 Myself, Mrs. Veena Sunil Patki, Assistant Professor, Department of Electronics Engineering, Valkan Institute of Technology, Solapur. Welcome you all for this session. At the end of this session, students can describe characteristics of DC motor and methods of speed control. So, three characteristics curves are considered for DC motor, torque versus armature current, speed versus armature current and speed versus torque. For these characteristics, following relations are considered, armature torque is directly proportional to flux and the armature current and speed is directly proportional to back PMF and inversely proportional to flux. You can see in this diagram, the voltage is connected across the armature and across the field winding. The voltage across field winding is constant. So, for this motor, the flux remain constant. So first, let us discuss about the torque versus armature current characteristics. So field flux are constant for the shunt motor and the relation for the torque and the flux armature current is, torque is directly proportional to flux and also directly proportional to armature current. So the state line, which is passing through the origin, that indicates the torque is directly proportional to armature current and the shaft torque is always less than the armature torque due to stray losses. So, these motors are not used for heavy starting torque because at the starting, the torque is less. Now let us discuss about the characteristics of the DC series motor. Armature is connected in series with the series winding and the total voltage is connected across the series combination and the armature torque is directly proportional to flux and armature current and the speed is directly proportional to back PMF and inversely proportional to flux. But here the current through armature is same as the series winding. So flux is directly proportional to armature current, that is IA equal to IF. So let us discuss about the torque versus armature current characteristics. Armature characteristics is also known as electrical characteristics and if we consider before magnetic saturation, the flux is directly proportional to armature current because IF equal to IA. So the flux is directly proportional to IA. So before magnetic saturation, armature torque is directly proportional to IA square. So at the starting, the graph is similar to parabola and the slightly curved graph will be there. But after magnetic saturation, the flux remain constant even though the armature current increases. Flux remain constant and the torque is directly proportional to armature current and here also the shaft torque is less than the armature torque due to stray losses and these motors are used where the high starting torque is required because the TA or the armature torque is directly proportional to square of the armature current. So starting torque is very high for these motors. Now the speed versus armature current graph, you can see here the speed is inversely proportional to armature current because the fluxes are directly proportional to armature current and the speed is inversely proportional to IA and phi is proportional to IA. So at the starting, if the armature current is less, the speed is very high and as the armature current increases, the speed decreases and the speed versus torque, these are again the mechanical characteristics and the speed and armature torque, the relation is again same as the speed and armature current graph. So generally these motors are not started without load because at the starting, the motor has a very high speed. So generally we cannot start these motors without load. Now pause the video and think about which motor can be preferable for high starting torque. So answer is the series motor because in the series motor, the torque is directly proportional to IA square at the starting. Now we will discuss about the speed control methods of DC motor. So back EMF is given by the formula P phi n z by 68 and where the P is the number of poles, phi is the flux per pole, n is the speed in rpm, z is the number of turns, number of armature conductors and A is the parallel paths. Back EMF is given by the formula V minus IARM. Speed is given by the formula EB 60A divided by P phi z, where 60A, P and z are the constant. So speed is directly proportional to back EMF and inversely proportional to phi, where k is the constant. So this shows the speed of the DC motor is directly proportional to back EMF and inversely proportional to flux per pole. So speed control of DC shunt motor has the two methods, field control method and armature control method. So let us first discuss about the field control method. For DC shunt motor, the speed is directly proportional to back EMF and inversely proportional to flux and flux is directly proportional to field current. If we insert one resistance in series with the field winding, we can only decrease the fluxes. So if we increase the resistance in series with the field winding, field current decreases. So flux decreases and speed increases. So here we can only increase the speed. We cannot decrease the speed by using this method. So advantages for this speed control method, it provides relatively smooth and easy control. Speed control above rated speed is possible as the field current is small. The size of rheostat required is small, but the disadvantages are there. The speed control below normal speed is not possible as the flux can be increased only up to rated value and as the flux reduces speed increases, but high speed affects the commutating making motor operation unstable. So there is limit to the maximum speed above normal possible by this method. So armature control of this DC shunt motor, so speed is again directly proportional to back EMF and inversely proportional to flux and K is the constant. So back EMF is given by V minus IRA. So one resistance is connected in series with the armature winding. We can increase the resistance so that we can increase the IRA drop. So speed is directly proportional to back EMF. As the back EMF increases, speed increases, but due to increase in resistance in series with the armature winding, we can simply decrease the back EMF. So we can achieve the speed below the normal speed only. We cannot increase the speed above the rated speed. So this is one disadvantage of the motor. So advantage of this speed control method is easy and smooth speed control below normal is possible. But disadvantages are as the entire armature current passes through the external resistance there are tremendous power losses and as the armature current is more than field current rheostat required is of large size and capacity. Speed above rated is not possible by this method and due to large power losses the method is expensive, wasteful and less efficient. You can refer these books. Thank you.