The modern electric power system is an alternating current, three phase system. Electricity is generated by synchronous generators which are machines which convert the rotational energy of a shaft into electrical energy.
The energy conversion is based on a phenomenon associated with magnetism and electricity called induction. If a stationary wire loop is placed in the field of a rotating magnet, an electric current will be induced in the wire.
The rotor of an electric generator is made to look like a magnet by energizing conductors embedded in it with a source of direct current.The system that provides direct current to the rotor windings is called the excitation system.
The energized windings on the rotor are conventionally called the field or field circuit. In modern generators the direct current excitation is derived from an alternating current source that has been rectified to provide dc.
The direct current excitation establishes a magnetic field in the metal of the rotor which extends across the air gap between the rotor and the stationary part of the generator (stator or armature). Electricity is induced in coils which are placed in slots in the stator.
The voltage induced in any one coil reflects the time varying characteristic of the magnetic field, as viewed by a stationary observer, caused by the rotation of the rotor. The magnitude of the induced voltage can be adjusted up or down by changing the magnitude of the direct current flowing in the rotor.
This is done by a voltage regulator in the excitation system which monitors the voltage at the terminal of the electric generator and adjusts the field voltage up or down as required to maintain the desired generator terminal voltage.
The voltage and current have a sinusoidal shape, that is, in each cycle of 360 degrees, it starts at a zero value at zero degrees, rises to a positive maximum at 90 degrees, falls to zero at 180 degrees, continues to fall to a negative maximum at 270 degrees and returns to zero at 360 degrees, where the process repeats as shown in figure below.
This sinusoidal shape reflects the rotating pattern of the magnetic field produced on the rotor. If the stator coil is connected to an external load, current will flow. The current will also be oscillatory in nature, hence the name alternating current. The number of full cycles that occur in a set time defines the frequency of the electricity.
In the United States and many other areas of the world, the frequency is 60 hertz or cycles per second. In other areas a frequency of 50 cycles is used. The frequency is set by the number of magnetic circuits that are established on the rotor.
The frequency of the electricity produced by a particular generator is defined as: where n is the speed in revolutions per minute (rpm) and p is the number of pairs of magnetic poles. Steam turbines rotate at high speeds.
For example, if one magnetic circuit is established, that is, there two magnetic poles established (a single pair consisting of a north and a south pole), a speed of 3,600 rpm will result in a frequency of 60 hertz.
Alternately, if two magnetic circuits are established using two pairs of poles, a speed of only 1,800 rpm is needed to produce a frequency of 60 hertz. Hydraulic turbines rotate at relatively low speeds and will have many poles to produce the required frequency.
Because of the oscillatory nature of the voltage and current, an “effective” voltage and current value is defined. These effective values are considered to be equivalent to the direct current voltage and currents that would produce the same power dissipation (as heat) in a resistance.
The effective value for a sine wave is equal to 0.707 x the peak value. In the United States, for example, the oft quoted household voltage of 120 volts is an effective value and corresponds to a peak value of 169.7 volts.
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