AN4121

Three-phase systems

Doc ID 023243 Rev 1

21/26

If the difference of the readings is computed,

which is 1/

√⎯3 times the total three-phase reactive power. This means that the two-

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

also the power factor.

A.2.2

Three-wattmeter method

This method is used in a three-phase four-wire balanced or unbalanced load. The

connections are made with one meter in each line as shown in

Figure 15. In this

configuration, the total active power supplied to the load is equal to the sum of the three

wattmeter readings.

Figure 15.

The wattmeter connections in the three-phase 4-wire loads

A.2.3

One-wattmeter method

This method is suitable only in three-phase 4-wire balanced loads. The connection of the

wattmeter is similar to the drawing given in

Figure 15. The total power is equal to three times

the reading of only one wattmeter that is connected between one phase and the neutral.

()

)

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

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line

line

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1

Total

θ

=

θ

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−

θ

−

°

=

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sin

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V

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30

cos(

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V

)

30

cos(

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V

P

P

line

line

line

line

line

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1

2

3

2

1

Total

P

P

P

P

+

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