Capacitor
draws a leading current and partly or completely neutralize the lagging
reactive component of load and current. This raises the power factor of the
load.
If
a capacitor is connected in parallel with the load, then the lagging reactive
power of the load will be partly neutralizes, thus improving the power factor
of the load.
Using
capacitors in a circuit changes the reactive power value whereas the active
power remains same in magnitude.
Usually
the value of capacitor is expressed in terms of farads but in power factor
improvements capacitor value is provided in KVAR.
TRUE
POWER - One watt of true power means that electric energy
is converted into heat energy at a rate of one J/sec.
APPARENT
POWER - One volt-ampere (VA) of apparent power converts
less than one J/sec.
How
much less depends on how far the voltage and current are out of phase.
Alternators
and Transformers and Circuit Breakers are rated in kVA because the power factor
of the load is not known when the machinery is manufactured in the factory.
CAUSES
OF LOW POWER FACTOR
The
low power factor is mainly due to the fact that most of the power loads are
inductive and, therefore, take lagging currents.
1.
Single phase and three phase induction motors which have low lagging p.f.
(Inductive load).
2.
The motors work at a power factor which is extremely small on light load (0.2
to 0.3) and rises to 0.8 to 0.9 at full load.
3.
Arc lamps, electric discharge lamps and electric furnaces and welding
equipments operate at low lagging power factor.
4.
The load on the power system is varying; being high in the morning and evening
and low at other times.
5.
Low load period, supply voltage is increased which increases the magnetization
current which leads to poor power factor.
DISADVANTAGES
OF LOW POWER FACTOR
1.
Large kVA rating of equipment
kVA
= [Active power / Power factor] = kW / cosθ
The
rating is inversely proportional to power factor. If the power factor is low
the kVA rating of the machine is high, hence the making the equipment larger
and expensive.
2.
Large copper losses
The
larger current at low power factor leads to more I^2 losses in all the elements
of the supply system. When losses are more results in poor efficiency.
3.
Large conductor size
To
carry higher current the size if the conductor will have more cross section which
increases the cost and size of the cable.
4.
Poor voltage regulation
The
large current at low lagging power factor causes greater voltage drop in
alternators, transformers, transmission lines and distributors. The decreased
voltage available at the supply end, thus affecting the performance of
utilization devices.
5.
KVA rating of the equipment increases
Power
factor and KVA rating are inversely proportional and hence if power factor
increases KVA rating decreases resulting in the reduction of capital cost.
The
apparent power S = √P2 + Q2 in volt-amperes (VA)
S
= P + jQ = VI* means the load is inductive
S
= P - jQ = V*I means the load is capacitive
Real
power (P) and Reactive power (Q) increases as the square of voltage magnitude.
If
frequency increases the real power decreases whereas reactive power increases.
POWER
FACTOR
In
a.c. circuit, there is generally a phase difference ɸ between voltage and
current.
If
the circuit is inductive, the current lags behind the voltage and the power
factor is referred to as LAGGING.
If
the circuit is capacitive, the current lead the voltage and power factor is
said to be LEADING.
The
ration of active power to the volt-amperes in an a.c. circuit is defined as
power factor (p.f).
Power
factor = Active power / Apparent power
Power
factor = P/S = [VIcosθ] / [VI] = cosθ
Power
factor = P/S = R/Z = Vr/V
The
term cosθ is called as power factor of the circuit.
The
cosine of angle between voltage and current in an a.c. circuit is known as
power factor.
The
angle ‘θ’ is called as power factor angle.
The
maximum value of power factor is one.
Power
factor of a purely resistive circuit is one.
Power
factor of a purely inductive circuit is zero.
Power
factor of a purely capacitive circuit is zero.
Lagging
or leading with the numerical value of power factor to signify whether the
current lags or leads the voltage.
If
the circuit has a p.f. of 0.6 and the current lags the voltage, we write p.f as
0.6 lagging.
Sometimes
p.f is expressed as a percentage. Thus 0.6 lagging p.f. may be expressed as 60%.
If
power factor value is one, then the real power is equal to the apparent power
i.e. P=S. That means the whole apparent power drawn by the circuit is being
utilized by it.
If
p.f is 0.5 or 50% means that it will utilize the 50% of the apparent power.
Thus
the power factor of a circuit is a measure of its effectiveness in utilizing
the apparent power drawn by it. The greater the power
factor of a circuit, the greater is its ability to utilize the apparent power.
The
three basic elements of electrical engineering are resistor, inductor and
capacitor.
Resistor
coverts electrical energy into heat energy when current is forced through a
material.
Inductor
and capacitor store in the positive half cycle and give away in the negative
half cycle of supply the magnetic field and electrical field energies
respectively.
INSTANTANEOUS
POWER
The
power supplied to a circuit is the product of instantaneous of voltage and
instantaneous current and it is measured in watts irrespective of the type of
circuit used.
The
instantaneous power may be positive or negative.
Positive
value means that power flows from the source to the load.
Negativevalue means that power flows from the load to the source.
APPARENT
POWER (S)
The
apparent power is the power that appears to be present when the voltage and
current in a circuit are measured separately.
The
apparent power, then, is the product of voltage and the current regardless of
the phase angle θ.
Apparent
power is denoted as S
S
= VI its unit is volt-ampere (VA) and its bigger units are kVA and MVA.
Apparent
power can be measured by using voltmeter and ammeter.
ACTIVE
POWER OR TRUE POWER
The
power which actually consumed in the circuit is called true power or active
power. A wattmeter is constructed so that it takes into account any phase
difference between current and voltage.
Active
power is denoted as P and its unit is watts and its bigger units are kW and MW.
P
= Voltage x component of total current in phase with voltage.
P
= V I cosθ in watts
True
power can be measured by using wattmeter.
True
power is used for producing torque in motors and supply heat, light etc. The
used true power cannot be recovered.
True
power does useful work in the circuit.
REACTIVE
POWER (OR) WATTLES POWER
This
power that flows back and forth in both directions in the circuit or reacts
upon itself. Hence it is called as reactive power.
The
product of voltage (V) and component of total current 90º out of phase with
voltage (I sinθ) is equal to reactive power.
Reactive
power is denoted as Q and its unit is VA and its bigger units are kVA and MVA.
Q
= Voltage x component of total current 90º out of phase with voltage.
Q
= V I sinθ in volt-amperes
I
sinθ is called the reactive component or wattles component.
A
wattmeter does not measure the reactive power.
Reactive
power does no useful work in the circuit and merely flows back and forth in
both directions in the circuit.
An
ac parallel circuit consists of two or more branches in parallel. Each branch
has either R or L or C. In parallel circuit voltage is constant, therefore
potential difference across all the branches in parallel is the same. It is convenient
to take voltage as the reference phasor in drawing the phasor diagram.
In
parallel R-L-C circuit, if the inductive current is more than that of the
capacitive current the circuit behaves as an inductive circuit. If the
capacitive current is more than that of the inductive current then the
circuit behaves as a capacitive circuit.
STEPS
FOR SOLVING SERIES-PARALLEL CIRCUIT
1.
Find the impedance in each branch.
2.
Find the total impedance by combining the impedance in
series and parallel circuit.
3.
Find the total impedance in the circuit.
4.
Find the branch currents.
5.
Find the apparent power, active and reactive power and
