output than a single-phase machine. ) For the same capacity, three-phase alternator is smaller in size when compared to single-phase alternator. ) Three-phase transmission system is cheaper. A relatively thinner wire is sufficient for transmission of three- phase power.
. TRANSFORMER Transformer is a stationary device used to transform electrical power from one circuit to another without changing its frequency . The applied alternating voltage is either increased or decreased with corresponding decrease or increase of current in the circuit. If the transformer converts an alternating current with low voltage into an alternating current with high voltage, it is called step-up transformer .
On the contrary, if the transformer converts alternating current with high voltage into an alternating current with low voltage, then it is called step-down transformer . . . Construction and working of transformer Principle The principle of transformer is the mutual induction between two coils.
That is, when an electric current passing through a coil changes with time, an emf is induced in the neighbouring coil. Construction In the simple construction of transformers, there are two coils of high mutual inductance wound over the same transformer core. The core is generally Figure . (a) Construction of transformer Primary winding of Secondary winding of Secondary current, Primary current, Primary voltage, N p turns N s turns I p I s V P Secondary voltage, s Transformer core Figure .
(b) Roadside transformer 12th - 12th - - - - - Unit ELECTROMAGNETIC INDUCTION AND ALTERNATING CURRENT Equation . is written in terms of amplitude of corresponding quantities, K s p s p p s i) If N K s p > > , then and s p s p > < This is the case of step-up transformer in which voltage is increased and the corresponding current is decreased. ii) If N K s p < < , then and s p s p < > This is step-down transformer where voltage is decreased and the current is increased. Efficiency of a transformer: The efficiency η of a transformer is defined