Transcript Unit 51: Electrical Technology

Unit 51: Electrical Technology
The Characteristics and Principles of
AC and DC Generators and the
features of a Range of difference
Power Station
Course Aims
•
NDGTA
At the end of this course the learner will be
able to…
1. Know the methods used to produce electrical
energy
2. Know the properties and applications of conductors,
insulators and magnetic materials
3. Know the physical arrangements of supply,
transmission and distribution equipment
4. Know how electrical energy is used to support
applications of electrical technology
Agenda
NDGTA
• At the end of the session the learner will
be able to…
– Describe DC Power systems
D.C. Power Systems
NDGTA
• D.C. Power systems have a multitude of
applications….
– Use in electroplating industry
– Use in cars
– Portable equipment
• Most electrical generation produced in the
UK is 3-phase A.C. which is converted to
D.C. for use.
D.C. Power Systems
NDGTA
• D.C. is also available in the form of
primary and secondary chemical cells.
• D.C. generators also produce electrical
power for specialist applications.
D.C. Production using
Chemical Cells
NDGTA
• An electrochemical cell is composed of two
dissimilar metals, which are immersed in a
conductive liquid or paste called an electrolyte.
• Chemical cells are classified as either primary or
secondary cells: primary cells are ordinarily not
usable after a certain time period i.e. the
chemicals are normally used up and cannot
produce any further electrical energy; secondary
cells can be renewed after they are used by
reactivating (i.e. charging) the chemical process.
• When two or more cells are connected in series
they form a battery
Characteristics of a
Primary Cell
NDGTA
Characteristics of a
Primary Cell
NDGTA
• When the chemicals that compose the cell are
brought together, their molecular structure is
altered.
• During this alteration atoms may gain additional
electrons or lose some of their electrons.
• This ionisation process develops a chemical
solution capable of conducting an electrical
current.
• The voltage of a primary cell depends upon the
electrode material and the type of electrolyte
used. (For a zinc-carbon dry primary cell this is
typically about 1.5 v)
Internal Resistance
NDGTA
• An important characteristic of a chemical
cell is its internal resistance
• V = I(R + r)
• R is the resistance of the load and r in the
internal resistance of the cell both
measured in Ohms.
Internal Resistance
I
V
R
r
NDGTA
Internal Resistance
NDGTA
• Given a 1.5 v battery has an internal resistance
of 0.8 Ω find the current when a load of 10.0 Ω is
connected to the battery?
• The no-load voltage of a battery is 9.05 v. Its
rated load is 200 mA and its internal resistance
is 0.1 Ω. Find the rated output voltage of the
battery?
• The rated output of a battery is 30 v and its noload voltage is 30.15 v. Its FLC is 350 mA. Find
the internal resistance of the battery?
Primary Cells
NDGTA
• Explore the applications of primary cells
Characteristics of a
Secondary Cell
NDGTA
• Chemicals of a secondary cell may be
reactivated by a charging process.
• Secondary cells are sometimes called
storage cells
• The most common types of secondary
cells are lead-acid cell, nickel-iron (Edison)
cell and the nickel-cadmium cell.
The Lead-Acid Cell
NDGTA
• The electrodes of the lead-acid cell are
made of lead (Pb is the negative plate)
and lead-peroxide (PbO2 is the positive
plate). The electrolyte is dilute sulphuric
acid (H2SO4).
• When a lead-acid cell supplies current to a
load connected to it the chemical process
can be expressed as…
• PbO2 + Pb + 2H2SO4 -> 2PbSO4 + 2H2O
The Lead-Acid Cell
NDGTA
• The sulphuric acid ionises to produce 4-positive
hydrogen ions (H+) and 2-negative sulphate (SO4-) ions.
• A negative charge is developed on the lead plate when
the SO4- ion combines with the lead plate to form lead
sulphate (PbSO4).
• The positive hydrogen ions (H+) combine with electrons
of the lead-peroxide plate and become neutral hydrogen
atoms. They next combine with oxygen (O) on this plate
to become water (H2O). The lead-peroxide plate thus
becomes positively charged.
• A fully charged lead-acid cell has an electrical potential
developed between the electrodes of around 2.0 volts.
