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Explain two wattmeter method for measurement of 3 - phase power.
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  • In Balanced 3-phase circuit, power can be measured by using 2-watt meters.
  • Let's consider, 2-wattmeter method to measure the power in a star connected balanced 3-phase load as shown in figure below-

     

  • As we know that for a stare connected balanced 3-phase load-
  1. Magnitude of Line Voltage is equal to3 times of magnitude of phase voltage. Means,|VL|=3|VR|=3|VY|=3|VB|(1)
  2. Line current s equal to phase current. Means,|IL|=|Iph|=|IR|=|IY|=|IB|(2)

From above connection diagram, 

(a) Current through wattmeter W1I1=IR

(b) Voltage across wattmeter W1V1=VRB

(c) Current through wattmeter W2I2=IY

(d) Voltage across wattmeter W2V2=VYB

 

  • As we know that the power measured by dynamometer type wattmeter in given by,

P=VIcosϕ.....(3)

Where, V is the voltage across the wattmeter

I is the current through wattmeter

ϕis the angle between V & I

  • Hence from above circuit, Power measured by wattmeterW1,

W1=|V1||I1|cos(angle between V1 and I1)=|VRB||IR|cos(angle between VRB and IR)....(4)

  • Similarly, Power measured by wattmeterW2,W2=|V2||I2|cos(angle between V2 and I2)=|VYB||IY|cos(angle between VYB and IY)....(5)
  • Lets, consider that load is working lagging factor ofcosϕ. Then the phasor diagram for above circuit is shown in figure below-

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