Electrical Engineering ⇒ Topic : Determination of p.f of Load by Twowattmeter Method (for balanced Y or delta load only)

David
 
If the 3phase 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 Yconnected load ; the pf. angle of load impedance being Φ lag. Let V_{RN}, V_{YN} and V_{BN }be the r.m.s. values of the three load phase voltages (phase sequence being RYB) and I_{R}, I_{Y} and I_{B} 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 V_{RY}, find the phasor sum of V_{RN} and V_{YN} as shown in Fig.(b). It is clear from the phasor diagram that phase angle between VRY and IR is (30^{0} +Φ). To obtain V_{BY}, find the phasor sum of V_{BN} and V_{YN} as shown in Fig. (B). It is clear from the phasor diagram that phase angle between V_{BY} and I_{B} is (30^{0}  Φ) (a) (b) figure i.e cos Φ = 1 (ii) Since the phase sequence is known, we can even tell the higherreading wattmeter from the circuit diagram. Thus refering to the circuit in Fig. (a), the phase sequence is RYB and that current coils of wattmeters W_{1} and W_{2 }are connected in R and B lines. Since V_{BN} leads V_{RN} (of course by 120°), the wattmeter in line B (i.e. W_{2}) is the higher reading wattmeter. Had the cunent coils of W_{1} and W_{2} been connected in R and Y lines, V_{RN} leads V_{YN} by 120°, the wattmeter in line R (i. e., W_{1} would have been the higherreading wattmeter.  
 
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