9.3 Motors and Generators
1. The motor effect
THE MOTOR EFFECT
A current carrying wire will
experience a force in the presence
of a magnetic field.
When the switch is closed a
current will flow in the aluminium
rod. It will suffer a force due to the
effect of the magnetic field and the
rod will move (you can use this as
an experiment)
If either the poles of the magnet or
the poles of the DC source are
changed around the direction of
movement will change.
The direction of movement (i.e. of
the force) can be found using the
right-hand-slap rule.
Magnitude of the force
The magnitude of the force is given by the formula:
F = B I L sinq
Where,
F = force(N); B = Magnetic field strength (T); I = current(A); L = length of conductor in
external magnetic field (m); and q = angle of conductor to magnetic field
It follows that:
*As the strength of the magnetic field in which the conductor is located increases, the
force increases
*As the magnitude of the current in the conductor increases, the force increases
*As the length of the conductor in the external magnetic field increases, the force
increases
* As the angle between the direction of the external magnetic field and the direction of
the length of the conductor approaches q = 90o force is max, when q = 0 (i.e. parallel to
magnetic field lines) force = 0
Torque
The product of the force applied to a lever
and its distance from the lever's fulcrum is
defined as the torque
t = Fd
Where, t = torque (Nm) ; F = force (N)
perpendicular to the axis of rotation and d
(m) = distance from the fulcrum to the force.
In the rectangular coil shown, because there
are two forces acting, the torque on the coil
is:
t = 2(F x 1/2a) = Fa
If the coil is a conductor and is placed in a
magnetic field, B (T) and a current I (A) flows
through the coil, then
F = BIL
Therefore torque will be
t = BILa,
But L x a =Area (m2) of coil,
therefore
t = BIA
For angles other than q = 0
t = BIA cosq
If the coil has 'n' turns then
t = nBIA cosq
Rotation
The two forces will
cause the coil to rotate
in a clockwise direction
As the coil rotates the
forces remain the same
but the torque on the
coil changes
When the coil is in position 1 torque
is maximum
In position 2 the torque will depend
on the angle q (t = nBIA cosq)
When coil is in position 3 the torque
is zero and the tendency to rotate
will stop.
If the momentum of the coil carries
it over the vertical position the
forces will reverse and return it to
the vertical position
To continue rotating clockwise the
direction of the current must be
reversed
DC motor
If a commutator is fitted to the coil as shown,
the current reverses every half cycle and the
coil will rotate continuously: this arrangement
is known as a DC motor.
The commutator is in contact with “brushes”
which carry the current to the coil.
In practical motors the coil has many turns (to
increase torque) and there are more than one
coil angled at regular intervals (to avoid the
momentary slow down when a single coil is
vertical)
The faces of the magnet are not flat but curved
so that the magnetic field is parallel to the
plane of the coil throughout the rotation.
The magnet can be a permanent magnet or an
electromagnet with its coil forming part of the
circuit.
The brushes are usually made of graphite as it
is a good conductor and is self lubricating
Some degree of sparking occurs at each gap in
the commutator, therefore these motors are
not safe in areas where inflammable solvents
are used.
The coils are called the rotor or armature and
the stationary magnet the stator.
Parallel wires
Parallel, current-carrying wires experience
forces between them because their respective
magnetic fields interact.
When the currents in both wires are in the
same direction the magnetic fields between
them are going in opposite direction and they
diminish each other and the two wires are
pulled together (attraction)
When the currents in both wires are in the
opposite direction the magnetic fields between
them are going in same direction and they
strengthen each other and the two wires are
pushed apart (repulsion)
In each case the force (F) experienced by each
wire is identical in magnitude and depends on
the magnitude of the currents in each (I1 and
I2), on the distance between them (d) and on
their length (L):

k in the equation is a constant which has the
value 2 x 10-7 T m A-1.
Some Calculations
I.
A 1 m long conductor, carrying a current of 0.5 A, is placed in a magnetic field of
0.003T at an angle of 30o to the magnetic field. If only 23 cm are in the magnetic
field. Calculate the force on the wire.
II.
A rectangular loop of wire consisting of 35 turns and having sided 2 cm x 4 cm is
placed between the poles of a magnet at an angle of 45o to their magnetic field of
0.0007 T . If the current in the loop is 1.6 A, calculate the torque on the loop.
III. Three parallel wires are 15 cm apart. Each carries a current of 2A, the middle wire
has the current going in the opposite direction to the other two. Calculate the force
per unit length on the outer wires. Estimate the force on the middle wire.
