 Myself, Mrs. Veena Sunil Patki, Assistant Professor, Department of Electronics Engineering, Valkan Institute of Technology, Solapur. I welcome to you all. At the end of this session, students can describe construction of DC motor and working principle of DC motor. The figure shows the constructional details of the simple four pole DC machine. A DC machine consists two basic parts stator and rotor. Basic constructional parts of DC machine are yoke, poles, field winding, armature core, armature winding, commutator and brushes. Now let us see all parts one by one. You can see in this diagram, yoke, the outer frame of a DC machine is called as yoke. It is made up of cast iron or steel. It not only provides the mechanical strength to whole assembly, but also carries the magnetic flux produced by the field winding. So, in motor, poles and field winding has the main role to create the magnetic field. It is just like a stator. Poles and pole shoots are joined to the yoke with the help of bolts. Poles consist of laminations to reduce eddy current losses they carry, field winding and pole shoes are fastened to them. Pole shoes serve two purposes. They support field windings and spread out the fluxes in the air gap uniformly. And field windings are usually made up of the copper. Field coils are placed on each pole and are connected in series. They are bound in such a way that when energized they form alternate north and south poles. Armature core is nothing but the rotor of the machine. It consists of laminated core slots, core winding, commutator and shaft. It is cylindrical in shape with slot to carry armature winding. The armature is built up of thin laminated sheets to reduce eddy current losses. It may be provided with air ducts because when current flows through armature winding that creates the heat in the armature core and for cooling purposes the air ducts are provided with the core and armature is keyed to the shaft. Armature winding of DC machine is always closed and of double layer type. Armature winding essentially means that all the coils are connected in series forming closed circuit. Each commutator segment is insulated from the adjacent segment by mica cell insulation. Armature winding can be wound by one of the two methods lap winding or wave winding. Using a lap winding number of brushes must always be equal to the number of poles. So number of parallel paths in lap winding is always equal to number of poles and lap winding is adopted for low voltage and high current DC machines. A look at those two coils suggests that the winding progresses like a wave hence the name wave winding is given. That the number of parallel paths in the simplex wave winding is always 2. The voltage generated in the armature placed in the rotating magnetic field is alternating in nature. The commutation in DC machine or more specifically commutation is the process in which generated alternating current in the armature winding of DC machine is converted into DC current after going through the commutator and the stationary brushes. Simply we can say that commutator converts AC to DC and the brushes are usually made from carbon or graphite. They rest on commutator segments and slide on segments when the commutator rotates keeping the physical contact to collect or supply the current. Now pause the video and think why the armature core is laminated. The answer is to reduce the ED current losses copper sheets or the laminations are used for the core. The working of DC motor is based on the principle that when a current carrying conductor is placed in magnetic field it experiences a mechanical force whose direction is given by Fleming's left hand rule. The magnitude of the force is given by F equal to BIL sin theta where B is the magnetic field density in Weber per meter square, I is the current in armature in ampere and L is the length of the conductor in meter, theta is the angle between the direction of the current in the conductor and electric field. If the current and the field are perpendicular then theta equal to 90 degree and the equation becomes F equal to BIL and it is the maximum force created on the conductor. In next diagram you can see if current carrying conductor is placed in magnetic field and having a direction of the current inside the plane two magnetic fields are created, one is due to current carrying conductor and another is due to field poles. Effective magnetic field is created as shown in the figure. So the magnetic lines of forces are more right to conductors and less left to conductor so forcing it to left side it is rather like the effect of stretched rubber sheet on an object put on it. We can also find the direction of the force acting on the armature conductors of the DC motor by Fleming's left hand rule which says that if we extend the index finger, middle finger and thumb of our left hand in such a way that the current carrying conductor is placed in a magnetic field represented by index finger is perpendicular to the direction of the current represented by the middle finger then the conductor experiences a force in the direction represented by the thumb mutually perpendicular to both the direction of the field and the current in the conductor. Now let us see types of DC motors according to connection of field winding and armature winding DC motors can be classified into two main categories separately excited and self excited. The diagram indicates the connection between the two windings in separately excited DC motor field coils are energized from an independent external DC source and in self excited DC motor the field coils are energized from the current produced by the motor itself. Initial EMF generation is due to residual magnetism in field poles and the generated EMF causes a part of current to flow in the field coils and self excited DC motors can further be divided into three types. First is the series wound the field winding is connected in series with the armature winding second one is the shunt wound the field winding is connected in parallel with the armature winding and the compound wound it is the combination of series and shunt winding again it has the two types long shunt and short shunt according to series and parallel winding connections. You can refer these books you can also refer the NPTEL courses.