# Measurement of 3-Phase Power by 2 Wattmeter Method

**Aim**: To measure the total 3-phase power of 3-phase balanced load using 2-wattmeter method and verify the result with its theoretical value.

## Apparatus Required

SL No. | Apparatus | Specification | QTY |

1 | Voltmeter | 0 – 600V | 1 |

2 | Ammeter | 0 – 5A | 1 |

3 | Wattmeter | 600V, 5A, UPF | 2 |

4 | 3-Phase balanced resistive load | 10KW | 1 |

## Theory

Two wattmeter method can be employed to measure the power in a 3 phase, three wire star and delta connected balanced load.

In two wattmeter method the current coils of the wattmeter are connected in series with two lines, say R and B and the potential coil of each wattmeter are joined to the third line i.e. Y as shown in the figure below.

## Procedure

### a) Star Connection

- Connect the circuit as shown in the circuit diagram 1
- Switch on the 3-phase supply using TPST switch
- Note down the line voltage, line current and readings of the voltmeter
- Compare the total power observed by the two wattmeter method with its theoretical value

### b) Delta Connection

- Connect the circuit as shown in the circuit diagram 2
- Repeat steps 2 to 4 of the star connection procedure

## Observations and Calculations

## Circuit diagram

### Star Connection

### Delta Connection

## Tabular Column

3 Phase Connection | Line Voltage V_{L} | Line Current I_{L} | Power factor cosϕ | W_{1} | W_{2} | Total 3 phase power (Practical) P=W _{1} + W_{2} (watts) | Total 3 phase power (Theoretical) P= √ 3 V _{L}I_{L }cosϕ (watts) |

STAR | 1 | ||||||

STAR | 1 | ||||||

STAR | 1 | ||||||

DELTA | 1 | ||||||

DELTA | 1 | ||||||

DELTA | 1 |

Before writing wattmeter reading multiply with wattmeter constant :

wattmeter constant K = VI cos**ϕ** / full scale deflection

where ‘cos**ϕ**‘ is power factor of the wattmeter

## Result

### Star connection

Theoretically Calculated Power | Practically Measured Power |

### Delta connection

Theoretically Calculated Power | Practically Measured Power |

## Inference

By conducting the experiment we found out practically that two wattmeters are sufficient to measure the power of 3 phase load.