ENGINEERING LESSON GUIDE 17

1
INTRODUCTION TO NAVAL ENGINEERING
FUNDAMENTAL ELECTRICAL THEORY
2
Definitions

• Current (I) flow of electric charges per unit
  time, measured in amperes or amps (A)
• Electromotive Force (emf) (E) a 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
  (?)
• Conductor a material which offers little
  resistance to current flow, e.g. silver, copper,
  iron, etc
• Insulator a material which offers high
  resistance to current flow, e.g. wood, paper,
  plastic, etc...

3
ELECTRICAL THEORY

4
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
• P I2 R E I

5
Ohms Law
6
Kirchoffs Laws

• The algebraic sum of all voltages in a circuit is
  zero

Series
Parallel
The sum of all currents entering a node sum
leaving
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

8
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
9
ELECTRICAL POWER GENERATION

10
Induced emf

• 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
3 Things Required to Generate Voltage

• Magnetic field
• Conductor
• Relative motion

12
Electromagnetic Induction
COIL (CONDUCTOR)
INDUCED CURRENT
RELATIVE MOTION
VOLTMETER
N
MAGNET
INDUCED CURRENT
S
13
Direction of Induced emf
MOTION OF CONDUCTOR
B
N
S
INDUCED CURRENT
LEFT HAND GENERATOR RULE
(electron flow)
14
Magnets

• Permanent magnets are usually too weak for any
  practical applications

IRON CORE

-
DC BATTERY
B
B ? (N x I)
ELECTROMAGNET
15
Generator Parts

• Prime mover mechanical work which turns the
  rotor, may be a steam turbine, gas turbine,
  diesel engine...
• Armature windings the conductor in which the
  output voltage is induced
• Field windings the conductors used to produce
  the electromagnetic field (needs a DC power
  supply)
• Stator stationary housing of the generator
• Rotor rotates inside the stator, moved by a
  prime mover (steam turbine, gas turbine, internal
  combustion engine, etc)
• 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
Two Types of AC Generators

• Revolving armature
• rotor is an armature which 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
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Relationship Between Generator Speed and Frequency

• Most electrical equipment in the United States
  operates on 60 Hz AC electrical power (some
  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 v. 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)

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

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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
QUESTIONS?

32
Example Problem 1

• Determine V1, V2, V3, V4, and I.

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

I
33
Example Problem 2

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

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

I1
34
Reading

• Introduction to Naval Engineering
• Chapter 17, pp. 319-322
• Principles of Naval Engineering
• Chapter 20, pp. 20-1 - 20-35
• HOME WORK HAND OUT 6

35
Questions??