Lecture
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An alternator is an electromechanical device that converts mechanical energy to alternating current electrical energy. Actually, DC and AC generators are similar in one important respect – they both generate alternating emfs. In the DC generator, the alternating voltage is rectified through the medium of a commutator and brushes, whereas the AC generator has no rectifier and delivers AC electric energy to its loads. General Types of Alternator 1. Synchronous Generator 2. Induction Generator 3. Inductor Alternator Two Possible Construction of an Alternator 1. Stationary field and a revolving armature 2. Stationary armature and a revolving field Prime Movers for Alternators 1. Large AC Generators a) b) c) d)
Steam Turbine Hydraulic Turbine Gas Turbine Internal Combustion Engine
2. Small AC Generators a) Internal Combustion Engine Frequency of Alternating-Current Generators Whenever the poles of a two-pole alternator revolve, the generated emf in the stationary armature winding changes direction every half revolution; therefore one complete positive and negative pulse, one cycle will occur in one revolution. So, the frequency per revolution is equal to the number of pairs of poles. Moreover, it is clear that the frequency of the emf in an alternator is proportional to: a) The speed in revolutions per second (rpm/60) b) The number of pairs of poles (p/2)
The relationship may be written in the form of the equation:
Where:
f = PNs 120 f = frequency, Hz or cps P= no. of poles Ns =synchronous speed, rpm
eqn. 1
Generated Voltage in an Alternator “an average of 1 volt is generated in one turn of wire if the flux passing through that turn changes at the rate of 100,000,000 (108) maxwells per second” This is the fundamental law of generator action or in equation form, this statement become: Eave = N Φ x 108 t Where:
Eave = average generated voltage N = number of turns in coil Φ = flux per pole t = time (seconds)
eqn. 2
Steam Turbine Hydraulic Turbine Gas Turbine Internal Combustion Engine
2. Small AC Generators a) Internal Combustion Engine Frequency of Alternating-Current Generators Whenever the poles of a two-pole alternator revolve, the generated emf in the stationary armature winding changes direction every half revolution; therefore one complete positive and negative pulse, one cycle will occur in one revolution. So, the frequency per revolution is equal to the number of pairs of poles. Moreover, it is clear that the frequency of the emf in an alternator is proportional to: a) The speed in revolutions per second (rpm/60) b) The number of pairs of poles (p/2)
The relationship may be written in the form of the equation:
Where:
f = PNs 120 f = frequency, Hz or cps P= no. of poles Ns =synchronous speed, rpm
eqn. 1
Generated Voltage in an Alternator “an average of 1 volt is generated in one turn of wire if the flux passing through that turn changes at the rate of 100,000,000 (108) maxwells per second” This is the fundamental law of generator action or in equation form, this statement become: Eave = N Φ x 108 t Where:
Eave = average generated voltage N = number of turns in coil Φ = flux per pole t = time (seconds)
eqn. 2
