- Three Phase Power Measurement Definition: Three phase power measurement involves using different methods to determine the total power in a three-phase circuit, depending on the number of wattmeters used.
- Three Wattmeters Method: Uses three wattmeters connected to each phase and neutral line in a four-wire system to measure total power by summing individual readings.
- Two Wattmeters Method: Employs two wattmeters and can be used for both star and delta load connections, summing the readings to determine total power.
- One Wattmeter Method: Suitable only for balanced loads, using one wattmeter and switching between phases to measure power.
- Balanced Load Limitation: The one wattmeter method cannot be used for unbalanced loads, making it less versatile.
Various methods are used for measurement of three phase power in three phase circuits on the basis of number of wattmeters used. We have three methods to discuss:
Let us discuss one by one each method in detail.
Measurement of Three Phase Power by Three Wattmeters Method
The circuit diagram is shown below-
This method is used for three-phase four-wire systems. The coils of the three wattmeters are connected to the respective phases, marked 1, 2, and 3. The pressure coils are connected to a common neutral point. Each wattmeter measures the product of phase current and line voltage (phase power). The total power is the sum of all wattmeter readings.
Measurement of Three Phase Power by Two Wattmeters Method
In this method we have two types of connections
- Star connection of loads
- Delta connection of loads.
When the load is star connected load, the diagram is shown in below-
For star connected load clearly the reading of wattmeter one is product of phase current and voltage difference (V2-V3). Similarly the reading of wattmeter two is the product of phase current and the voltage difference (V2-V3). Thus the total power of the circuit is sum of the reading of both the wattmeters. Mathematically we can write
but we have 
, hence putting the value of 
.
We get total power as 
.
When delta connected load, the diagram is shown in below
The reading of wattmeter one can be written as
and reading of wattmeter two is
but 
, hence expression for total power will reduce to 
.
Measurement of Three Phase Power by One Wattmeter Method
Limitation of this method is that it cannot be applied on unbalanced load. So under this condition we have 
.
Diagram is shown below:
Two switches are used, labeled 1-3 and 1-2. Closing switch 1-3 gives the wattmeter reading as…
Similarly the reading of wattmeter when switch 1-2 is closed is