Answering Physics calculation-type questions:
Draw a quick, but accurate sketch of the problem
Enter all the data given, make sure you change each to standard units
Choose from the formula-list supplied and write down the formula that applies to
the problem
If necessary, change the subject of the formula to obtain the answer requested
(following algebraic rules)
Substitute correctly in the formula
Calculate your result and give the answer to the same significance level as was
used in the question
If more than one calculation is necessary , do not round-off any numbers until the
end
It is very important that you right
down the correct formula and
then do the correct substitution
of values.
The actual answer, although
important, is secondary to those
two crucial steps.
Generally, full marks will be
awarded for the correct
substitution in the correct
formula.
The
galvanometer
A galvanometer uses the motor effect to
measure the magnitude of an electric current.
The current flows through a coil wound
around a soft iron core, suspended in the
field of a permanent magnet.
The resulting motor effect produces a torque
on the coil which is opposed by a circular
spring. When the restoring force in the spring
exactly balances the electrically induced
torque the pointer stops and the current is
read from a previously calibrated scale.
The pole faces of the magnet are curved to
ensure that the magnetic field is even and so
produces a torque of approximately constant
value within the range of the needle swing.
The torque on the coil is directly
proportional to the current as the coil rotates,
allowing an evenly divided scale to be used.
The
Loudspeaker
The electric current flowing through
the coil varies with respect to the
signal generated by the equipment
attached to the speaker wires
(radio, telephone, amplifier etc).
The motor effect sets the coil
vibrating 'in concert' with the
changes in the current supplied by
the signal acting in the strong
magnetic field from the permanent
magnet.
The vibrating cone amplifies the
vibration of the coil producing
sound waves as the air is set
vibrating.
Revision questions - 1
1) Discuss the effect on the magnitude of the force on a current-carrying
conductor of variations in: the strength of the magnetic field in which it is
located; the magnitude of the current in the conductor; the length of the
conductor in the external magnetic field; the angle between the direction of the
external magnetic field and the direction of the length of the conductor
2) Describe a first-hand investigation you performed to demonstrate the motor
effect
3) Describe the application of the motor effect in: the galvanometer and the
loudspeaker
4) Describe qualitatively and quantitatively the force between long parallel
current-carrying conductors.
5) Define torque.
6) Define the motor effect.
7) Describe the forces experienced by a current-carrying loop in a magnetic field
.
8) Describe, using a sketch and labeling, the main features of a DC electric motor
and the role of each feature
9) Identify how the required magnetic fields in DC motors can be produced.
2. Generators
Faraday's law
In 1831 English chemist and physicist Michael Faraday (1791-1867) believed
that if an electric current could cause a magnetic field (as had been shown by
Oersted and Ampere) then, a magnetic field should be able to produce an
electric current.
After many experiments, Faraday found that only a moving or changing
magnetic field would induce a current in a conductor
At first, he induced an electric current in a rotating disc of copper metal
between the poles of a strong magnet, essentially he had invented the first
generator. Later, he induced currents using coils wound on the same soft iron
ring and not only deduced his Law of electric induction, but also invented the
first transformer.
His law states:
“The induced electromotive force or emf (e) in a closed circuit is given by the time
rate of change of the magnetic flux (F) through the circuit.”
Note: the minus charge is related to Lenz's law
Magnetic Flux
Faraday described magnetic flux as
the total number of magnetic flux lines
that pass through a given area (he was
the first to introduce the concept of
lines to represent electric and
magnetic fields).
The magnetic flux through a surface is
proportional to the net number of
magnetic field lines that pass through
the surface.
Magnetic flux is given by:
F = BA
The units for magnetic flux are the
Weber (Wb)
It follows that:
B=F/ A
The magnetic field strength (B) is thus
defined as the magnetic flux density
and 1T = 1 Wb m-2
Lenz's Law
Heinrich Lenz was a Russian physicist
who in 1833 formulated a law concerning
electromagnetic induction. It is now known
as Lenz's law:
“The induced electric current flows in a
direction such that the magnetic fields
generated by the current will oppose the
change that induced the current”
A small amount of work is done in pushing
the magnet into the coil and in pulling it out
against the magnetic field formed by the
induced current.
Lenz's law is a consequence of the Law of
Conservation of Energy.
If the current were induced in the opposite
direction, its action would spontaneously
draw the bar magnet into the coil, thus
increasing the induced emf which, in turn
would increase the strength of the induced
field, further accelerating the magnet, and
so on.
Energy would be created, contravening the
Law of Conservation of Energy.
