Category: THREE-PHASE AC CIRCUITS

For pure inductive or capacitive loads, the power factoris zero lagging or zero leading, that is, cos ɸ = 0 or ɸ = 90°. The two wattmeter readings will be   W1 = VL ILcos (30° − ɸ) = VL IL(30° − 90°) = VL ILcos (− 60°) = VL ILcos 60° (positive) W2 = VL ILcos (30° + ɸ) = VL ILcos (30° + 90°) = VL ILcos 120° = −VL ILcos 60° (negative) Hence, when the…

Under such conditions, the readings of the wattmeters will be   W1 = VL ILcos (30° − ɸ )        (positive, larger) W2 = VL ILcos (30° + ɸ )        (negative, smaller) Hence, when the power factor is less than 0.5 but more than zero, one of the wattmeters W2 gives negative downscale reading, while the other wattmeter W1 gives positive upscale reading.In order to obtain upscale reading on W2, either the connections of…

Under such conditions, the readings of the wattmeters will be   W1 = VL ILcos (30° − ɸ )        (positive, larger) W2 = VL ILcos (30° + ɸ )        (negative, smaller) Hence, when the power factor is more than 0.5 but less than one, both the wattmeters give positive (upscale) readings. However, wattmeter W1 gives larger reading than wattmeter W2. Now, total power, P = W1 + W2; P = larger reading + smaller readings

For pure resistance load, the power factor is unity, that is,cos ɸ= 1 or ɸ= 0°. Then, two wattmeter readings will be   W1 = VL ILcos (30° − 0) = VL ILcos 30° W2 = VL ILcos (30° + 0) = VL ILcos 30°, i.e., W1 = W2 Hence, when power factor is unity, both the wattmeters give equal reading. 8.17.2  Power Factor Is 0.5 (cosɸ= 0.5) or ɸ= 60° Under such conditions,…

The two-wattmeter method can be explained somewhat more clearly by considering a balanced load. In this case, we shall prove that power measured by the two wattmeters (i.e.,sum of two wattmeter readings) is equal to VLIL cosɸ, which is the actual power consumed in a three- phase balanced load. The connection diagram for a three-phase balanced load…

Similar to three-phase supply, the three-phase loads may also be connected in star or delta. The three-phase loads connected in star and delta are shown in Figure 8.8(a) and (b), respectively. The three-phase loads may be balanced or unbalanced. If the three loads (impedances) Z1, Z2, and Z3 are having same magnitude and phase angle, then the three-phase load is said to be…

In delta (∆) or mesh connections, the finish terminal of one winding is connected to start terminal of the other winding and so on, which forms a closed circuit. The three line conductors are run from three junctions of the mesh called line conductors, as shown in Figure 8.6. Fig. 8.6  (a) Three phases connected in delta…

In star or wye (Y) connections, the similar ends (either start or finish) of the three windings are connected to a common point called star or neutral point. The three line conductors are run from the remaining three free terminals called line conductors. Ordinarily, only three wires are carried to the external circuit giving three-phase,…

In a three-phase AC generator, three are three windings. Each winding has two terminals (start and finish). If a separate load is connected across each phase winding as shown in Figure 8.3, then each phase supplies an independent load through a pair of leads (wires). Thus, six wires will be required in this case to connect…

An alternating quantity is generally represented by a double-subscript notation. In this notation, two letters are placed at the foot of the symbol for voltage or current, as shown in Figure 8.2. This conveys the following two scenarios: Fig. 8.2  Circuit to represent double subscript notation For instant, the current is represented as Iab. It means that The double-subscript…