3-phase systems

AN3157

20/26

Doc ID 17105 Rev 1

In the balanced loads, the sum of the two wattmeter readings gives the total power. This can

be proven in a star-connected load mathematically using the power reading of each meter

as:

Equation 10

Equation 11

Equation 12

The difference between the readings is computed as follows:

Equation 13

which is 1/

two-wattmeter method can also indicate the total reactive power in the 3-phase loads and

also the power factor.

Figure 14.

The two-wattmeter method in star- or delta-connected load.

()

)

30

cos(

I

V

30

cos

I

V

P

line

line

1

12

1

θ

+

°

=

θ

+

°

=

()

)

30

cos(

I

V

30

cos

I

V

P

line

line

3

32

2

θ

−

°

=

θ

−

°

=

θ

=

+

=

cos

I

V

3

P

P

P

line

line

2

1

Total

θ

=

θ

+

°

−

θ

−

°

=

−

sin

I

V

)

30

cos(

I

V

)

30

cos(

I

V

P

P

line

line

line

line

line

line

1

2