The power in three phase, which is equal to √3VLILcos Φ.
Three phase Power Measurement Methods
It can be measured by these following methods:
- By single wattmeter of single phase,
(i) when neutral is available,
(ii) when neutral is not available. - By two wattmeters method.
- By three wattmeters of single phase method.
- By one wattmeter of three phase method.
By single wattmeter method
This method is adopted in case of balanced load. The total power consumed will be equal to the three multiplied by one wattmeter reading.
Fig. 1. Single wattmeter method.
When neutral is available: In that case the current coil of the wattmeter is connected in series with load and pressure coil across one phase and neutral as shown in Fig. 16.9. The wattmeter reading is taken and multiplied by three. Thus total power
W = 3 x Wattmeter reading.
Fig. 2. Singe wattmeter method making artificial neutral.
When neutral is not available: In that case the artificial neutral is made. Generally in case of delta connections the neutral is not available. The artificial neutral is made just by connecting the two similar resistances having equivalent value of the same value as that of the resistance of pressure coil of the meter, as shown in Fig. 2.
The total power is equal to
WT = 3 x Wattmeter reading.
Show how will you connect three single phase wattmeters and one three phase wattmeter for measuring the power in three phase power load.
Three phase powercan also be measured by these following methods:
By using three wattmeter of single phase
In this case three single phase wattmeters are used. This method can be used for balanced or unbalanced loads. The c.c. of each wattmeter is connected in series with the load and the pressure coil across each phase in star as shown in Fig. 16.123. In this case neutral may or may not be available. The total power is the sum of all the three wattmeters readings.
WT WI + W’3 Watts.
Fig. 3. 3 wattmeters method Of power measurements.
By using single wattmeter of three phase
In this system of power measurement it can be directly read from the meter. It can be used for balanced or unbalanced loads and also for three phase four wire or three phase three wire systems as shown in Fig. 16.13.