Page 241 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Power – The rate of doing work is called power.
Work done
Time
The unit of power is joule/second or watt in M.K.S. or S.I. system of unit.
Energy – The energy is as the capacity for doing work.

The unit in M.K.S. system is joule or watt/second and KWHr.
Power factor
In an alternating current (AC) electrical supply, a mysterious thing called “Power Factor” comes into play.
• Power Factor is simply the measure of the efficiency of the power being used,

• so, a power factor of 1 would mean 100% of the supply is being used efficiently.
• A power factor of 0.5 means the use of the power is very inefficient or wasteful
It is defined as the
1 Cosine of the angle Ø of current lag or lead from the applied voltage ,i.e. cos Ø
2 It is the ratio of Resistance and impedance of the AC circuit, cos Ø=R/Z

3 And it is the ratio of True power and apparent power. Cos Ø = KW/KVA
• Ideal power factor is unity
• It is the measure how efficiently the electrical power is converted into useful work.
Let us consider a 230V,50Hz, 1Hp motor. Let us assume that it is 100% efficient so that it draws a true power of
746W. Such a motor has a typical power factor of 0.75 lagging.
• To deliver 746W from 230V at a power factor of 0.75 requires a current of
• I=P/V X cosØA , 746W/230V X 0.75=4.33A

• Let us assume that we can modify the motor in some way to make the power factor unity. The current now
required is
I=P/V*cosØ

• 746W/230V*1 =3.24A
• It requires a higher current to deliver a given quantity of true power if the power factor of the load is less than
unity.

This higher current means that more energy is wasted in the feeder wires serving the motor. In fact if an industrial
installation has a power factor less than 85%, a power factor penalty is assessed by the electric utility company.
It is the reason for power factor correction.

Sources of reactive power
Transformer, Induction motor, Alternator, Discharge lighting etc
Causes and effects of lowe power factor
i In industrial and domestic fields, the induction motors are widely used. The induction motors are always taking
lagging current which constitute or results low power factor.
ii The industrial furnaces have low power factor.
iii The transformers at substations have lagging power factor because of load and magnetising currents.
iv Because of inductive load in houses like tubes, traction h.p. motors etc.

Effect of low power factor
i For the same power to be transmitted over a distance, it will have to carry more current at low power factor. As
a result the area of cross-section of the conductors is more, causing more cost, labour cost, line losses, line
drops and lowering the efficiency of transmission lines.

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CITS : Power - Electrician & Wireman - Lesson 38-40 CITS : Power - Electrician & Wireman - Lesson 38-40