 I am Mrs. Veena Sunilpathy, Assistant Professor, Department of Electronics Engineering, Valkan Institute of Technology, Solapur, welcome you for this session. At the end of this session, students can explain 3 point starter and 4 point starter. So, let us see about the necessity of starter. Starters are used to start and stop the motor. Starters are used to limit the inrush current when necessary. So, generally the starting current for motor is very high. We are going to limit that starting current by using starters. It gives automatic control when required. It protects the motor from the fault overload and no hold. So, in general the motor EMF equation is given by V equal to EB plus IARA, where V is the supply voltage, EB is the back EMF, IA is the armature current and RA is the armature resistance. Since at the starting back EMF is 0 because the conductors are stationary and the back EMF is developed due to cutting of flux by the conductor. So, if the conductors are stationary means if the speed is 0 back EMF is 0 then the V is given by IARA. So, we can find out the armature current IA equal to V by RA. As the armature resistance is very small, very high current flow through the armature winding and due to that the armature winding may burn and to limit the starting current through armature winding we are going to use the starters. That means the resistance is inserted in series with the armature winding. So, this excessive armature current will blow out the fuses and may damage the armature winding or commutator brush arrangement. It produces very high starting torque as the torque is directly proportional to armature current and this high starting torque will produce huge centrifugal force which may throw off the armature winding. So, these starters avoid the above two drawbacks starters are used for starting of DC machine. There are various starters like 3 point starter, 4 point starter, no load release coil starters, thyristor starters etcetera. The main concept behind every DC motor starter is adding external resistance to the armature winding at starting. So, you can see this is the 3 point starter. Why the name 3 point is given? You can see here the L, F and A. These are the 3 points where L is connected to the supply voltage, F is connected to the field winding of the motor and A is connected to the armature winding. So, here you can see the different parts. This is the resistance which is connected in series with the armature winding. This is the arm or the starter handle spring lever and the triangular iron is provided for this overload coil. This is the no volt coil and this is the off position. So, when we move this handle to the first position, this total resistance is connected in series with the armature. Due to this series resistance, we can limit the starting current and then afterwards we move this handle from this first to second, second to third like that here at the run position. When current flows through this no volt coil, this no volt coil creates the magnetic field and this arm is hold by this no volt coil because here the magnetic material is connected and here the overload coil is connected. So, pause the video and think what is the necessity of starter? Yes, what is the answer? To limit the starting current, starters are used that is the answer. Now, let us see about the working of no volt coil. Here the no volt coil is there, when current flows through this coil, this no volt coil creates the magnetic field and when we move this arm from first position to run position due to this magnetic field, this arm remains at this position. But now if the no volt supply or if the supply voltage is suddenly interrupted, then this no volt coil de-magnetizes and due to this spring action, this handle moves to off position. But if the no volt coil were not used, what will happen? This arm remains at this position and when suddenly again the supply comes back, the total voltage is connected across the armature winding and very large current flows through the armature winding and armature winding may burn. So, for that the no volt coil is used in the starter. Now let us see about the working of overload coil. So, if the motor is overloaded or fault occurs, very large current flows through the motor and this is the overload coil, very large current flows through this overload coil and this overload coil creates the magnetic field for that current only. When this arm lifts up due to this triangular portion, this no volt coil de-magnetizes and this handle goes to off position. So, here this overload coil protects the motor from overload current. Now let us see this by experiment. This is the experimental setup. You can see here this is the starter, so number of studs are given, so these are the resistances. Here the emitter and voltmeters are connected. You can see here when we move this handle to start the motor, here you can observe the deflection of this meter, very large current flows through the motor, through the armature winding. You can observe that very large current flows through that armature winding and then it settles down. So, to limit that starting current, this resistance is inserted in series with the armature winding and here you can see here the working of overload coil, we are going to move that plate upwards and that motor turns off. Now, we can see the drawback of three point starters. The no volt coil is connected in series with the shunt field circuit, while exercising the speed control through field regulator, the field current may be weakened because we are going to insert one resistance in series with the field winding due to that the field current decreases as the field current decreases, flux decreases and speed increases. So, generally we use the speed control method where we required very high speed above the normal speed. So, this field current becomes the no volt coil magnetic field and at very high speed the motor goes to off position due to demagnetization of the no volt coil. So, this may disconnect the motor from the supply when it is not desired. So, motor is at very high speed and it is not desired that to disconnect the motor from the supply. So, this drawback is overcome in the four point starter. In the four point starter, the field winding is separated from the no volt coil. You can refer the books, Electrical Engineering by B. L. Thareja, Principles of Electrical Machines by B. K. Mehta and Rohit Mehta. Thank you.