 I am Mrs. Veena Sunil Patki, Assistant Professor, Department of Electronics Engineering, Vulturen Institute of Technology, Sallapur. Welcome you for this session. At the end of this session, students can derive the EMF equation of DC generator and describe the characteristics of DC generator. DC generator is the device which converts mechanical energy into electrical energy, when we rotate the armature winding in magnetic field according to Faraday's law, EMF is induced in the armature winding. So, P is the number of poles of the machine, phi is the flux per pole in Weber, Z total number of armature conductors, N is the speed of armature in revolutions per minute, capital A is the number of parallel paths in armature winding. The flux cut by one conductor in one revolution is given by P into phi measured in Weber. According to Faraday's law, the average induced EMF in one conductor is given by d phi by d t, time taken to complete one revolution T equal to 60 by N. So, induced EMF E is given by P phi divided by 60 by N. The number of conductors connected in series in each parallel path is given as Z by A. For lap winding value of A is P that is equal to number of poles and for wave winding A equal to 2. The average induced EMF across each parallel path is given as P phi N by 60 into Z by A volts. So, generated induced EMF is given by EG equal to phi ZN by 60 into P by A volts, where the phi is the flux per pole and N is the speed and Z, 60, P, A all are the constant terms. So, generated induced EMF is proportional to phi and the speed. Now pause the video and find out the answer for this question. EMF generated by shunt generator is E, if pole flux remain constant and speed of generator is doubled then EMF generated will be E by 2 to E slightly less than E or E. So, what is the answer? Answer is 2E because the generated EMF is directly proportional to speed and here the speed of generator is doubled. So, the EMF is 2E, following are the most important characteristics of DC generator, no load saturation characteristics. These characteristics are also called as open circuit characteristics or magnetic characteristics. Second is the internal or total characteristics. And third is the external characteristics or that is also called as the performance characteristics. So, this is the circuit which is used to find out the characteristics. Ammeter is connected in series with the field winding to measure the field current and the voltmeter is connected across the armature. So, this is the graph for all types of generators this OCC is same. So, here the no load voltage is EG and IF is the field current, graph between these two is nothing but the no load saturation characteristics. These characteristics are also called as the magnetization characteristics. So, you can see in this graph OA that indicates the induced EMF in the armature if the field current is 0 that OA is due to residual magnetism in the field winding. After then if we increase the field current, the flux increases and the induced EMF is directly proportional to flux, so the no load induced EMF increases. So, here we will get the straight line and after then even though we increase the field current practically flux remain constant after saturation of the field coils then induced EMF remain constant. You can also see for N1, N2, N3 as we increase the speed that induced EMF also increases. For internal and external characteristics we are going to connect the load across the generator. So, AB indicates the no load voltage if we increase the load current that no load voltage remain constant AC indicates the internal characteristics that voltage drops due to demagnetizing effect of the armature reaction as we increase the IL and AD indicates the external characteristics that terminal voltage decreases due to ohmic voltage drop across the armature. For decision generator armature winding and field winding is connected in parallel, emitter and voltmeter are connected as shown in the figure, variable load is connected across armature to find out the internal and external characteristics. So, AB is the no load voltage that remain constant for any load for internal characteristics you can see that AC due to demagnetizing effect of armature reaction voltage decreases. So, here for external characteristics AD if we decrease the load resistance IL increases as the load current increases that terminal voltage starts decreasing. Now, if we decrease the load resistance beyond some limit that AD curve turns back and as shown in the figure by dotted line that indicates that as we increase the load current that terminal voltage drastically decreases due to armature reaction and due to ohmic voltage drop. So, here there is some limit to decrease in RL, so we cannot decrease the load resistance for this shunt generator. For series generator the field winding and armature winding is connected in series, load is connected across that as we decrease the load that IL increases as IL increases that field current also increases and if the field current increases the induced AMF also increases. So, here the graph indicates that OA is the induced AMF if the load current is 0 or if the field current is 0 due to residual magnetism OA is present and AB is the open circuit characteristics and due to armature reaction drop that induced AMF decreases and the graph between generated voltage and the load current is the internal characteristics is indicated by OC and again the terminal voltage decreases if we increase the load current, if we decrease the load resistance load current increases and at the starting that induced AMF is directly proportional to field current that is nothing but load current because IL equal to field current both are in series, but if we further increase in IL that decreases the terminal voltage for external characteristics OD you can see that by dotted line that if we increase IL that terminal voltage starts decreasing due to demagnetizing effect of the armature reaction. You can refer the books Electrical Engineering by B. L. Thareja, Principles of Electrical Machines by V. K. Mehta and Rohit Mehta, thank you.