DC Generator

Parts:  Fixed field magnet (stator), windings with commutator switch attached (armature), bearings and mount, brushes.

19th century scientists and engineers determined that cutting a magnetic field at a right angle with a coil of wire to generate a continuous supply of electricity was best accomplished using rotary motion. By 1850, the basic generator configuration was established.  The coils were wound on an armature that rotated within a frame housing that contained the fixed field magnets.  In 1866, a better method was discovered using self excited electromagnets to provide the fixed field.  A steam engine provided the mechanical power to rotate the generator armature. By 1870, a Belgian Company by the name of Zenobe  Theopile Gramme built the first generator that could produce current for practical use. 

Trigonometric functions are used to calculate the electrical output associated with
the circular rotation of the generator armature within the fixed magnetic field.

 

 

 

 

 

 

 

 

In the figure above, on the left is a cross section of a armature coil that rotates on its axis within a magnetic field.  The center line represents the starting point where current will be zero.  As the coil turns counterclockwise, the current will reach a peak at 90 degrees and reduce to zero at 180 degrees, reversing direction and reaching a peak at 270 degrees and finally reducing back to zero at 360 degrees (360 and zero are the same).  The sine wave to the right represents the output from the coil as it is rotated one complete revolution (one cycle) through the magnetic field.  Continuous rotation of the coil will provide a continuous sine wave output from the generator.

The magnetic field excitation is produced by the magnets attached to the frame (stator).  The rotating coil is called the armature.  To obtain the electricity that is generated, an AC Generator has two carbon brushes that contact two Slip Rings that are attached to the armature.

For a DC generator, the armature coil is connected at one end of the armature to a rotary switch called the commutator. In reality, there are several separate windings in the armature of a DC generator. Two carbon brushes rub against the commutator to make the electrical connection with each winding of the armature.  The commutator is a special rotary switch that reverses the current path in each of the individual windings every 180 degrees of rotation of the armature. The rotor turns to provide a pulsating DC output. 

The small dynamo that is powered by the turning rim on the front wheel of a bicycle is an example of a modern small DC Generator.  It will provide enough current and voltage to illuminate the headlight on the handlebars.  The light burns brighter when you speed because the voltage output of the generator increases as it turns faster.

 

AC Generator

Components: Armature, Stator, case, bearings, slip rings and brushes, copper wire, insulating material.

AC Generators are called Alternators.  Like the DC Generator, they convert mechanical energy into a varying voltage and current. The output electricity varies according to the rotation of the armature through the magnetic field.  This circular rotation follows a sine wave pattern.  Unlike DC Generators, the output power is developed in the stator windings and the field current flows through the rotating armature. It is easier to insulate the stationary stator windings for high voltage output. Brushes rub against slip rings to provide an electrical connection to the armature...

Automobiles use AC alternators with built in rectifiers and voltage regulators to operate the electrical system in a car and also to recharge the battery.

The largest alternators are used by the utility industry to generate power for the homes and industry in our cities.  SteamTurbines provide the mechanical energy to operate these alternators.  Hydroelectric plants use the gravity flow of water to turn the turbines.  Nuclear power plants and coal fired plants use steam driven turbines.  There are also turbines that operate from natural gas.  Utility plant alternators provide 3 phase power with voltage outputs exceeding 25,000 volts.  Using high voltage transformers, this voltage can be stepped up to over 115,000 volts to transport it over long distances to other cities.

Electrical power plants place the power they produce onto a managed electrical distribution grid.  During President Carter’s administration in the late 1970’s, the government encouraged firms to develop wind driven generators.  Presently they make a minimal contribution to the electric grid as compared to the other types of power plants; their good feature is the fact that they don’t pollute the atmosphere as the other types do.