Dynamo
The Dynamo was the first electrical generator capable of delivering power for industry. The dynamo uses electromagnetic principles to convert mechanical rotation into a pulsing direct electric current through the use of a commutator. A dynamo machine consists of a stationary structure, which provides a constant magnetic field, and a set of rotating windings which turn within that field. On small machines the constant magnetic field may be provided by one or more permanent magnets; larger machines have the constant magnetic field provided by one or more electromagnets, which are usually called field coils.
The first dynamo based on Faraday's principles was built in 1832 by Hippolyte Pixii, a French instrument maker. It used a permanent magnet which was rotated by a crank. The spinning magnet was positioned so that its north and south poles passed by a piece of iron wrapped with wire. Pixii found that the spinning magnet produced a pulse of current in the wire each time a pole passed the coil. Furthermore, the north and south poles of the magnet induced currents in opposite directions. By adding a commutator, Pixii was able to convert the alternating current to direct current.
Unlike the Faraday disc, many turns of wire connected in series can be used in the moving windings of a dynamo. This allows the terminal voltage of the machine to be higher than a disc can produce, so that electrical energy can be delivered at a convenient voltage.
The relationship between mechanical rotation and electric current in a dynamo is reversible; the principles of the electric motor were discovered when it was found that one dynamo could cause a second interconnected dynamo to rotate if current was fed through it.
The transformative ability of a dynamo to change energy from electrical power to mechanical power and back again could be exploited as a current-compensation and balancing device to even out power distribution on interconnected, unbalanced circuits.
Concepts
The generator moves an electric current, but does not create electric charge, which is already present in the conductive wire of its windings. It is somewhat analogous to a water pump, which creates a flow of water but does not create the water inside. Other types of electrical generators exist, based on other electrical phenomena such as piezoelectricity, and magnetohydrodynamics. The construction of a dynamo is similar to that of an electric motor, and all common types of dynamos could work as motors.
Excitation
A generator that uses field coils instead of permanent magnets requires a current flow to be present in the field coils for the generator to be able to produce any power at all. If the field coils are not powered, the rotor can spin without the generator producing any usable electrical energy.
For older and very large power generating equipment, it has been traditionally necessary for a small separate exciter generator to be operated in conjunction with the main power generator. This is a small permanent-magnet generator which produces the constant current flow necessary for the larger generator to function.
Most modern generators with field coils feature a capability known as self-excitation where some of the power output from the rotor is diverted to power the field coils. Additionally the rotor or stator contains a small amount of magnetizable metal, which retains a very weak residual magnetism when the generator is turned off. The generator is turned on with no load connected, and the initial weak field creates a weak flow in the field coils, which in turn begins to slightly affect the rotor to begin to produce current that then further strengthens the field. This feedback loop continues to increase field voltage and output power until the generator reaches its full operating output level.
This initial self-excitation feedback process does not work if the generator is started connected to a load, as the load will quickly dissipate the slight power production of the initial field buildup process.
It is additionally possible for a self-exciting generator either turned off or started with a load connected to result in dissipation of the residual magnetic field, resulting in complete non-function of the generator. In the case of a 220v portable generator commonly used by consumers and construction contractors, this loss of the residual field can usually be remedied by shutting down the generator, disconnecting all loads, and connecting what are normally the high-voltage/amperage generator outputs to the terminals of a common 9-volt battery. This very small current flow from the battery (in comparison with normal generator output) is enough to restore the residual self-exciting magnetic field. Usually only a moment of current flow, just briefly touching across the battery terminals, is enough to restore the field.
Terminology
The parts of a dynamo or related equipment can be expressed in either mechanical terms or electrical terms. Although distinctly separate, these two sets of terminology are frequently used interchangeably or in combinations that include one mechanical term and one electrical term. This causes great confusion when working with compound machines such as a brushless alternator or when conversing with people who work on a machine that is configured differently than the machines that the speaker is used to.
Mechanical
Rotor: The rotating part of an alternator, generator, dynamo or motor.
Stator: The stationary part of an alternator, generator, dynamo or motor.
Electrical
Armature: The power-producing component of an alternator, generator, dynamo or motor. The armature can be on either the rotor or the stator.
Field: The magnetic field component of an alternator, generator, dynamo or motor. The magnetic field of the dynamo or alternator can be provided by either electromagnets or permanent magnets mounted on either the rotor or the stator.
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