ELECTRICAL BASICS (Chapter 8)

Electrical termsElectricity & magnetism

Electricity Circuits

MagnetismElectrical units

Electric potential or eletromotive force Electric current Resistance

resistance of a conductor is affected by (additional information): type of material length: directly proportional to the length of a conductor cross-section area or thickness temperature: directly proportional to the temperature of a conductor

Electrical power Energy (additional information)

The term energy is used to express work Energy = Power*Time Unit of energy is kilowatt-hour (kWh)

ELECTRICAL BASICS

Calculations and examples Ohm's Law (additional information)

Current (I or A), electric potential (V or E), and resistance (R) are related to each other, and a variance in one will affect the others. This relationship is known as Ohm's Law.

Current (I or A) is directly proportional to voltage (V or E) Current (I or A) is inversely proportional to resistance (R)

Power formulas

GENERATION AND DISTRIBUTION (Chapter 9)

Power, work, and energy

Demand and consumption

Electric current: DC or AC

Generating efficiency

Cogeneration

GENERATION AND DISTRIBUTION (Chapter 9)

Direct current (DC) (additional information) In DC, electrons always move in the same direction Polarity of the generator remains always the same.

Alternating current (AC) (additional information) polarity of the generator or alternator reverses periodically current varies periodically in value and directions, first flowing in one

direction in the circuit and then flowing in the opposite direction Generation of AC (additional information)

generated using a combination of physical motion and magnetism simplest form of AC generator is constructed using a single loop of wire placed

between the poles of a permanent magnet and then rotating it by some suitable mechanical means.

magnetic lines of force are interrupted with the rotation of loop an electromagnetic force is induced in the loop EMF thus produced exists between the two ends of the loop slip rings and brushes attached to each end of the loop apply the generated EMF

to an external circuit.

GENERATION AND DISTRIBUTION

How the current alternates (additional information) When the loop is parallel to the magnetic lines of force, no magnetic flux is interrupted and no

EMF is induced to the loop. As it begins to rotate, it cuts the magnetic flux at an increasing rate reaching a maximum when it has rotated a quarter turn (i.e. 90°).

As the rotation continues, the EMF is still in the same direction but is decreasing in value. A half revolution (i.e. 180° ), the loop is again parallel to the magnetic lines and no magnetic flux is interrupted; EMF at this point equals 0. As the rotation continues further, the sides of the loop reverse position and the induced EMF reverses polarity; therefore, direction of the flow of current also reverses. The EMF increases to a maximum again at 3/4 turn (i.e. 270° ) and declines to 0 when the rotation is completed.

Cycle of AC (additional information) Each complete rotation by the loop of wire (or armature coil) within the poles of magnet is

called a cycle. Frequency of AC (additional information)

The number of cycles per second is known as the frequency of the voltage or current The unit used to measure this frequency is Hertz

In the United States, the frequency for alternating current is 60 Hertz. Use of AC results in the reduction of transmission loss.

GENERATION AND DISTRIBUTION (Chapter 9)

Example of transmission loss reduction Watt = Current2 x Resistance If current (A or I) = 20 and resistance (R) = 4, then

transmission loss (W) = 202x4 = 1600 The loss can be reduced by decreasing R. If R were

decreased by half to 2, then W = 202x2 = 800 The loss can also be reduced by decreasing current. If it

were decreased by half to 10, then W = 102x4 = 400 It is obvious that if current is decreased by half, transmission

loss would be reduced by a factor of 4. In case AC, the stepping up and stepping down of current

and voltage can be achieved by using a transformer.

GENERATION AND DISTRIBUTION (Chapter 9) Single-phase and three-phase power

Single-phase (additional information) a single armature coil creates a complete cycle of voltage

and current requires one 'hot' wire and a neutral wire

Three phase (additional information) use of three separate coil conductors equally spaced (@

120° ) around generator armature in order to obtain the value of line voltage, phase voltage

(either neutral-to-phase or phase-to-phase) has to be multiplied by 3 or 1.73 (e.g. if single-phase voltage is 208, then the corresponding three-phase voltage would be 208* 3 = 360)

a three-phase system with an Y-connection requires 3 'hot' wires and a neutral wire.

GENERATION AND DISTRIBUTION (Chapter 9)

Transformers Power distributionTransformers (additional information)

Used for stepping up or stepping down voltages and current

Consists of iron core surrounded by two circuit loops (windings)

Capacity rated in kVA

Power factor (PF)Grounding

ELECTRICAL RATING OF EQUIPMENT (Additional information)

Voltage rating Maximum voltage that can be safely applied continuously to an equipment.

The rating is primarily determined by: Type and quantity of conductor insulation used Physical spacing between electrically energized parts of the equipment

Current rating Determined by the maximum operating temperature at which the

components of an equipment can operate continuously and properly maximum operating temperature is determined by the type of insulation of

the conductor the maximum safe operating temperature of conductors having cotton braid

as insulation would be 65° C Current which will be producing this temperature would be the maximum

permissible current for the conductor the maximum safe operating temperature of conductors having silicone or

glass compound would be 150° C Consequently, the maximum permissible temperature for the same conductor

would increase