The basic operation of all these generator types can be explained using two simple rules, the first for magnetic circuits and the second for the voltage induced in a conductor when subjected to a varying magnetic field.

The means of producing a magnetic field using a current in an electric circuit have shown that the flux Φ in a magnetic circuit which has a reluctance Rm is the result of a magneto-motive force (mmf ) Fm, which itself is the result of a current I flowing in a coil of N turns.

Φ = Fm/Rm and Fm = IN

The main magnetic and electrical parts of a salient-pole generator are shown in Fig. 5.4. In Fig. 5.4(a), dc current is supplied to the rotor coils through brushes and sliprings.

The product of the rotor or field current I and the coil turns N results in mmf Fm as in eqn 5.2, and this acts on the reluctance of the magnetic circuit to produce a magnetic flux, the path of which is shown by the broken lines in Fig. 5.4(b).

As the rotor turns, the flux pattern created by the mmf Fm turns with it; this is illustrated by the second plot of magnetic flux in Fig. 5.4(b). When a magnetic flux Φ passes through a magnetic circuit with a cross section A, the resulting flux density B is given by B = Φ/A Figure 5.4(a) also shows a stator with a single coil with an axial length l.

As the rotor turns, its magnetic flux crosses this stator coil with a velocity v, an electromotive force (emf ) V will be generated, where V = Bvl (5.4)

The direction of the voltage is given by Fleming’s right-hand rule, as shown in Fig. 2.6. Figure 5.4(b) shows that as the magnetic field rotates, the flux density at the stator coil changes. When the pole face is next to the coil, the air gap flux density B is at its highest, and B falls to zero when the pole is 90° away from the coil.

The induced emf or voltage V therefore varies with time (Fig. 5.5) in the same pattern as the flux density varies around the rotor periphery. The waveform is repeated for each revolution of the rotor; if the rotor speed is 3000 rpm (or 50 rev/s) then the voltage will pass through 50 cycles/second (or 50 Hz).

This is the way in which the frequency of the electricity supply from the generator is established. The case shown in Fig. 5.4 is a 2-pole rotor, but if a 4-pole rotor were run at 1500 rpm, although the speed is lower, the number of voltage alternations within a revolution is doubled, and a frequency of 50 Hz would also result.

The general rule relating the synchronous speed ns (rpm), number of poles p and the generated frequency f (Hz) is given by f = nsp/120 The simple voltage output shown in Fig. 5.5 could be delivered to the point of use (the ‘load’) with a pair of wires as a single-phase supply.

If more coils are added to the stator as shown in Fig. 5.4(a) and if these are equally spaced, then a three-phase output as shown in Fig. 5.6 can be generated. The three phases are conventionally labelled ‘U’, ‘V’ and ‘W’. The positive voltage peaks occur equally spaced, one-third of a cycle apart from each other.

The three coils either supply three separate loads, as shown in Fig. 5.7(a) for three electric heating elements, or more usually they are arranged in either ‘star’ or ‘delta’ arrangement in a conventional three-phase circuit (Fig. 5.7(b)).

In a practical generator the stator windings are embedded in slots, the induced voltage remaining the same as if the winding is in the gap as shown in Fig. 5.4(b). Also, in a practical machine there will be more than the six slots shown in Fig. 5.6(a). This is arranged by splitting the simple coils shown into several subcoils which occupy separate slots, each phase still being connected together to form a continuous winding. Figures 5.1 and 5.2 show the resulting complexity in a complete stator winding.

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