Monday, November 3, 2008

Basic Electrical Theory

On this theory we will learn about:
  1. Understand of basic electrical theory by definition, DC and AC.
  2. Understand How to use Ohm and Kirchoff law to apply on simple network.
  3. Understand of how electric current is merged
  4. Understand of generator part basic and the function.
  5. Understand of generator classification
  6. Understand of how the electric appliance below work :
  • Motor
  • Battery
  • Transformer


  • Current (I): flow of electricity per time unit, in Ampere or A unit.
  • Electromotive Force (emf) (E): potential difference "electric pressure" that is force electric current, measure in "volts" (V)
  • Resistance (R): electrical network that is against of electric current, measure in "ohms" (W)
  • Conductor: material that have little resistant of electric current, such silver, copper, iron, etc.
  • Insulator: material has high resistant to electric current such as wood, paper, plastic, etc.

Direct Current (DC)

Electric current having same big and same direction

Ohm law : Electric current is directly proportional to potential difference and inversely proportional to the network resistance.

V = I . R

P = I2 . R = V . I

Kirchoff Law:

1. Sum of all potential difference in close network is zero.

In Series Network: Rc = R1 + R2 + R3 + ...

Problem Example:

On closed circuit is installed 3 resistances in serial order, each of this resistance 5 ohm, What is the Resistance total on that circuit?


Rc = R1 + R2 + R3 = 5 + 5 + 5 = 15 ohm

In Parallel Network: 1/Rp = 1/R1 + 1/R2 + 1/R3 + ...

Problem Example:

On closed circuit is installed 3 resistances in parallel order, each of this resistance 5 ohm, What is the Resistance total on that circuit?


1/Rp = 1/R1 + 1/R2 + 1/R3 = 1/5 + 1/5 + 1/5 = 3/5

Rp = 5/3 = 1 2/3 ohm = 1.667 ohm

2. The sum of all electric current in = sum of all electric current out

Alternating Current (AC)

  • Electric Current will change the big and its direction for the same interval.
  • Current have time function and always vary as sinus function.
Induksi emf (electromotive force)
  • Faraday founded that emf being inducted to the conductor if the magnet passing conductor, so that the moving between magnet and conductor will cut band flux of magnetic.
  • Direction of emf induction depend on the direction magnetic moving between magnet and conductor.
  • Current big inducted depend on the magnet flux, magnet speed when passing conductor and the number of flux magnet that is cut.

3 ways should be attention on produce potential difference

  • Magnetic field
  • Conductor
  • Relative movement

Electromagnetic Induction

Emf (Electromotive Force) inducted direction


Permanent magnet always too weak for any application

Parts of Generator:

  • Prime mover: mechanic force that move rotor, such as turbine gas, diesel engine, etc.
  • Armature windings: conductor in which is the potential out inducted
  • Field windings: conductor used for resulting magnetic field (need DC current)
  • Stator: static generator housing
  • Rotor: rotate inside stator, moved by prime mover (steam turbine, gas turbine, internal combustion engine, etc.)
  • Sliding contacts (slip-rings and brushes): Used for electric transfer in magnetic field or armature to and from rotor.

Simple AC Generator

There are two kind of AC Generator

Revolving armature

  • Rotor is armature that is rotate inside of static electromagnetic field.
  • Seldom to use because power out should be transfer through slip-ring and brush

Revolving field

  • DC electric is supplied by motor cause the magnetic field rotate inside stator.
  • More simple because supply current need by field smaller than current out from armature.

Revolving Armature

Revolving Field

Relationship of generator speed and frequency.

N x P = 120 x f

"Nuclear Power is 120 times as Fun"

Three Phase Electric Power

  • Using 3 armature circumference to yield three different output.
  • Armature circumstance physically separated 120o each others, so every phase separated of 120o.
  • Power resulted by generator higher by certain weight and dimension.
  • Continuously giving energy to electric appliance even broken of one phase.

1- Fase VS. 3- Fase

Generator Classifications

  • Number of phases: most shipboard electrical power is 3 phase
  • Frequency: most shipboard electrical power is60 Hz, some electronic equipment operate at 400 Hz or higher
  • Voltage: usually 450 V, smaller appliances use 120 V
  • Power rating: measured in kW, most shipboard generators are 2,000 - 3,000 kW


  1. If you want all the definitions regarding electrical terms please click

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