Electrical Engineering ⇒ Topic : Determination of p.f of Load by Two-wattmeter Method (for balanced Y or delta load only)
If the 3-phase load (Y or delta) is balanced, we can also determine the power factor of the load from the wattmeter readings. Fig. (a) shows a balanced Y-connected load ; the pf. angle of load impedance being Φ lag. Let VRN, VYN and VBN be the r.m.s. values of the three load phase voltages (phase sequence being RYB) and IR, IY and IB the r.m.s. values of line currents. These currents will lag behind their respective phase voltages by (Φ) as shown in the phasor diagram in Fig. (b).
To obtain VRY, find the phasor sum of VRN and VYN as shown in Fig.(b). It is clear from the phasor diagram that phase angle between VRY and IR is (300 +Φ).
To obtain VBY, find the phasor sum of VBN and VYN as shown in Fig. (B). It is clear from the phasor diagram that phase angle between VBY and IB is (300 - Φ)
i.e cos Φ = 1
(ii) Since the phase sequence is known, we can even tell the higher-reading wattmeter from the circuit diagram. Thus refering to the circuit in Fig. (a), the phase sequence is RYB and that current coils of wattmeters W1 and W2 are connected in R and B lines. Since VBN leads VRN (of course by 120°), the wattmeter in line B (i.e. W2) is the higher reading wattmeter. Had the cun-ent coils of W1 and W2 been connected in R and Y lines, VRN leads VYN by 120°, the wattmeter in line R (i. e., W1 would have been the higher-reading wattmeter.
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