Electricity generation

Electricity generation

Electricity generation

Electricity generation is the process of generating electric energy from other forms of energy.The fundamental principles of electricity generation were discovered during the 1820s and early 1830s by the British scientist Michael Faraday...

 requires the connection of large numbers of alternator

Alternator

An alternator is an electromechanical device that converts mechanical energy to electrical energy in the form of alternating current.Most alternators use a rotating magnetic field but linear alternators are occasionally used...

s in parallel

Series and parallel circuits

Components of an electrical circuit or electronic circuit can be connected in many different ways. The two simplest of these are called series and parallel and occur very frequently. Components connected in series are connected along a single path, so the same current flows through all of the...

 and additional alternators must be switched in when demand rises.

Before one alternator is connected in parallel with others, the voltage

Voltage

Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...

s and frequencies

Frequency

Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

 must be identical and they must be synchronised so that they are in phase

Phase (waves)

Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...

.

If one machine is slightly out of phase it will pull into step with the others but, if the phase difference is large, there will be heavy cross-currents which can cause voltage fluctuations and, in extreme cases, damage to the machines.

Electric motors

Electric motor

Electric motor

An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...

s for alternating current fall broadly into three categories:

• Universal motors (AC/DC motors), whose speed is not related to frequency
• Synchronous motors, whose speed has a precise relationship to frequency, i.e. they run at synchronous speed
• Asynchronous motors, which run at slightly less than synchronous speed, the exact speed depending on the amount of "slip" between the rotor
  Rotor (electric)
  The rotor is the non-stationary part of a rotary electric motor, electric generator or alternator, which rotates because the wires and magnetic field of the motor are arranged so that a torque is developed about the rotor's axis. In some designs, the rotor can act to serve as the motor's armature,...
  
   speed and the speed of the rotating magnetic field
  Magnetic field
  A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
  
  . Asynchronous motors typically run with a "slip" of about 4%.

Synchronous speeds

Synchronous speeds for synchronous motors and alternators depend on the number of poles on the machine and the frequency of the supply. In the following table, frequencies are shown in hertz

Hertz

The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....

(Hz) and rotational speeds in revolutions per minute (rpm):
>

No. of poles	Speed (rpm) at 50 Hz	Speed (rpm) at 60 Hz
2	3,000	3,600
4	1,500	1,800
6	1,000	1,200
8	750	900
10	600	720
12	500	600

Formula

The relationship between the supply frequency, f, the number of poles, p, and the synchronous speed (speed of rotating field), n_s is given by:
.

Sources

• The Electrical Year Book 1937, published by Emmott and Company Limited, Manchester, England, pp 53-57 and 72