Generating a
current
The set-up shown is one that will
answer the requirements of the
syllabus first-hand investigation. If a
CRO is not available it maybe
substituted with a sensitive
galvanometer or a data logger.
The coil is connected to a CRO which
will give a read-out of the emf induced
when the magnet is in motion.
The movement of the magnet should
be kept fairly constant for the first
parts of the experiment.
I. The distance between the coil and
magnet is varied by lowering and
raising the magnet
II. The strength of the magnet is
varied by using stronger magnets
(e.g. neodymium magnets)
III. The relative motion between the
coil and the magnet is varied by
oscillating the magnet faster.
Eddy currents
The French physicist Léon Foucault in 1851
discovered eddy currents (or Foucault
currents) when he exposed a conductor to a
changing magnetic field.
The currents formed are similar to the
circulating eddies in a river, hence their name.
They create induced magnetic fields that
follow Lenz's law and oppose the change
caused by the original magnetic field.
Like all electric currents, eddy currents
generate heat depending on the resistance of
the material. The heat may be harnessed in a
form of heating called induction heating.
The opposing magnetic fields induced by eddy
currents can be used for a form of braking
called induction braking.
Eddy currents are increased by: stronger
magnetic fields; faster changing fields; thicker
materials and lower resistivity materials
(aluminium, copper, silver etc.).
Eddy currents are decreased by the opposites
to the conditions mentioned above, as well
as: slotted materials and laminated materials
so that currents cannot circulate freely.
Induction
braking
An eddy current brake or induction brake, like a
conventional friction brake slows a moving
object. (e.g. train, fly wheel or roller coaster).
Induction brakes slow an object by creating
eddy currents which oppose the movement.
Induction braking is virtually fail-safe because it
relies on the basic properties of magnetism and
requires no electricity. Magnetic brakes are also
completely silent and are much smoother than
friction brakes.
In roller-coasters, magnetic brakes are made up
of one or two rows of very strong Neodymium
magnets. When a metal fin (typically copper or a
copper/aluminum alloy) is raised between the
rows of magnets, eddy currents are generated in
the fin, which creates a magnetic field opposing
the roller-coaster's motion.
The resultant braking force is directly
proportional to the relative speed between the
magnets and the metal fin.
This property is also one of the disadvantages
of induction braking: in that the eddy force itself
cannot completely stop a train, it can only slow
it down.
Induction heating
Induction heating is the process of heating a metal object by electromagnetic
induction. Eddy currents are generated within the metal and its resistance leads
to heating of the metal.
An induction heater (for any process) consists of an electromagnet, through
which a high-frequency alternating current (AC) is passed. The frequency of AC
used depends on the object size and type of material.
In induction cooking, an induction coil in the cook top heats the metal base of the
pot by inducing alternating eddy currents of high frequencies. Glass or ceramic
pots cannot be heated in this way as they are non-conducting. The heat induced
in the base is transferred to the food via conduction and convection.
Cookware must generally be made of ferrous materials. Copper or aluminum pans
will not work on a typical induction cooker as their resistance is too low. Newer
cook tops are being developed that are designed to work with any metal,
including copper and aluminum,
Benefits of induction cookers include efficiency, safety (the cook-top remains
cool to the touch) and speed.
According to the Electric Glass Company, a producer of induction cook tops,
"power savings of 40-70% are realistically achievable in comparison to
conventional cook tops."
Other uses of induction heating are: Induction furnaces and Induction welding.
Back emf
Example problem:
(i) Calculate the internal
resistance of a motor if it draws
1.5 A on start-up when connected
to 240 V.
(ii) Once the motor reaches full
speed it draws only 0.05 A at
constant speed. Calculate the
back emf at this point.
Answer:
(i) R = V / I
= 240 / 1.5
= 160 W
(ii) enet =
eapplied - eback
0.05 x 160 = 240 - eback
eback = 240 - 80
= 160 V
As the rotor of an electric motor rotates in
the stator's magnetic field an electric current
is induced in its coils (Faraday's Law). This
induced current will flow in the opposite
direction to the current applied externally
(Lenz's law)
This current will produce an emf in the coils
that is opposite in direction to the applied
emf and hence is known as back-emf.
The net emf driving the motor is the
difference between the applied emf and the
back emf. The electrical resistance of the
motor will determine the current through the
coils.
enet = eapplied - eback and enet = RI
As the motor increases in speed the back
emf also increases in value but, the applied
emf remains constant, (as long as it isn't
changed externally).
At the point where the two emf's are
'balanced' the motor reaches constant speed
for that supplied voltage (emf). The net emf
and current flowing are just enough to
overcome the frictional and load forces.
