Fundamental Electrical and Electronic Principles, Third Edition

Chapter 7D.C. MachinesLearning OutcomesThis chapter covers the operating principles of d.c. generators and motors, their characteristicsand applications. On completion you should be able to:1 Understand and explain generator/motor duality.2 Appreciate the need for a commutator.3 Identify the different types of d.c. generator, and describe their characteristics. Carry outpractical tests to compare the practical and theoretical characteristics.7.1 Motor/Generator DualityAn electric motor is a rotating machine which converts an electricalinput power into a mechanical power output. A generator converts amechanical power input into an electrical power output. Since oneprocess is the converse of the other, a motor may be made to operate asa generator, and vice versa. This duality of function is not confined tod.c. machines. An alternator can be made to operate as a synchronousa.c. motor, and vice versa.To demonstrate the conversion process involved, let us reconsidertwo simple cases that were met when dealing with electromagneticinduction.Consider a conductor being moved at constant velocity, through amagnetic field of density B tesla, by some externally applied force Fnewton. This situation is illustrated in Fig. 7.1 .Work done in moving the conductor,W Fd newton metre233