The Direct Current Generator

Most common electrical appliances (e.g., electric light-bulbs, and electric heating elements) work fine on AC electrical power. However, there are some situations in which DC power is preferable. For instance, small electric motors (e.g., those which power food mixers and vacuum cleaners) work very well on AC electricity, but very large electric motors (e.g., those which power subway trains) generally work much better on DC electricity. Let us investigate how DC electricity can be generated.

Figure 41: A split-ring commutator.

A simple DC generator consists of the same basic elements as a simple AC generator: i.e., a multi-turn coil rotating uniformly in a magnetic field. The main difference between a DC generator and an AC generator lies in the manner in which the rotating coil is connected to the external circuit containing the load. In an AC generator, both ends of the coil are connected to separate slip-rings which co-rotate with the coil, and are connected to the external circuit via wire brushes. In this manner, the emf ${\cal E}_{\rm ext}$ seen by the external circuit is always the same as the emf ${\cal E}$ generated around the rotating coil. In a DC generator, the two ends of the coil are attached to different halves of a single split-ring which co-rotates with the coil. The split-ring is connected to the external circuit by means of metal brushes--see Fig. 41. This combination of a rotating split-ring and stationary metal brushes is called a commutator. The purpose of the commutator is to ensure that the emf ${\cal E}_{\rm ext}$ seen by the external circuit is equal to the emf ${\cal E}$ generated around the rotating coil for half the rotation period, but is equal to minus this emf for the other half (since the connection between the external circuit and the rotating coil is reversed by the commutator every half-period of rotation). The positions of the metal brushes can be adjusted such that the connection between the rotating coil and the external circuit reverses whenever the emf ${\cal E}$ generated around the coil goes through zero. In this special case, the emf seen in the external circuit is simply

$${\cal E}_{\rm ext} = \vert{\cal E}\vert = {\cal E}_{\rm max}\,\vert\sin (2\pi\, f\, t)\vert.$$ (218)

Figure 42 shows ${\cal E}_{\rm ext}$ plotted as a function of time, according to the above formula. The variation of the emf with time is very similar to that of an AC generator, except that whenever the AC generator would produce a negative emf the commutator in the DC generator reverses the polarity of the coil with respect to the external circuit, so that the negative half of the AC signal is reversed and made positive. The result is a bumpy direct emf which rises and falls but never changes direction. This type of pulsating emf can be smoothed out by using more than one coil rotating about the same axis, or by other electrical techniques, to give a good imitation of the direct current delivered by a battery. The alternator in a car (i.e., the DC generator which recharges the battery) is a common example of a DC generator of the type discussed above. Of course, in an alternator, the external torque needed to rotate the coil is provided by the engine of the car.

Figure 42: Emf generated in a steadily rotating DC generator.