The Lead-Acid Cell
NDGTA
• After discharging by supplying a current to
a load for a certain period of time, it no
longer is able to develop an output
voltage.
• The cell may be recharged by causing
direct current to flow through the cell in the
opposite direction
• 2PbSO4 + 2H2O -> PbO2 + Pb + 2H2SO4
Other
Secondary Cells
NDGTA
• Explore the construction of the nickel-iron
and the nickel-cadmium cells.
• Explore the applications of secondary cells
A Battery
NDGTA
• A battery is a combination of more than one cell.
• The cells in a battery may be connected in
series or in parallel.
– In series:
• The total emf = sum of each of the cell’s emfs
• The total internal resistance = sum of each of the cell’s
internal resistances
– In parallel:
• If each cell has the same emf and internal resistance, then..
• The total emf = emf of one cell
• The total internal resistance of n cells = 1/n = internal
resistance of one cell
Problems
NDGTA
• Eight cells, each with an internal resistance of 0.2 Ω and an
emf of 2.2 v are connected (a) in series and (b) in parallel.
Determine the emf and the internal resistance of the
batteries?
• A cell has an internal resistance of 0.02 Ω and an emf of
2.0 v. Calculate its terminal p.d. if it delivers (a) 5 A (b) 50 A
• The pd at the terminals of a battery is 25 v when no load in
connected and 24 v when a lad taking 10 A is connected.
Determine the internal resistance of the battery.
• Ten 1.5 v cells, each having an internal resistance of 0.2
Ω are connected in series to a load of 58 Ω. Determine the
current flowing in the circuit and (b) the pd at the battery
terminals
Further information
on Batteries
NDGTA
• As cells can be placed in series within one
another to form a battery; so to can
batteries be placed in series with one
another.
Note: the
1.5 V
1.5 V
3
internal
resistance of
the series of
batteries is the
sum of the
internal
resistance of
each of the
batteries
NDGTA
Batteries in Series
• Consider the following arrangement of
batteries…
1Ω
2Ω
1Ω
Q
P
4V
5V
What would be the total emf and
internal resistance across P – Q?
3V
Batteries in Parallel
NDGTA
• Consider the following arrangement of
batteries…
2Ω
P
1.5 V
2Ω
Q
1.5 V
What would be the total emf and
internal resistance across P – Q?
Safe disposal
of batteries
NDGTA
• Battery disposal has become a topical subject in
the UK because of greater awareness of the
dangers and implications of depositing up to 300
million batteries per annum – a waste stream of
over 20,000 tonnes – into landfill sites.
• Certain batteries contain substances which can
be a hazard to humans, wildlife and the
environment, as well as posing a fire hazard.
• Other batteries can be recycled for their metal
content
Safe disposal
of batteries
NDGTA
• Waste batteries are a concentrated source of toxic heavy
metals such as mercury, lead and cadmium.
• If batteries containing heavy metals are disposed of
incorrectly, the metals can leach out and pollute the soil
and groundwater, endangering humans and wildlife.
– Long-term exposure to cadmium, a known carcinogen (i.e. a
substance producing cancerous growth), can cause liver and
lung disease.
– Mercury can cause damage to the human brain, spinal system,
kidneys and liver.
– Sulphuric acid in lead acid batteries can cause severe skin burns
or irritation upon contact.
• It is increasingly important to correctly dispose of all
types of batteries.
Safe disposal
of batteries
NDGTA
• Refer to Handout 1 and note the disposal
recycling options
• Battery disposal has become more regulated
since the Landfill Regulations 2002 and
Hazardous Waste Regulations 2005.
• From the Waste Electrical and Electronic
Equipment (WEEE) Regulations 2006,
commencing July 2007 all producers
(manufacturers and importers) of electrical and
electronic equipment will be responsible for the
cost of collection, treatment and recycling of
obligued WEEE generated in the UK.
Alternative and
Renewable Energy Sources
NDGTA
• Alternative energy sources are sources of
energy that could replace the coal, oil or gas –
all of which release carbon when burned.
• Renewable energy implies that it derives from a
source which is automatically replenished (or
one which is effectively infinite) so that it is not
depleted when used
• Coal, oil and gas are not renewable – their
supplies will eventually run out.