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Generally AC electrical power drives most common electrical appliances like electric heating elements and electric light bulbs but there are some situations in which DC power is preferable. For example in electric motors (eg. Which power vacuum cleaners and food mixtures) AC electricity is used but for very large electric motor, those powering subway trains, use DC electricity as they work much better on DC electricity. Now let us see how DC electricity is generated.
Fig (1): A split ring commutator.
A simple DC generator consists of a multi-turn coil rotating uniformly in a magnetic field, similar is the arrangement in a simple AC generator. The major difference between DC generator and AC generator is in the way the rotating coil is connected to the external circuit having load. In case of an AC generator, both ends of the coil are connected to separate slip rings co-rotating with the coil and are connected to the external circuit through the brushes. Hence, the emf, Eext, seen by the external circuit is same as emf, 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 co-rotating with the coil. Metal brush is used for connecting split ring to the external circuit (see the above figure). A commutator is the combination of stationary metal brushes and rotating split ring. The purpose of the commutator is to ensure that the emf, Eext, seen by the external circuit is equal to the emf, E, generated by the rotating coil for half the rotation period, but is equal to minus this emf for the other half (as the connection between external circuit and rotating coil is reversed by the commutator every half period of rotation). The position of the metal brushes can be adjusted such that the connection between the rotating coil and the external circuit reverses whenever the emf , Eext, generated around the coil goes through zero. In this special case, the emf seen in external circuit is given by:
The below plot shows Eext plotted as a function of time according to the above equation. The plot of emf is very similar to AC except for the fact that the plot never goes negative because of the commutator. The commutator reverses the polarity of the coil with respect to the external circuit so thet the negative half of the AC signal is converted to positive. The resulting emf is a bumpy emf which rises and falls but never changes the direction to negative. This type of pulsating emf can be smoothed out by using more than one rotating coil about the same axis or by other electrical techniques to give a good imitation to direct current delivered by battery. One example of the DC generator of the type discussed above is the alternator of the car. In an alternator, the external torque needed to rotate the coil is given by the engine of the car.
Fig (2): DC generator EMF plot.
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