HOW AN ELECTRIC GENERATOR WORKS



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Generators by definition are devices that produce electricity from mechanical energy. The mechanical energy in turn is converted from chemical or nuclear energy in various types of fuel, or obtained from renewable sources, such as wind or falling water.

Turbines and internal-combustion engines are the common systems that supply the mechanical energy for such devices. Generators are made in a wide range of sizes, from very small machines with a few watts output to very large power plant devices providing gigawatts of power.

The animation below demonstrates how a generator works to produce electricity. Two magnets represent the source of magnetic field directed from north pole to south pole. The two brushes connected to the wire loop contact two fixed slip rings, which go to an external load.

To view this animation you need to install Adobe Flash Player plug-in and allow blocked content

   Courtesy of Joe Wolfe at Physclips Project at UNSW.

THEORY OF OPERATION


Operation of power generators is based on the phenomenon called electromagnetic induction:




whenever an electric conductor moves relative to magnetic field, voltage (called electromotive force, emf) is induced in the conductor. Particularly, if a coil is spinning in a magnetic field, the two sides of the coil move in opposite directions, and the voltages induced in each side add. Numerically the instantaneous value of the resulting emf is equal to the minus of the rate of change of magnetic flux "Φ" times the number of turns: V=−N•∆Φ/Δt. Originally this relationship has been found experimentally. It is referred to as Faraday's law. The minus sign here is due to what is known as Lenz law, which states that the direction of the emf is such that the field from the induced current opposes the change in the flux which produces this emf. Lenz law is explained by the conservation of energy.

For clarity the above animation shows a single rectangular wire loop. Normally, there is an armature with a set of windings on an iron core. Since ∆Φ/Δt through the wire that spins at a constant rate varies sinusoidally with the rotation, the voltage generated at its terminals is also close to sinusoidal. If an external circuit is connected to these terminals, electric current will flow through this circuit, resulting in energy being delivered to the load. Note that the this current in turn creates "Φ" that opposes the change in the flux of the winding, so it opposes the motion. The higher current, the larger force must be applied to the armature to keep it from slowing down. Thus, the rotational mechanical energy is converted into electrical energy. If you use a commutator, such system is called dynamo. Its operation is similar as described above, except the output voltage becomes pulsating (unipolar). The above animation illustrates the basic concepts of generator operation. In reality, such configuration is seldom used except as an educational project. In practice, the mechanical energy that rotates the coil is produced by turbines or engines called prime movers. In small AC generators for residential use the prime mover is a rotary internal-combustion engine.

Note that the production of the voltage depends only on the relative motion between the conductor and the magnetic field. EMF is induced by the same physics law whether the magnetic field moves past a stationary coil, or a coil moves through a stationary field. In our example, the "Φ" is produced by a stationary permanent magnet while the winding is revolving. Its terminals are connected to slip rings, which are in contact with two brushes. The main disadvantage of revolving power-producing armature is the load current has to flow via a slip rings and brushes, which wear out with use. Today's AC gensets normally have spinning field and a stationary armature. This armature comprises of a set of coils that form a cylinder. Also, in practice, the magnetic flux is usually induced by an electromagnet rather than a permanent magnet. An electromagnet consists of so called field coils mounted on an iron core. A current flow in these coils may be driven either from an external source or from the system's own armature. Many modern AC sources are self-excited: the field current is supplied by an additional winding in the armature. For the operation theory of practical residential generators see this tutorial. It explains how does self excitation work and how to do "field flashing" when a genset does not start.



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