Back emf and motors
When more load is applied to the motor, the coils slow
down and the back emf is reduced. A greater current
will now flow through the coils. It will result in an
increased torque to handle the increased load.
However, if the load is too great, too much current will
flow through the coils. Their temperature will increase
and eventually they will melt - the motor will have
“burned out”
Larger DC motors, to be efficient, need a lot of
windings with a minimum of mass. Hence, thin wire is
used in their armatures. These thin wires run the
chance of burning-out at the low start-up speeds
(when the back emf is low).
For this reason, motor coils are usually protected by a
series resistors. These 'switch-off' at higher speeds
when the back emf reduces the net current .
Revision questions - 2
1) Outline Michael Faraday's discovery of the generation of an electric current by a moving
magnet
2) Describe how magnetic field strength B can be defined as magnetic flux density
3) Describe the concept of magnetic flux in terms of magnetic flux density and surface area
4) Describe the generated potential difference as the rate of change of magnetic flux
through a circuit
5) Account for Lenz's Law in terms of conservation of energy and relate it to the production
of back emf in motors
6) Explain back emf in electric motors.
7) Explain using appropriate sketches the production of eddy currents in terms of Lenz's
Law
8) Describe an investigation you performed to model the generation of an electric current
by moving a magnet and a coil.
9) Describe a first-hand investigation you performed to verify the effect on a generated
electric current when: the distance between the coil and magnet is varied; the strength
of the magnet is varied and he relative motion between the coil and the magnet is varied
10) Explain how induction is used in cook tops in electric ranges
11) Explain how eddy currents are used in electromagnetic braking
3. Power Production
The DC generator or dynamo
A generator converts mechanical energy to
electric energy by spinning a coil of wire in a
magnetic field - electromagnetic induction.
In a DC generator a commutator and brushes
carry the generated electric current to where
it is needed e.g. a light bulb
In practice, the rotor is made up of many
windings around a laminated soft iron core
on a rotating axle and the assembly is known
as the armature
The magnets are rounded for optimum
induction (they hug the rotation of the
armature). They can either be permanent or
electromagnets and are known as field
magnets.
The brushes are graphite and pick up the
current from the coil through a commutator
Essentially it is identical to a DC motor
except that electric power is generated. It is
not used-up as in a motor i.e. motors convert
electric energy to mechanical/rotational
energy. Generators convert
rotational/mechanical energy into electrical
energy
Generating an AC current
The same equipment that was used
on a previous occasion can be set
up.
As the permanent magnet
oscillates in and out of the coil an
AC current is produced that can be
detected by the CRO.
Varying the period of oscillations
will produce AC currents of
different frequencies (The AC
current from a normal domestic
power point has a frequency of
50Hz)
Other variables that can be
investigated are: the strength of
the magnet ; the speed of the
magnet; the relative position of the
magnet to the coil
The AC generator or alternator
A generator converts mechanical energy to
electric energy through the process of
electromagnetic induction by spinning a coil of
wire in a magnetic field.
In an AC generator slip rings and brushes
carry the generated electric current to where
it's needed e.g. a light bulb.
In practice the rotor is made up of many
windings around a laminated soft iron core on
a rotating axle and the assembly is again
known as the armature
The magnets are rounded for optimum
induction (they hug the rotation of the
armature) and they can either be permanent or
electromagnets.
The brushes are graphite and pick up the
current from the coil through a set of slip rings
An alternative AC generator may have the rotor
as the magnet (permanent or electromagnetic)
and the stator as the coil in which the current
is induced, This arrangement requires no
brushes or slip-rings and is sometimes
referred to as the 'brushless generator'.
DC and AC generators
The only real difference between AC and
DC generators is in the type of connection
used to source the induced current from
the coils in the rotor (slip or split rings).
This difference results in a very different
type of current signal being produced by
each as is shown here.
In a single coil DC generator the current is
induced in the coil for a half a turn. The
maximum voltage is reached when the coil
is completely parallel to the magnetic field.
When the split in the commutator touches
the brushes no current flows or is
generated, hence the voltage available
drops to zero to start again as the
commutator moves on. The result is a
current flowing in one direction only.
In a single coil AC generator, with the
brushes running on slip rings a constant
connection is maintained with the external
circuit. This means that the polarity
changes every half-turn and a current that
changes direction is produced.
AC and DC generators
DC
DC generators use a split-ring
commutator.
In DC generators the output current is
always induced in the rotor.
Brushes and commutators wear out
quickly and short circuits in the
commutator are possible due to sparking.
