# PHYSICS –Power generation.

PHYSICS – Power generation

A generator requires a relative motion between a magnetic field and a coil. In power stations, an electromagnet is fed with a current to produce an induced EMF.

Electromagnetic induction refers to how magnetic fields can be used to produce electric currents and the effect caused if, either the force is applied externally or the magnetic field is changed.

When rotating a coil in a magnetic field, the flux through the coil varies as shown in a the below diagram:

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A representation of this variation is as follows:

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According to Faraday’s Law, induced EMF          E= -do/dt, which can be determined by the negative gradient of the previous graph.

Therefore induced EMF varies with times as follows:

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To provide a more even supply of power, generators are designed with three sets of coils, which produce three separate supplies of alternating current known as a three-phase generator. It is these three phases that are carried by the three conductors in the high voltage power lines we see coming from power stations.

Three-phase generators have three pairs of stator coils and an electromagnet set at 120º to each other.

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Similar voltages are generated in each stator coil, delayed or out of phase with each other by one-third of a period. The voltage in any coil of wire is alternating, with a maximum value of Um volts. The potential difference between any two of the phase wires is not zero and is alternating.

In power stations a powerful electromagnet called a rotor is mounted on a shaft supported between bearings. This is made to rotate inside a cylindrical iron shell (stator) containing slots through which the coils are wound. Various types of power sources such as superheated steam or water may be used to rotate the motor.

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The generators at Lay Yang B Power Station generate at an EMF of 20 kV and have a current of 17.1 kA when supplying power of 500 Megawatts with a rotating electromagnet drawing a direct current of 5000 amperes.

At a step-up transformer, the voltage is stepped up to 400kV for transmission. The current used varies from time to time but it’s kept to a minimum as possible for smaller power losses through the cables.

After transmission, the voltages are stepped down to 66kV at a terminal station and then down to 11kV at a substation. Pole ...