as T s by T f equal to i s by i f bracket square into S f. Now at the time of starting, the per phase voltage is reduced to X v1. The per phase starting current is also reduced to X i s. So substituting the value of i s as X i s in this equation, here we can write down the starting torque and full load torque ratio as i s by i f bracket square S f into X square and the equation shows the variation of starting torque with value of X. If we add series resistor, then the energy losses are increased. So it is better to use series reactor in place of resistor because it is more effective in reducing the voltage. However, series reactor is more costly than series resistance. Now pause the video and think about this question. Starters are required in the induction motor because A, high starting current, B, they are not self-starting, C, torque produced is very low at starting to overcome inertia and D, all of the mentioned. So what is the answer? The answer is A, high starting current because starters are needed in induction motors to limit the starting current. So auto transformer starting also known as auto starters used for both star or delta connected squirrel cage motors, it is basically a three phase step down transformer with steps provide that permit the user to start the motor at say 50 percent, 65 percent or 80 percent of line voltage. So at the time of starting, per phase voltage is reduced to X v1 and per phase starting current is also reduced to X s. So we can write down the starting torque equation to full load torque as i s by i f bracket square S f into X square. So here also we will get the starting torque that is proportional to X square. So this shows the variation of starting torque with the value of X. So this is the diagram you can see here, the supply is connected to the stator winding these are the stator windings and here at the starting the start position here the steps are provided. So we can reduce the voltage by increasing the number of turns here and here the dropped voltage or the small voltage is connected to the stator at the starting and then afterwards we can change over the switch so that the supply is then total supply is then connected to the stator. So auto transformer starting has several advantages auto transformer starter limits significantly the inrush current for large machine over 25 HP this method of starting is often used and this method can be used for both star and delta connected motors. So you can refer the book electrical machines by B. L. Tareja principles of electrical machines by V. K. Mehta and Rohit Mehta thank you.