Arcing is likely after prolonged use.
DC generators will require regular
maintenance.
In a DC generator the current is generated
in the rotor. If larger the currents are
required, heavier rotor coils must be
used. This will increase maintenance
problems.
A DC generator can be made using many
coils in a regular pattern around the
armature resulting in a smooth output
with just a small 'ripple'. This is an
advantage with equipment that needs a
steady voltage rather than a fluctuating
one.
AC
AC generators use slip-rings.
in an AC generator the output current can be
induced in the rotor or the stator.
In an AC generator the slip-rings have continuous,
smooth surfaces, allowing the brushes to remain
continuously in contact with the slip ring surface.
Thus the brushes in an AC generator do not wear
as fast as in a DC generator and there is little or no
possibility of creating an electrical short circuit.
An AC generator will requires less maintenance
and will be more reliable than a DC generator.
In an AC generator the rotor can be used to create
the magnetic field that induces AC current in the
stator (as the rotor rotates). The current will be
drawn using a fixed connection to the stator which
is much more reliable than through a moving
commutator.
An advantage of AC generators is that they can
easily be designed to produce three-phase
electricity. This makes them ideal for generating
electricity on a large scale for distribution over a
wide area.
Edison and Westinghouse
(the "War of Currents" 1880s)
Edison
Westinghouse
Edison aggressively promoted generating
and supplying direct current (DC)
Westinghouse and Nikola Tesla advocated the use
of alternating current (AC) electricity.
Edison carried out a campaign to
discourage the use of alternating current,
including spreading information on fatal AC
accidents and publicly killing animals
including a circus elephant
Westinghouse, using Tesla's AC system, won the
international Niagara Falls Commission contract.
On November 16, 1896, electrical power was sent
from Niagara Falls to industries in Buffalo.
Edison opposed capital punishment, but his
desire to disparage the system of alternating
current led to the invention of the electric
chair to promote the idea that AC was
deadlier than DC.
When the chair was first used, on August 6,
1890, the technicians on hand misjudged the
voltage needed to kill the condemned
prisoner, William Kemmler and the
procedure had to be repeated a number of
times.
Westinghouse commented:
"They would have done better using an axe."
AC replaced DC for central station power
generation and power distribution, enormously
extending the range and improving the safety and
efficiency of power distribution.
The Chicago World's Fair in 1893 exhibited a
complete poly-phase generation and distribution
system installed by Westinghouse and invented
by Tesla.
The successful Niagara Falls system was the
turning point in the “War of Currents” and the
acceptance of AC.
The final irony is that Tesla had worked for Edison
but Edison had discounted his AC system saying:
“Tesla's ideas are splendid, but they are utterly
impractical”
Why is AC better for power
transmission than DC?
DC
DC cannot be stepped up or down in
voltage easily (especially in the 1880s)
hence it had to be distributed at the
voltages used by the consumer
Since the resistance of the wire is pretty
much constant low voltages meant high
currents. High currents resulted in large
power losses over relatively short
distances ( less than 2 km)
This meant that a lot of power or boost
stations were needed to supply large cities
and from locations where hydroelectric
power could be generated.
Today, through the use of solid state
electronics it has become easy to switch
from DC to AC and if high temperature
superconductors are developed,
transmission of power as DC may prove to
be more economical.
AC
AC can easily be stepped up and down in
voltage using transformers
AC could be generated at a low voltage,
stepped up to very high voltages for
transmission and then stepped down again for
consumer use.
Higher voltages-low currents are better for
power transmission because the power lost in
transmission due to the wire resistance is
proportional to the square of the current
(P=I2R) but only directly proportional to the
voltage (P =VI)
With AC the power loss per kilometer was a lot
lower hence power or boost stations could be
spaced far apart, resulting in a much cheaper
and more attractive grid system
AC had the disadvantages that it caused
power loses through electromagnetic radiation
and induction and the frequencies used were
apt to be more deadly.
Power
losses
Sample calculation of power-loss
Compare the energy losses in the
transmission of 1 MW of power
over a line with a resistance of 1.5
ohm per kilometer between using
high voltages/low currents and low
voltages/high currents.
P=VI
1, 000,000 = 200,000 x 5 (HV/LA)
or
= 5000 x 200 (LV/HA)
Power lost / km in the 1st case is:
P = I2R = 52 x 1.5 = 37.5 W
Power lost / km in the 2nd case is:
P =I2R = 2002 x 1.5 = 60 000 W
Power is lost during transmission because of the
natural resistance of the wire and because of
electromagnetic induction of eddy currents.
