Electrical Engineering ⇒ Topic : Phase Difference

Phase Difference

When an alternating voltage is applied to a circuit, an alternating current of the same frequency flows through the circuit. In most of practical circuits, for reasons we will discuss later, voltage and current have different phases. In other words, they do not pass through a particular point, say *zero point, in the same direction at the same instant. Thus voltage may be passing through its zero point while the current has passed or it is yet to pass through its zero point in the same direction. We say that voltage and current have a phase difference.

Hence, when two alternating quantities of the same frequency have different zero points, they are said to have a phase difference.

The angle between zero points is the angle of phase difference (Φ). It is generally measured in degrees or radians. The quantity which passes through its zero point earlier is said to be leading while the other is said to be lagging. It should be noted that those zero points of alternating quantities are to be considered where they pass in the same direction. Thus if voltage has passed through its zero point and is rising in the positive direction, then zero point considered for the current should have similar situation. Since both alternating quantities have the same frequency, the phase difference between them remains the same.

                                                                                 figure (a)

Consider an a.c. circuit in which current i lags behind the voltage v by Φ⁰. We say that phase difference between voltage and current is Φ⁰. This phase relationship is shown by waves in Fig. (a). Note that in determining the phase difference, those zero points have been considered where waves pass in the same direction. Thus in Fig. (a), voltage v is passing through its zero point o and is rising in the positive direction. Similarly, current i passes through its zero point 'a' and is rising in the positive direction. Therefore, phase difference between voltage and current is oa (= Φ⁰). Similarly, phase difference can be determined by considering points b and c (bc = Φ⁰). The equations of voltage and current are :

Note. Although voltage and current have been considered to explain the concept of phase difference, it is equally valid for two or more currents or voltages