Electrical generators by definition are devices that convert mechanical energy into electric energy. The mechanical energy in turn is produced from chemical or nuclear energy in various types of fuel, or obtained from renewable sources such as wind or falling water.
Steam turbines, internal-combustion engines, gas combustion turbines, electric motors, water and wind turbines are the common methods to supply the mechanical energy for such devices. Generators are made in a wide range of sizes, from very small machines with a few watts of output power to very large power plant devices providing gigawatts of power.
The electric generator animation below demonstrates an example on how a generator works to produce energy. Two black arrows show the direction of the coil rotation. The blue lines represent magnetic field directed from north pole to south pole. The red arrows show the instantaneous direction of the induced AC current.
The applet is courtesy of Walter Fendt, URL: http://www.walter-fendt.de/ph14e/ ©Walter Fendt, May 8, 1998
ELECTRIC GENERATORS: HOW THEY WORK
Operation of power generators is based on the phenomenon of electromagnetic induction:
whenever a conductor moves relative to magnetic field, voltage is induced in the conductor. Particularly, if a coil is spinning in a magnetic field, then 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 voltage (called electromotive force, emf) is equal to the minus of the rate of change of magnetic flux Φ times the number of turns in the coil: V=−N•∆Φ/Δt. This relationship has been found experimentally and is referred to as Faraday's law. The minus sign here is due to Lenz law. It states that the direction of the emf is such that the magnetic field from the induced current opposes the change in the flux which produces this emf. Lenz law is connected to the conservation of energy.
For clarity in the above animation a single rectangular conductor loop is shown instead of an armature with a set of windings on an iron core. Since the rate of magnetic flux change through the coil that spins at a constant rate changes sinusoidally with the rotation, the voltage generated at the coil terminals is also sinusoidal. If an external circuit is connected to the coil's terminals, this voltage will create current through this circuit, resulting in energy being delivered to the load. Note that the load current in turn creates a magnetic field that opposes the change in the flux of the coil, so the coil opposes the motion. The higher current, the larger force must be applied to the armature to keep it from slowing down. Thus, the mechanical energy that rotates the coil 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). In our animation the coil is rotated by the hand crank. In practice, the mechanical energy is produced by turbines or engines called prime movers. In a small AC generator a prime mover is a rotary internal-combustion engine. In commercially available devices for residential use an alternator is integrated with such an engine into a single appliance. The resulting device is referred to as or genset, although casually it is often called just a generator (you can find information on types and selection of home generators here).
Note that the production of the voltage depends only on the relative motion between the coil and the magnetic field. EMF is induced by the same physics law whether the magnetic field moves past a stationary coil, or the coil moves through a stationary magnetic field. In the animation, the magnetic field is produced by a fixed magnet while the coil is revolving. Today's AC gensets are usually brushless. They have spinning field and a stationary power-producing armature. This armature comprises of a set of coils that form a cylinder. Also, in practice, the magnetic field is usually induced by an electromagnet rather than a permanent magnet. The electromagnet consists of so called field coils mounted on an iron core. A current flow in the field coils produces the magnetic field. This current may be obtained either from an external source or from the system's own armature. Regulation is achieved by sensing the output voltage, converting it to a DC, and comparing its level to a reference. An error is then used to control the field in order to maintain a constant output. Most modern AC sources with field coils are self-excited: the current for field coils is supplied by an additional exciting winding in the armature.
How does self excitation works? The voltage from exciter coil is rectified and fed into a regulator. When output AC current is generated, a portion of it flows into field coil to generate magnetic field. The initial magnetic field before the device started is produced by residual magnetism in electromagnet's cores. In the models equipped with electric start it is created by a electric current driven from a battery during engine cranking. The residual magnetism of the exciter's core may be lost or weakened by external magnetic fields from any source, or by non-operation for a long time. Some devices provide automatic field flashing. Otherwise, if the core lost its residual magnetism, the rotor will spin, but no AC output voltage will be produced. In this case, to start the device you may need to do so-called field flashing. Here is a typical procedure for field flashing a generator: stop the engine, disconnect exciter field leads from the voltage regulator (note the polarity of the leads), and turn the circuit breaker off. Then briefly apply voltage from an external battery or another DC source in series with a 10-20 Ohm 25W limiting resistor or a bulb to the field coil while observing polarity. Allow the field to be flashed for some 10 seconds, then remove the external voltage source. Finally, you need to reconnect the exciter coil. For the recommendations for a particular model consult your owner's operation manual.