The losses occurring because of electrical resistance
result in heating of the wire and loss of energy to the
environment, they can be minimized by using:
Very high voltage AC and low currents
Good conductors like copper or aluminium
Lines with large cross-sectional area (this must
be off-set by the weight of the line hence
aluminium is often preferred as it has a lower
density)
Losses due to the induction of eddy currents can be
minimized by:
Supporting the wires away from metal towers
Using laminated iron cores in transformers used
to step up and step down the voltage
Eddy currents result in overheating of
transformers. Cooling transformers with
circulating oils and radiative fins reduces over
heating. This also further energy losses due the
increase in resistance which comes with
increasing temperature.
Power transmission insulators
Insulators used for high-voltage power transmission are
made from very high resistance materials like glass,
porcelain, or composite polymer materials and are covered
with a smooth glaze to shed dirt and water. This keeps
surface conductivity to a minimum.
High voltage insulators are shaped into a series downward
facing, cup-shaped surfaces that act as umbrellas. This not
only maximizes the length of the leakage path along the
surface from one end to the other but also avoids overwetting in rainy weather.
Even higher voltage transmission lines use modular cap
and pin insulator designs . The wires are suspended from a
'string' of identical disk-shaped insulators which attach to
each other with metal links. The advantage of this design is
that series of insulator disks can be easily assembled to
handle different wire voltages.
Protection from lightning
Power lines, being high metal structures, are natural attractors of
lightning strikes. These can cause severe damage to the lines, the
towers and to the distribution network , due to high surges in current.
To protect against the effects of lightning strikes, power lines are often
equipped with arresters or with overhead ground wires called shield
wires that are well grounded (with large areas of metal buried
underground)
An arrester behaves like an open circuit when the line is in normal
operation. When the line is hit by lightning, the arrester acts more like a
fuse by diverting the lightning current to the ground and holding the
voltage at safe value.
The shield wires are placed above the lines and are designed to be 'hit'
before the lines by the lightning.
Shield wires are connected directly to the transmission towers without
the use of insulators, when they are struck by the lightning, the current
is conducted safely to earth.
Some very tall transmission lines may have two sets of shield wires.
“The effects of AC generators on society and the
environment” 1
Any paradigm changing invention or technology is bound to have good and bad
effects on the way society develops. It will have its admirers and its detractors. There
is little doubt that the generation of freely available AC power has had a major
influence on how society is structured today.
On the whole, its effect has to be recognised as having been beneficial. It has allowed
for the ability to feed, clothe, keep warm in winter and cool in summer, transport,
entertain and care for a large number of people.
Without full scale power transmission it is doubtful whether we would have factories
to make all affordable, labour saving and life saving goods we take for granted every
day. The humble refrigerator, for example, has probably saved a countless number of
people from sore tummies and even from deadly food poisonings.
Hospitals would be unmanageable without electrical equipment for monitoring and
carrying out surgical procedures, storing of pharmaceutical and important biological
samples.
The world would be a very different place without AC power. Life would be a lot
harder for the great majority of people. Death and sickness rates would be a lot
greater.
With no electricity, the burning of high pollution fuels could be widespread - at the
start of the industrial revolution coal burning caused terrible smog in places like
London and many people died during long, cold winter nights.
(cont.)
“The effects of AC generators on society and the
environment” 2
The relative cheapness of electricity has promoted the development of a wide
range of machines, processes and appliances that depends on electricity. Many
tasks that were once performed by hand are now accomplished with electrical
appliances and most domestic and industrial work requires less labour. Other
new tasks can now be achieved that were formerly impossible, such as
electronic communication.
Some believe that this has caused widespread unemployment. Unemployment was
not any better before electricity - with no large factories, no electrical goods and
automotive production, little or no entertainment industry. Before cheap
electricity, poverty was widespread and child-labour a common practice.
Air pollution from fossil-fuel burning power stations has contributed to acid rain
and contributed to the global warming. At the time that Power generation was
introduced, there was no technology to harness solar energy or wind energy or
anything else but: wood, coal and gas. Global warming might be even worse by
now without AC generators.
AC electricity has helped us with all the technological advancement that have
occurred, now we have the ability, the know-how to and the need look for
alternatives.
