To Measure Single Phase Power by Three Voltmeter Method.
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Power may be defined as the rate at which energy is transformed. In almost all cases the
power in a d.c. circuit is best measured by separately measuring quantities, V and I and
by computing P=VI. In case of a.c. circuits the instantaneous power varies continuously
as the current and voltage go through a cycle of values. The fact that the power factor is
involved in the expression for the power means that a wattmeter must be used instead of
merely an ammeter and voltmeter.
However, single phase power may be measured by 3 Voltmeter method. Power and
power factor are calculated using following relations.
3 Voltmeter methods have following disadvantages.
•Even small errors in measurement of voltages may cause serious errors in the
value of power,
•Supply voltage higher than normal voltage is required.
This method is not of practical importance due to high degree of errors.
1. Connect the circuit as shown in circuit diagram.
2. Switch on the power supply and measure I, V1, V2, V3and P’L as shown by
3. Calculate PL and Power factor by using formulae.
4. Find %error between P’L and PL.
5. Vary resistance or inductive load and repeat above steps.