Monday, 3 November 2014

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Power factor is simply a name given to the ratio of “actual” power (active power) being used in a circuit, expressed in watts or more commonly kilowatts (kW), to the power which is “apparently” being drawn from the mains, expressed in volt-ampere or more commonly kilo volt-ampere (kVA).
All modern industries utilize electrical energy in some form or other. Two basic categories of load are encountered in alternate current (AC) networks.

1.  Resistive Loads
Devices containing only resistance e.g. incandescent lamps, heaters, soldering irons, ovens, etc.

The current drawn from the supply is directly converted into heat or light.  Since the voltage is assumed to be constant, the actual power (kW) being used is identical to the apparent power (kVA) being drawn from the line.  The power factor is therefore unity or 1.  In these purely resistive circuits, the current and voltage sinewave peaks occur simultaneously and are said to be “in phase”.

2.  Inductive Loads 
All motors and transformers depend on magnetism as the basis of
their operation.  Magnetism is a force and in the physical sense is not
consumed.  In AC motors and transformers, magnetic forces are only
required periodically.  Consequently, a permanent magnet cannot be
used and the necessary magnetism is produced by electrical means.
The electrical current needed for this purpose is not fully utilised.
Having produced the magnetic force, the current flows back to the
power station again.  This current is called the reactive current in con-
trast to the active current which performs work and is fully utilised in
so doing.  Although the reactive current is not utilised, it imposes a
load on the electrical distribution system and supply authorities
demand payment for this load according to specific tariffs.
The current drawn from the supply is made up of two separate kinds
of current “power producing current”  and “magnetising current”.
Therefore the current flowing in an AC circuit (unless corrected) is
generally larger than is necessary to supply the power being by the
expended point.
What does Cosϕ mean?

Reactive power and active power flow through the motor or trans-

former.  Geometrical calculation of these two powers yield the apparent power.  The ratio of the active and apparent power is denoted by cosϕ and indicates what fraction of apparent power flowing is actually used by the motor.The apparent power is greater than theactive power and hence the power factor is a value considerably lessthan unity.
Disadvantages of Low Power Factor

1.    Increased authorities cost since more current has to be transmitted, and this cost is directly billed to consumers on maximum demand kVA systems.
2.      Causes overloaded generators, transformers and distribution lines within a plant, resulting in greater voltage drops and power losses, all representing waste, inefficiency and needless wear and tear on industrial  electrical equipment
3.    Reduces load handling capability of the plants electrical system.Most electrical supply authorities have changed to kVA demand systems from the inefficient kW demand system.  Consumers are now billed and penalised for their inefficient systems according to the apparent power being used.  In future, consumers will be penalised for plants with power factor below a pre-determined value.
Power Factor Improvement

The term power factor comes into picture in AC circuits only. Mathematically it is cosine of the phase difference between source voltage and current. It refers to the fraction of total power (apparent power) which is utilized to do the useful work called active power.
Need for Power Factor Improvement
Real power is given by P = VIcosφ. To transfer a given amount of power at certain voltage, the electrical current is inversely proportional to cosφ. Hence higher the pf lower will be the current flowing. A small current flow requires less cross sectional area of conductor and thus it saves conductor and money.
• From above relation we saw having poor power factor increases the current flowing in conductor and thus copper loss increases. Further large voltage drop occurs in alternator,electrical transformer and transmission & distribution lines which gives very poor voltage regulation

How to calculate the capacitor for motor
Qc  =0.9x1.73xUnI0        
Un  is motor voltage
I0  is no load current of induction motor
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