Revision questions - 3
1)
2)
3)
4)
5)
6)
7)
8)
9)
Describe using sketches and labeling the main components of a generator
Compare the structure and function of a generator to an electric motor
Describe the differences between AC and DC generators
Discuss the energy losses that occur as energy is fed through transmission
lines from the generator to the consumer
Assess the effects of the development of AC generators on society and the
environment
Describe a first-hand investigation you performed to demonstrate the
production of an alternating current
Discuss advantages/ disadvantages of AC and DC generators and relate these
to their use
Describe the competition between Westinghouse and Edison to supply
electricity to cities
Explain why transmission lines are: i) insulated from supporting structures
and ii) protected from lightning strikes
4. Transformers
Transformers
Electrical transformers are used to change
AC voltage from one value to another. They
do this through magnetic induction.
Magnetic induction demands a changing
magnetic field, hence transformers only
work with AC current.
Transformers can either be classed as
step-up or step-down
Step-up transformers have:
More turns in the secondary coil than
the primary coil
Higher output voltage than input
voltage
Lower output current than input current
Step-down transformers have:
More turns in the primary coil than the
secondary coil
Lower output voltage than input
voltage
Higher output current than input
current
Voltage transformations
Sample calculation:
A transformer manufacturer has to
construct a transformer to output 9V
when connected to domestic power
supply of 240V. If their standard
primary coil has 1000 turns, find how
many turns they will need in the
secondary coil. They know that the
current flowing through the primary
coil will be 0.05 A, transformer what
will be the maximum current possible
in the secondary coil?
vp/vs=np/ns
240/9=100/ns
ns=38
Maximum current will be if
transformer is ideal
Ip/Is=ns/np
0.05/Is=38/1000
Is=1.32 A
The relationship between the number of turns in each coil
and the voltage transformation is given by:
Vp / Vs = np / ns
In the ideal transformer, according to the Law of
Conservation of Energy, energy-in must equal energy-out ,
hence power-in must equal power-out :
Pp = Ps
Hence in the ideal transformer:
Ip / Is = ns / np
In reality, transformers show power/energy losses in the
transformation of voltage. This is because heat is produced
by the induced eddy currents in the iron core due to its
significant electrical resistance. The power output is always
lower than the power input.
The formation of eddy currents (and hence their heating
effect) can be reduced by using laminated iron cores (many
thin lamina or sheets of iron pressed together and separated
by thin insulating layers of oil, lacquer or wax. This restricts
the formation of eddy currents to the thickness of one
lamina).
Once hot, transformers need to be cooled. Large
transformers are oil cooled and have cooling fins and are
located outside where air circulation helps to dissipate the
heat produced.
Modeling a transformer
The set-up shown below would illustrate how a transformer works.
Various AC voltages could be tried and the outputs recorded.
If the ring is connected to the DC side of the TR unit a secondary signal
should be absent, but this is not so, why?
Try connecting the primary coil to a battery and check to see that there
is no secondary signal if true DC is used. (there will be a momentary
signal as the battery is connected and disconnected, why?
NOTE: DO NOT CONNECT THIS DIRECTLY TO A 240V OUTLET
Transformers in
sub-stations
Power stations generate electricity at voltages between 2000 volts and 30,000 volts, depending on
their size. To minimise energy loss due to resistance in transmission wires the voltage is steppedup by the power station to 115kV to 765 kV (typically about 500kV) for transmission over long
distances.
Electricity distribution is accomplished with a combination of sub-stations and pole mounted
transformers that reduce the voltage to a lower level for distribution to commercial and residential
users.
At each stage, the output voltage is chosen to match the power demand and the distances over
which supply is needed. Finally, it is transformed to low voltages of 240V single phase or 415 V
over three phases to operate most industrial and domestic equipment and appliances
Electric power transmission to rural and remote areas, in contrast to urban systems, tend to use
higher voltages because of the longer distances covered by those distribution lines.
Transformers in the home
Many appliances like washing machines and fridges are
designed to run on a supply of 240V and do not need further
use of transformers. If they have computer controlled
circuits and digital displays that run at much lower voltages
like 1 to 9 V, then step-down transformers are needed
The older style television and computer tubes were based
on cathode ray tubes which required high voltages, hence
these appliances incorporated step-up transformer units
that produced highly dangerous voltages of 20,000 V or
more.
Many commonly used electronic equipment like mobile
phones require a recharge transformer coupled to an AC-DC
rectifier to be able to run or re-charge
Leaving transformers (e.g. mobile re-chargers) connected to
the power point, when not in use, is dangerous and
wasteful. The transformer continues to operate for as long
power flows through it.
The impact of transformers on
society
Transformers have made possible the economic transmission of electrical power, hence most of the
discussion points that were used for the effect of generators on society can be adapted to this
discussion, as both relate to the proliferation of AC electricity throughout our society.
