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1. INTRODUCTION
1.1
Objective
(1) To measure the power consumed and current drawn by a domestic electric fan
at various speed settings.
(2) To study the significance of power factor and the use of a power correction
capacitor to adjust the power factor.
1.2
Theory
The instantaneous power consumed in an electric circuit is the product of the
instantaneous voltage and current. In linear a.c. circuit, the average power consumed in
the circuit is given as :
P = IV cos
φ
(1)
where
V = rootmeansquare (r.m.s) voltage
I = rootmeansquare (r.m.s) current
= phase angle difference between the voltage and current
cos
is defined as the power factor (pf) of the circuit.
Hence, to deliver a certain amount of power to a load at a given voltage, the current will
be small if the power factor (cos
)
is high or ideally 1.
If the power factor is low, the
current will have to be bigger.
Such a case is not desirable because we need better and
more expensive power transmission cables and equipment to transmit and distribute
higher currents.
We can therefore see that it is important for electrical appliances to have
a high power factor.
All common electric motors are inductive loads, causing the current flowing in it
to be lagging in phase to that of the voltage. This lagging current can be corrected by
connecting a capacitor in parallel with the load as shown in figure 2 & 3 :
Energy, Power and Power Factor of A Fan Motor
1
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View Full Document The current through a capacitor leads the voltage by 90
°
.
The phasor diagram
shows that the current thru the capacitor (I
C
) causes the angle
φ
to be smaller and hence
the power factor (cos
φ
) of the circuit to be bigger.
In our experiment, we use a conventional household Energy Meter to measure the
power consumed by the fan motor.
Average power, P
= Energy consumed / time taken
=
(N/K) x 1000
W
t / (60 x 60)
=
N x 3.6 x 10
6
W
K x t
(2)
where,
K = energy meter constant (number of disc rotations per kWh energy consumed)
N = number of rotations by the disc in time t
t
= time taken for N rotations
1.3
Apparatus and Equipment
3speed 230V fan
Energy, Power and Power Factor of A Fan Motor
2
The fan used in this experiment has a permanentcapacitor motor. Different tappings in
the main winding of the fan motor give three different speeds, which can be selected by
the speed selector.
1
Energy (kWh) meter
This meter has a voltage coil which is connected across the load
and a current coil which
is connected in series with the load. Energy consumed in a given time is proportional to
the number of rotations by the disc during that time.
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This note was uploaded on 10/18/2009 for the course ECONS 111 taught by Professor Yo during the Spring '09 term at Nassau CC.
 Spring '09
 Yo

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