INTRODUCTION

As discussed earlier, the flow of current in the circuits was steady and in only one direction, that is, direct current (DC). The use of DC is limited to few applications, for example electroplating, charging of batteries, electric traction, electronic circuits, etc. However, for large-scale power generation, transmission, distribution, and utilization, an alternative current (AC) system is invariably adopted. In AC system, voltage acting in the circuit changes polarity and magnitude at regular interval of time, and hence, the flow of current in the circuits will reverse the direction periodically.

The most important advantages of AC system over DC system are the following:

1. An alternating voltage can be stepped up and stepped down efficiently by means of transformer. To transmit huge power over a long distance, the voltages are stepped up (up to 400 kV) for economical reasons at the generating stations, whereas they are stepped down to a very low level (400/230 V) for the utilization of electrical energy from safety point of view.
2. The AC motors (i.e., induction motors) are cheaper in cost, simple in construction, more efficient and robust as compared to DC motors.
3. The switchgear (e.g., switches, circuit breakers, etc.) for AC system is simpler than DC system.

Thus, AC system is universally adopted for generation, transmission, distribution, and utilization of electrical energy. In this chapter, we shall confine over attention to the fundamentals of alternating currents.