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Introduction to Alternating Current

A. C. means alternating current-The current or voltage which alternates its direction and magnitude every time. Now a days 95% of the total energy is produced, transmitted and distributed in A. C. supply.

The reasons are the following :

(i)                 More voltage can be generated (upto 33000 V) than D.C. (650 V only).

(ii)               A. C. voltage can be increased and decreased with the help of a static machine called the ‘transformer’.

(iii)             A.C. transmission and distribution is more economical as line material (say copper) can be saved by transmitting power at higher voltage.

(iv)             A.C. motors for the same horse power as of D.C. motors are cheaper, lighter in weight, require less space and require lesser attention in operation and maintenance.

(v)               A.C. can be converted to D.C. (direct current) easily, when and where required but D.C. cannot be converted to A. C. so easily and it will not be economical.


However, D.C. entails the following merits and hence finds wide applications.

(i)                 D.C. series motors are most suitable for traction purposes in tramway, railways, crains and lifts.

(ii)               For electroplating, electrolytic and electrochemical processes (battery charging etc.), D.C. is required.

(iii)             Arc lamps for search lights and cinema projectors work on D.C.

(iv)             Arc welding is better than on A.C.

(v)               Relay and operating time switches, etc., and circuit-breakers, D.C. works more efficiently.

(vi)             In rolling mills, paper mills, colliery winding, etc., where fine speed control of speeds in both directions is required, D.C. motors are required.