generator A generator converts any form of energy into electrical energy. A coil called armature rotates in a magnetic field. The magnetic flux linked to the coil changes continuously producing emf in the coil. This emf drives the current in the external circuit. The mechanical energy to rotate the coil is obtained from diverse sources, e.g. steam engine driven by burning of coal, diesel engine, potential energy contained in water in a reservoir, motion of wind, nuclear fission etc.

Fig. g4 shows schematically the arrangement in an ac generator. A rectangular coil of N turns, rotated in a magnetic field, B with constant angular velocity w produce emf V given by,

V = - dF/ dt

where F = NBA cos w t (see fig. g4)

V = NBA w sin w t = E₀ sin w t

ac generators of large capacity are revolving field type, because the transmission of current through slip rings and brushes, is difficult. The stationary part of the machine is called the stator. It carries three armature coils. The moving part, called the rotor has field coil carrying steady current.emf is induced in the three armature coils of the stator when the rotor moves producing the revolving magnetic field (see fig. g5). An arrangement of brush and slip ring supplies current from the armature to the field coils. A rectifier converts the alternating current from the armature to direct current for the field coils.

Three armature coils of a three phase generator ( fig. g5) may be represented in a circuit diagram by three voltage generators. There are two ways in which these sources may be connected together so as to transmit power with less than six conductors. These are shown in fig. g6a and g6b. In each case the individual sources are known as phases.

In a dc generator connection to the external circuit is made by split ring or commutator (fig. g7a), which make current flow unidirectional. The connection in the external circuit is reversed every time the direction of emf changes in the coil. Even though the current is unidirectional, it has large variation (fig. g7b) . Further improvement in design of dc generators was achieved by winding the coil on a cylinder (fig. g7c). The output of each coil is tapped when it reaches the peak. This makes the current almost constant.

The magnetic field of a generator is generally produced by an electromagnet, which draws current from generator itself (self excited).

Power loss in a generator are due to (i) Joule’s heating* of the armature coils and the field coils called copper losses; (ii) hysteresis* loss and eddy current loss in the core of the armature called iron losses; and (iii) frictional energy loss.