BY ‘DC MACHINES’ we mean either generators which convert mechanical energy into electrical energy and deliver a unidirectional current, or motors which do the converse. Usually a DC machine can serve either purpose. AC machines are more widely used than DC machines, but in certain applications DC machines used to be preferred, especially in traction and whenever speed control over a wide range was desired. With the development of high-power semiconductor devices such as the thyristor and gate-turn-off thyristor (GTO) it has now become much easier to control the speed and torque of AC motors so that these are now preferred even for traction. However, the same devices have made possible the development of brushless DC motors of up to 50 kW. Regardless of the mode of excitation, what is indispensable in all electrical machines is relative motion of an electrical conductor and a magnetic field and we shall start by looking at a very simple example.
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