1. Title: Measurement of active and reactive power of three phase
balanced load by one wattmeter method and two wattmeter
method
Presented By
1) Vishal Bholaji Satone
2) Rohit Purushottam Lichade
3) Suraj devanand Gilorkar
4) Gauri Madhavi
2. INTRODUCTION
Power in a three-phase System can be measured by using
the following methods:
1) Two wattmeter method
2) One wattmeter method
. 3) Three wattmeter method
General Power:
In d.c. circuits, Power= Voltage× Current.
But in a.c. circuits, Power Voltage x Current x p.f.
where, p.f. is a power factor which depends on the phase
difference between voltage and current.
Power factor 1 i.e. unity if the circuit is purely resistive.
But if the circuit is inductive the p.f. is less than 1 and
lagging. In capacitive circuit, p.f. is leading.
3. Concept of Active and Reactive power :
Active Power:
The instantaneous power drawn by an a.c. circuit is
equal to the product of instantaneous values of voltage and
current.
This power consumed by load
Active power = VI cosΦ watts
Reactive power:
The reactive component of current is equal to I sinΦ. The power
drawn by the circuit due to reactive component of current is called as
reactive power.
Reactive power= VI sinΦ VA
4. One Wattmeter Method of Measuring Power of Three-
Phase Circuit.
This method is to be used for balanced loads only, and it is
a modification of two wattmeter method of measuring the
power.
The wattmeter current coil is connected in one line and
the readings of wattmeter are taken by connecting its
pressure coil alternately across the line voltages.
In balanced 3-wire, 3-phase load circuit the power in each
phase is equal. Therefore, the total power of the circuit can
be determined by multiplying the power measured in any
one phase by three.
Total power in balanced load = 3 x Power per Phase
= 3 x Wattmeter reading
8. Two wattmeter method of measuring power of
three phase Circuit
In a three phase three wire system we require 3 elements.
But if we make the common points of the pressure coils
coincide with one of the lines, then we will require only
n- 1= 2 elements.
Power consumed by load = V1I1 +V2I 2 +v3 I 3
Instantaneous power P1 = I1 (V1-V3)
Instantaneous power P2 =I2 (V2-V3)
10. Sum of the instantaneous reading= Pi + P₂
= I 1(V1-V3) + I 2 (V2- V3)
= I1V1- I1V3 + I2V2 – I2V3
=V1I1+V2 I2 – V3 (I1+I2)
= V1 I1 +V2 I2+V3 I3
Therefore, the sum of the two wattmeter reading
is equal to the power consumed by the load.
This is irrespective of whether the load is
balanced or unbalanced.