The ready access to an economical power supply has had a great influence on the growth of cities
into suburbs (e.g the continuous spreading of Sydney in a westerly direction) and the growth of nonurban localities in far-away rural locations.
Relatively cheap power has seen the explosion in temperature control (both heating and cooling) of
our living environment. It has been especially useful in the our the storage of food and medical
supplies.
Easy availability of electrical power has lead to a increasingly de-centralised society, feeing a lot of
space previously occupied by factories for human habitation and entertainment.(e.g. Sydney's mid
west suburbs and inner city areas)
Many of the labour saving devices, leisure and entertainment equipment, scientific and life-saving
medical apparatus would not have been possible without an economic power supply provided by the
use of step-up and step-down transformers.
Our society has relied and is relying on the availability of power to every home, every factory, every
office to such an extent that most of us cannot conceive a world without it.
A very good depiction of what might happen to a society like ours if all power were suddenly to be
switched off can be found in the semi-fictional novel “One second after” by William R Forstchen ). It is
an exciting read, especially because its premise is based on scientific fact.
Revision questions -4
1) Describe the purpose and principles of transformers in electrical
circuits
2) Compare step-up and step-down transformers
3) Identify the relationship between the ratio of the number of turns in the
primary and secondary coils and the ratio of primary to secondary
voltage
4) Explain why voltage transformations are related to conservation of
energy
5) Explain the role of transformers in electricity sub-stations
6) Discuss the use of transformers in the home.
7) Discuss the impact of the development of transformers on society
8) Describe investigation you performed to model the structure of a
transformer to demonstrate how secondary voltage is produced
9) Discuss how difficulties of heating caused by eddy currents in
transformers may be overcome
10)Discuss the need for transformers in the transfer of electrical energy
from a power station to its point of use
5. Motors
Demonstrating the principle of an AC
induction motor
The set-up should spin easily once it is
balanced on the watch glass and small cuts
are made in the Balsa to fix it in position
The embedded tacks are there to keep the
neodymium magnets in position (check that
the tacks are attracted to the magnets first).
Once the magnets are spinning on the
watch glass gently lower a disc of
aluminium foil and note that it starts
rotating in the same direction .
Eddy currents form in the aluminium disc
that try to stop the rotation by forming
poles of opposite polarity to the magnets,
and hence are dragged with it. Notice that it
'lags' behind the rotation of the magnets.
Check that the aluminium disc is not
affected if there are no magnets.
Once the experiment is over try to make
cuts on the disc with a pair of scissors to
reduce the circulation of eddy currents and
then test your efforts by comparing it to
how it behaved before you cut it.
AC motors
There are different types of AC motors which run at
constant speeds depending on the frequency of the AC
source (about 3000 rpm for 50Hz):
The universal motor, which is basically a DC motor run
with AC; The synchronous motor which is like an AC
generator with slip rings but is run as a motor by
connecting it to AC.
The most common type of AC motor is the induction
motor. In 1882, Nikola Tesla invented the rotating
magnetic induction motor, where the rotor moves because
of a current induced by a rotating magnetic field produced
by the stator. It has no brushes or slip rings. The current
in the stator maybe single or poly-phase
The windings in the stator of an induction motor set up a
rotating magnetic field around the rotor. The relative
motion between this field and the rotation of the rotor
induces electric current in the conductive bars.
These induced currents in the conductor bars react with
the magnetic field of the motor to produce force acting at a
tangent to the rotor, resulting in torque to turn the shaft.
The rotor is carried around with the magnetic field with a
slightly slower rate of rotation, called slip, which increases
with load.
The iron core spreads the magnetic field evenly across the
rotor. The thin laminations, separated by varnish
insulation, reduce stray eddy currents.
Some common energy transfers
Type of electrical
equipment
transfer of energy
from
to
Typical location
Motor
Electrical
Mechanical (plus
some heat & sound)
Washing machine,
record player, DVD
players
Generator
Mechanical
Electrical (plus
some heat & sound)
Power stations, car
Transformer
Electrical
Chemical
Battery chargers
Lights, microwave
ovens, TV
Electrical
Electromagnetic
radiation (light etc.)
lighting, cooking, Xray machines
Loudspeakers
Electrical
Sound
Radio, TV, stereo
players
Batteries
Chemical
Electrical
Car, portable Mp3,
laptops
Revision questions - 5
1) Describe the main features of an AC electric motor
2) Describe an investigation you performed to
demonstrate the principle of an AC induction motor
3) Identify some of the energy transfers and
transformations involving the conversion of electrical
energy into more useful forms in the home and
industry