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The study of the electrical engineering, basically involves the analysis of the energy transfer from one form to another. An electrical machine, deals with the energy transfer either from mechanical to electrical form or from electrical to mechanical form This process is called electromechanical energy conversion. An electrical machine which converts mechanical energy into an electrical energy is called an electric generator. While an electrical machine which converts an electrical energy into the mechanical energy is called an electrical motor. Such electrical machines may be related to an electrical energy of an alternating type called ac. machines or may be related to an electrical energy of direct type called dc. machines The d.c. machines are classified as d.c. generators and d.c. motors. The construction of a dc. machine basically remains same whether it is a generator or a motor. In this chapter constructional features of a de machine, working principle and types of dc-generator are discussed. But before beginning the study of the d.c. machines, it is necessary to revise the basic concepts of magnetism and electromagnetism. Magnetism is a property by virtue of which a piece of solid body attracts iron pieces and pieces of some other metals. Such a piece of solid body is called a natural magnet. The two ends of a magnet are called its poles. When such a magnet is suspended freely by a piece of a silk fiber, it tums and adjusts itself in the direction of North and South of the earth. The end adjusting itself in the direction of North is called N pole while other is called S pole. When such two magnets are brought near each other, their behaviour is governed by some laws called laws of magnetism.
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