In alternator

Question

Question

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BPSC AE Paper 4 (General Engineering Science) 10 Nov 2022 Official Paper

having three-phase circuit, the power in watt is W = \(\left(\frac{V}{\sqrt{3}}\right)\) Icos ϕ
the amount of heat produced is dependent on square of the current which in turn varies directly with the power factor
of three - phase type, if windings are connected with star system, phase voltage is \(\frac{V}{\sqrt{3}}\)
generation of single-phase power involves less losses compared to three-phase power generation

where

V = line voltage

I = current in ampere

cos ϕ = power factor

Answer 1

Synchronous generator or alternator is an electrical machine that converts the mechanical power from a prime mover into an AC electrical power at a particular voltage and frequency.
The synchronous machine always runs at a constant speed called synchronous speed.
To excite the field winding of the rotor of the synchronous machine, a direct current is required.
Thus, an alternator is a polyphase synchronous machine in which excitation is provided by rotor winding connected to dc supply.
With D.C excitation, there exists magnetic locking between stator poles and rotor pole. So that alternator rotates at synchronous speed.
In a three-phase load, three different impedances are connected together in a star or delta fashion.
The delta in a three-phase system is formed by connecting one end of the winding to the starting end of other winding and the connections are continued to form a closed loop.
The star in the three-phase system is formed by connecting one end of all three impedances are connected together.

The relationship between line and phase voltages and currents in star connected and delta connected networks are: