A voltage can be developed in a coil of wire in one of the three ways:

1. By changing the flux through the coil.

2. By moving the coil through the magnetic field.

3. By altering the direction of the flux with respect to the coil.

The first one is that voltage is said to be induced emf and in accordance with Faraday's law, its magnitude at any instant of time is given by the formula as shown below:

e = N(dΦ/dt) x 10

^{-8}volts

where N is the number turns in a coil

dΦ/dt = rate at which the flux in maxwells changes through the coil

Please take note that in this method of developing an emf, there is no physical motion of coil or magnet; the current through the exciting coil that is responsible for the magnetism is altered to change the flux through the coil in which the voltage is induced. For the second and third method mentioned above, there is actual physical motion of coil or magnet, and in altered positions of coil or magnet flux through the coil changes. A voltage developed on these ways is called a generated emf and is given by the equation:

e = Blv x 10

^{-8}volts

where B is the flux density in lines per square inch

l is the length of the wire, in., that is moved relative to the flux

v is the velocity of the wire, in.per sec., with respect to the flux

Two-pole single AC Generator |

Rotated 90 degree |

1. The voltage changes from instant to instant.

2. The electrical polarity (+) and minus (-) changes with alternating positions under north and south poles.

In actual, ac generator rotate a set of poles that is placed concentrically within a cylindrical core containing many coils of wires. However, a moving coil inside a pair of stationary poles applies equally well to the rotating poles construction; in both arrangements there is a relative motion of one element with respect to the other.

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