3 What is the significance of a zero indication on a threeph

3. What is the significance of a zero indication on a three-phase wattmeter?

Solution

3.Ans) Assuming you are using 2-Wattmeter method to measure 3 phase power

Wattmeter 1 measures P1=V₁₂I₂cos(30°+)

Wattmeter 2 measures P1=V₁₂I₂cos(30°-)

P=P1+P2

Total 3-Phase Power P = V₁₂I₂cos(30°+) + V₁₃I₃cos(30°-)

Where V₁₂=V₁₃=V_L (Line Voltage)

I₂=I₃=I_L (Line Current)

P=0 when V₁₂I₂cos(30°+) + V₁₃I₃cos(30°-) =0

cos(30°+) =-cos(30°-) =cos(180+30°-)=cos(210°-)

30°+=210°-

=90 deg (Phase angle between phase voltage and current)

Power Factor=Cos=0 PF

So total 3 phase power zero indicates either zero(0) power factor lag or leading load

or else if one of the wattmeter reading is zero means

P1=V₁₂I₂cos(30°+)=0 when =60 deg

power factor is cos60=0.5