Three Phase power measurement using two watt-meters

Objectives:

• To measure the total power consumed in a balanced three phase star / delta connected load by using two wattmeters.
• To calculate the power factor of the load connected to the three phase system.

Theory:

A wattmeter consists of two coils: a low resistance current coil which is connected in series with the line carrying the current, and a high resistance  potential coil which is connected across the two points whose potential difference is to be measured. Thus, a wattmeter shows a reading which is propoprtional to the product of the current through its current coil, the potential drop across its voltage coil and the cosine of the angle between this voltage and the current.

In order to measure the power consumed in a three phase ac circuit (whether wye or delta) by two wattmeters, the current coils of the wattmeters are inserted in series in any of the two lines and the potential coils are shorted and connected to the third line.

1. Wye or star connection

Circuit Diagrams:

Apparatus used:

Procedure:

• The connections are made as shown in the circuit diagram (1 or 2).
• The supply is switched on and the variac is gradually increased to a fixed value.