D.C. Machines1.1 IntroductionAn electrical machine is an electro- mechanical energy conversion device. The device which converts electrical energy into mechanical energy is called a motor. The device which converts mechanical energy into electrical energy is called a generator. The basic principle behind the working of an electrical machine is electromagnetic induction. Fig 1.1 shows the schematic representation of an electrical machine. Fig.1.1. Schematic representation of electromechanical energy conversion devicesThe classification of electrical machines is shown in Fig. 1.2 Fig. 1.2 Classification of electrical machines1.2 D.C. GeneratorA dc generator converts mechanical energy to electrical energy. The energy conversion is based on the principle of the production of dynamically induced emf. Whenever a conductor cuts magnetic flux, dynamically induced emf is produced in it according to Faradays Laws of Electromagnetic Induction.Hence, the two basic essential parts of a dc generator are (i) a magnetic field and (ii) a conductor or conductors which can move to cut the flux.1.2.1 Simple Loop D.C. Generator Fig. 1.3. Elementary dc generator Fig. 1.4. Fig. 1.5.Consider a single turn coil ABCD rotating about its own axis in a magnetic field. The two ends of the coil are connected to a slip ring splitted into two parts as shown in Fig. 1.3. The two halves of slip ring are separated from each other by mica insulation and brushes are placed diametrically opposite on each half. Let us consider the coil is rotating in the clock-wise direction. An e.m.f. is induced in the coil which is proportional to the rate of change of flux linkage. When the plane of the coil is at right angle to the direction of field, the flux 1inked with the coil is maximum, whereas the rate of change of flux linkage is minimum Hence there is no induced e.m.f. This position of the loop is known as neutral position. When the plane of the coil is in the direction of the field, the flux linked with the coil is minimum whereas the rate of change of flux linkage in maximum. Hence maximum emf is induced in the coil at this position of loop.The direction of current in the coil reverses after every half revolution as in Fig. 1.4. In order to get a unidirectional current in the external load resistance, split ring (i.e. commutator) arrangement is used. The function of commutator in a d.c. generator is to convert the alternating current produced in the armature into direct current in the external circuit. Its wave shape is shown in Fig. 1.5.1.2.2 Main Constructional FeaturesA d.c. machine has two main parts, field (stator) and armature (rotor). The stator is the stationary member and consists of field system. The rotor of a d.c. machine houses armature winding. Fig 1.6 shows the simplified cross- sectional view of such a d.c. machine. Fig. 1.6. Cross-sectional view of a dc machineThe major parts of a dc machine are described as follows. Pole Core, Pole Shoe and YokeThe pole cores are usually made of a number of steel laminations stacked and rivetted together. The pole cores are then bolted to a hollow cylindrical frame called the yoke. The yoke may be made of cast steel or fabricated rolled steel. It provides mechanical support for the poles and acts as a protecting cover for the whole machine. The pole shoes serve two purposes.1) They spread out the flux in the air gap and reduce the reluctance of the magnetic path.2) They support the field coils.The field coils or pole coils are made of copper wire or strip and are former wound for the correct dimensions. The wound coil is put into place over the core. The field poles when excited by d.c. current produces magnetic field.Armature Core and WindingsThe armature is a laminated cylinder and is mounted on a shaft. The air gap between armature and field poles is kept very small to minimise the reluctance of magnetic circuit. The laminations are about 0.4-0.6 mm thick and are insulated from one another. Laminations reduce eddy-current loss in the core. Armature windings are housed in the slots on the periphery of laminated armature and are usually former wound.Commutator and BrushesCommutation is the process of conversion of alternating current induced in the armature into d.c. at the output circuit. The commutator and brush arrangement is the connecting link between the armature winding and external circuit. The cross-sectional and longitudinal view of commutator is as shown in Fig. 1.7. Fig. 1.7.Commutator is of cylindrical shape and is made up of wedge-shaped hard drawn copper segments. These segments are insulated from one another by thin sheet of mica. Each armature coil is connected to the commutator segment through a riser. The brushes are usually made of high grade carbon because carbon is a conducting material and also provides lubricating effect on the commutating surface. The brushes are held in particular position around the commutator by brush holders.