Fundamental Electrical Theory

1
Fundamental Electrical Theory

• Lesson 18

2
Objectives

• Understand basic definitions related to
  electrical theory
• Understand direct current, alternating current,
  and the differences between them
• Be able to calculate current, voltage, and
  resistance in basic circuits
• Understand single- and three-phase power

3
Definitions

• Current (I)
• Flow of electric charges per unit time
• Measured in amperes or amps (A)
• Analogous to flow of water in a pipe
• Electromotive force (emf, E, V)
• Potential difference or electric pressure which
  drives the flow of charges
• Measured in volts (V)
• Resistance (R)
• An electrical circuits opposition to current
  flow
• Measured in ohms (?)

4
Definitions (contd)

• Conductor
• Material that offers little resistance to current
  flow
• e.g. silver, copper, iron, etc.
• Insulator
• Material that offers high resistance to current
  flow
• e.g. wood, paper, plastic, etc.

5
Electrical Theory
6
Direct Current (DC)

• Current flow is unidirectional and of constant
  magnitude
• Ohms Law
• Current in a circuit is directly proportional to
  the applied voltage and inversely proportional to
  the circuit resistance
• E I R

I
t
7
Alternating Current (AC)

• Current is constantly changing in magnitude and
  direction at regular intervals
• Current is a function of time and usually varies
  as a sine function

I
t
8
Useful Equations

• E I R
• P I2 R E I
• Es E1 E2 (series)
• Rt R1 R2 (series)
• 1/Rt 1/R1 1/R2 (parallel)

9
Electrical Power Generation
10
Induced emf (Voltage)

• Faraday discovered that emf is induced in a
  conductor if a magnet passes by the conductor
  since relative motion between the magnet and
  conductor cut through magnetic lines of flux
• The direction of the induced emf depends on the
  direction of relative motion between the magnet
  and the conductor
• The magnitude of the induced current depends on
  the magnitude of magnetic flux, velocity at which
  the magnet passes by the conductor, and the
  number of magnetic lines of flux that are cut

11
Electromagnetic Induction
COIL (CONDUCTOR)
INDUCED CURRENT
RELATIVE MOTION
VOLTMETER
N
MAGNET
INDUCED CURRENT
S
12
Direction of Induced emf
MOTION OF CONDUCTOR
B
N
S
INDUCED CURRENT
LEFT HAND GENERATOR RULE
(electron flow)
13
Magnets

• Permanent magnets are usually too weak for any
  practical applications

IRON CORE

-
DC BATTERY
B
B ? (N x I)
ELECTROMAGNET
14
Generator Parts

• Prime mover
• Mechanical work that turns the rotor
• e.g. steam, gas, diesel, etc.
• Armature windings
• Conductor in which the output voltage is induced
• Field windings
• Conductor used to produce the electromagnetic
  field
• Needs a DC power supply

15
Generator Parts (contd)

• Stator
• Stationary housing of the generator
• Rotor
• Rotates inside the stator
• Moved by a prime mover
• Sliding contacts (slip-rings and brushes)
• Used to conduct the field or armature current to
  and from the rotor

16
A Simple AC Generator
17
Types of AC Generators

• Revolving armature
• Rotor is an armature that is rotating inside a
  stationary electromagnetic field
• Seldom used since output power must be
  transmitted through slip-rings and brushes
• Revolving field
• DC current is supplied to the rotor, which makes
  a rotating electromagnetic field inside the
  stator
• More practical since the current required to
  supply a field is much smaller than the output
  current of the armature

18
Revolving Armature
19
Revolving Field
http//people.howstuffworks.com/hydropower-plant2.
htm
20
Relationship Between Generator Speed and Frequency

• Most electrical equipment in the United States
  operates on 60 Hz AC electrical power (most
  foreign countries use 50 Hz)
• How fast must a 2-pole generator be rotating to
  produce a 60 Hz output?
• N x P 120 x f

21
Three-Phase Electrical Power

• Uses three sets of armature windings to produce
  three separate outputs
• Armature windings are physically separated 120º
  from each other, and therefore, each phase is
  120º apart from another
• More power may be generated by a generator of a
  given size and weight
• Provides continuous power to electrical equipment
  even if one phase is damaged

22
Single-Phase vs Three-Phase
23
Classifying Generators

• Number of phases
• Most shipboard electrical power is 3 phase
• Frequency
• Most shipboard electrical power is 60 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

24
Miscellaneous Electrical Devices
25
DC Motors

• Similar in construction to DC generators
• A DC generator may be made to act as a DC motor
  by applying a suitable voltage across its output
  terminals (a DC motor acts as a DC generator
  operating in reverse)
• Operates based on the principle that a current
  carrying conductor placed in, and at right angles
  to, a magnetic field tends to move in a direction
  perpendicular to the magnetic lines of force
  (right-hand rule)
• http//electronics.howstuffworks.com/motor5.htm

26
AC Motors

• Widely used for constant speed applications
  (speed depends only upon the frequency for a
  given number of poles)
• Most AC motors are synchronous, 3-phase,
  induction motors
• Rotor is a cage with conductors arranged in a
  cylinder with short circuited ends
• Rotor currents are supplied by electromagnetic
  induction, and a rotating magnetic field is
  established by 3-phase stator windings

27
Batteries

• Dry-cell batteries
• Cylindrical zinc container, carbon electrode, and
  ammonium chloride/water electrolyte
• Wet-cell batteries
• Lead-acid battery is the most common, can be
  charged by forcibly changing the direction of
  electrical current
• Bigger version of your car battery

28
Lead-Acid Battery

-
Pb
PbO2
H2SO4
?
Pb PbO2 2H2SO4
2PbSO4 2H20
?
29
Transformers

• A device that transfers energy by electromagnetic
  induction
• Primary and secondary windings (insulated from
  each other electrically) are mounted on opposite
  sides of a ferromagnetic core
• Used to raise voltage (step-up transformer) or
  lower voltage (step-down transformer)
• Voltage is raised when the primary winding has
  fewer turns than the secondary winding, and
  voltage is lowered when the primary winding has
  more turns than the secondary winding

30
A Simple Transformer
PRIMARY WINDING
SECONDARY WINDING
CORE
31
Rectifiers

• Uses diodes to convert alternating current into
  direct current
• Diodes have a small resistance to current flow in
  one direction and a very large resistance to
  current flow in the opposite direction (act as a
  conductor for half of the cycle and as an
  insulator for the other half)

32
Rectifying Device Output
I
I
t
t
INPUT
OUTPUT
DIODE
33
Example Problem 1

• Determine V1, V2, V3, V4, and I

20?
5?
10?
10?
V1
V2
V4
V3
90V
-

I
34
Example Problem 2

• Determine I1, I2, I3, I4, and total circuit
  resistance

20?
I4
30?
I3
20?
I2
75V
-

I1
35
Summary

• Understand basic definitions related to
  electrical theory
• Understand direct current, alternating current,
  and the differences between them
• Be able to calculate current, voltage, and
  resistance in basic circuits
• Understand single- and three-phase power

36
Questions?