Definition of Alternating Current
Alternating current changes its direction at a periodic interval. The magnitude of the current may or may not change at every instant but the current must alter its direction periodically. Alternating current
is represented by a periodic function. The waveform of the current may sinusoidal, square, triangular or sawtooth type etc. The average value of an pure alternating current
is zero. These are the waveforms of different alternating currents.
RMS Values of Alternating Current
The value of alternating current is generally represented with its RMS value
The RMS value of sinusoidal alternating current is
The RMS value of square alternating current is
The RMS value of triangular alternating current is
The RMS value of sawtooth alternating current is
Advantage of Alternating Current or AC
In modern age the current we use is alternating current. This is because generation of alternating current is more practical than producing direct current in generator. For producing alternating current the electrical generator
does not require any commentator or rectifier. Alternating current has the induction effect due to which transformer
action is based. By using power transformers
(step up transformer
and step down transformer
) electrical power can easily be steped up and down for electrical transmission
and electrical distribution
purposes. Alternating currents can be easily stepped down for relaying and measuring purposes in electrical power systems
. Another minor but remarkable advantage of AC is that we can measure AC by using tong tester or clamp meter without disturbing the live circuit.
Disadvantage of Alternating Current
Thea main disadvantage of alternating current is that it has skin effect
and proximity effect, when it flows through a conductor. Because of skin effect, an alternating current always tries to flow through the outer surface of the conductor which increases the effective resistance
of the conductor. To minimize skin effect, generally, conductors of alternating current are made stranded to increase the surface area of the conductor. Proximity effect is similar to skin effect and due to proximity effect, current distribution across the cross section of a conductor becomes nonuniform. The proximity effect causes nonuniform current distribution in one conductor due to the induction effect of current in nearby parallel current carrying conductor.
Measurement of Alternating Current
Alternating current can be measured by using moving iron type ammeter
. Although pmmc instrument
can be used to measure alternating current but in this case the alternating current first been rectified before it is measured by pmmc instrument.
Alternating Current in Three Phase System
In balanced three phase system the vector sum of all three phase current is always zero. The phase angle between two conjugative currents in a three phase system is 120° (electrical).