DC GENERATORS
A direct-current (DC) generator is a rotating machine that supplies an electrical output with unidirectional voltage and current.
Principle: An electrical generator is a machine that converts mechanical energy into electrical energy. The energy conversion is based on the principle of the production of dynamically induced emf, where a conductor cuts magnetic flux, dynamically induced emf is produced in it according to Faraday’s Laws of electromagnetic Induction. This emf causes a current to flow if the conductor circuit is closed. Hence, two basic essential parts of an electrical generator are (i) a magnetic field and (ii) a conductor or conductors which can so move as to cut the flux. The following figure shows a single-turn rectangular copper coil rotating about its own axis in a magnetic field provided by either permanent magnets or electromagnets. The two ends of the coil are joined to two slip-rings ‘a’ and ‘b’ which are insulated from each other and from the central shaft. Two collecting brushes (of carbon or copper) press against the slip rings. Their function is to collect the current induced in the coil and to convey it to the external load resistance R. The rotating coil may be called ‘armature’ and the magnets as ‘field magnets’.
As the coil rotates in a clockwise direction and assumes successive positions in the field the, flux linked with it changes. Hence, an emf is induced in it which is proportional to the rate of change of flux linkages (e = NdΦ /dt).
1) When the plane of the coil is at right angles to lines of flux i.e. when it is in position 1, then flux linked with the coil is maximum, but the rate of change of flux linkages is minimum. Hence, there is no induced emf in the coil.
2) As the coil continues rotating further, the rate of change of flux linkages (and hence induced emf in it) increases, till position 3 is reached where θ= 900, the coil plane is horizontal i.e. parallel to the lines of flux. The flux linked with the coil is minimum but the rate of change of flux linkages is maximum. Hence, maximum emf is induced in the coil at this position.
3) From 900 to 1800, the flux linked with the coil gradually increases but the rate of change of flux linkages decreased. Hence, the induced emf decreases gradually till in position 5 of the coil, it is reduced to zero value.
4) From 1800 to 3600, the variations in the magnitude of emf are similar to those in the first half revolution. Its value is maximum when the coil is in position 7 and minimum when in position 1. But it will be found that the direction of the induced current is the reverse of the previous direction of flow.
For making the flow of current unidirectional in the external circuit, the slip-rings are replaced by split rings. The split rings are made out of a conducting cylinder which is cut into two halves or segments insulated from each other by a thin sheet of mica or some other insulating material. As before, the coil ends are joined to these segments on which rest the carbon or copper brushes. It is seen that in the first half revolution current flows along (ABMLCD) i.e. brush No.1 in contact with segment ‘a’ acts as the positive end of the supply and ‘b’ as the negative end. In the next half revolution, the direction of the induced current in the coil has reversed. But at the same time, the positions of segments ‘a’ and ‘b’ have also reversed with the result that brush No.1 comes in touch with the segment which is positive i.e. segment ‘b’ in this case. Hence, the current in the load resistance again flows from M to L. The waveform of the current through the external circuit is shown below. This current is unidirectional but not continuous like a pure direct current.
1) The position of brushes is so arranged that the changeover of segments ‘a’ and ‘b’ from one brush to the other takes place when the plane of the rotating coil is at right angles to the plane of the lines of flux. It is so because, in that position, the induced emf in the coil is zero.
2) The current induced in the coil sides is alternating as before. It is only due to the rectifying action of the split rings (also called a commutator) that it becomes unidirectional in the external circuit.
USES OF DC GENERATORS
1. Shunt generators with field regulators are used for ordinary lighting and power supply purposes. They are also used for charging batteries because their terminal voltages are almost constant or can be kept constant.
2. Series generators are not used for power supply because of their rising characteristics. However, their rising characteristic makes them suitable for being used as boosters in certain types of distribution systems, particularly in railway service.
3. Compound generators: The cumulatively-compound generator is the most widely used dc generator because its external characteristic can be adjusted for compensating the voltage drop in the line resistance. Hence, such generators are used for motor driving which requires a dc supply at constant voltage, for lamp loads, and for heavy power services such as electric railways. The differential-compound generator has an external characteristic similar to that of a shunt generator but with a large demagnetization armature reaction. Hence, it is widely used in arc welding where a larger voltage drop is desirable with an increase in current.
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