Teaching Notes for Unit 9.3 Motors and Generators Topic of NSW HSC Physics Course

1. Topic 9.3
3.3.1 Generators

2. Generating emf
● When a coil is rotated in a
uniform magnetic field an emf is
induced across the coil.
● When the coil is perpendicular to
the field the induced emf is the
greatest.
● When the coil is parallel to the
field the induced emf is zero.
● If the coil is rotated at constant
speed, then the induced emf will
vary sinusoidally with the same
frequency as the rotation.

3. AC Generators
● When a coil is turned in a
magnetic field it cuts lines of
magnetic flux.
● This causes a current to be
induced in the coil.
● This current flows around the
coil to the ends where it is
attached to a slip ring
commutator.
● As the coil turns through the
perpendicular, the direction of
the current is reversed.
● This causes an alternating
current to be produced.

4. Induced EMF
(beyond Syllabus)
● The flux linkage of a coil of N turns and cross
sectional area A in a uniform magnetic field B is
given by:
Φ=NBA
● If this coil is rotating then the flux linkage when
the normal to the plane of the coil makes an
angle θ to the field becomes:
Φ=NBA cosθ

5. Induced EMF
(beyond Syllabus)
● The induced emf is therefore given by the rate
of change of flux linkage:
Δ t =NBA Δ cosθ
Δ t
ε=Δ Φ
● It can be shown that this is the same as:
ε=NBAsin θ Δ θ
● Where is the angular velocity (ω) in rads-1
Δ θ
Δ t
● Therefore:
Δ t
ε=NBA(sin ωt)ω

6. Induced EMF
(beyond Syllabus)
● The maximum emf is therefore when sin(ωt) =
1
● Therefore
εmax=ω NBA
● Note that this implies that if the frequency of
rotation is doubled then the frequency of the
emf will double as will the peak emf.

7. AC Generators
● The higher the frequency of
revolution, the greater the rate
of change of flux, and the
higher the induced emf.
● Using an iron core in the coil
“amplifies” the magnetic field
and increases the induced emf.
● Using more coils increases the
flux linkage increasing the
induced emf.
● The higher the frequency of
revolution the higher the
frequency of the alternating
current produced.

8. AC Generator Construction
● A simple generator is exactly the same in
construction as a simple motor. It consists of:
● A magnetic field – created by either a
permanent magnet or an electromagnet.
● An armature onto which the coils of wire are
wound.
● A commutator to pass the current out of the
rotating rotor.
● Spring-loaded carbon brushes to make
contact with the commutator.

9. AC Generator Construction
● For practical reasons, most real
world generators are AC and use
stator coils and use the magnet as
the rotor.
● This simplifies the construction of
the generator and allows for more
powerful generators to be made.
● The stator windings can be made
from thicker wires reducing their
resistance and increasing the
maximum current carrying ability.
● The rotor is often a simple
electromagnet to allow for control
of the output emf.

10. AC vs DC Generators
The primary difference between a simple AC and simple DC generator is the
commutator.
In a DC generator a split ring commutator is used so that the current is always
coming out in the same direction.
An AC generator uses a slip ring commutator so that each end of the coil is always
attached to the same brush. This means that the output current changes direction
every 180°.

11. AC vs DC Generators
Generator Advantages Disadvantages
AC ●Simpler construction
●More powerful 3 phase
generators can be made.
●AC voltage is more easily
distributed with transformers
●The electricity grid needs fine
co-ordination to ensure that all
generators are in phase with
each other.
●AC current is signifficantly more
dangerous than the equivalent
DC current in an electric shock.
DC ●A lot of devices rely on DC
currents for their operation.
●At a given voltage a DC current
can be more powerful than the
equivalent AC current.
●More complex construction with
split-ring commutator.
●Sparking occurs in the gap of the
commutator wasting energy.
●DC is more difficult to distribute
efficiently.

12. Power Losses in Transmission Lines
● Electricity is generated at around 10000V, 50Hz.
● If the electricity was transmitted, at this voltage then
the currents in the transmission wires would be very
large.
● This will cause heating effects in the wires due to
P=I2R.
● For this reason electricity is stepped up to around
400kV using a transformer for transmission thereby
reducing the current and the power losses.

13. Power Losses
● As well as the heating effect in the transmission
wires due to the current and the wires'
resistance, there are other power losses.
● Dielectric losses – The insulation material acts as a
capacitor and causes energy to be lost as heat.
● Skin effect – the alternating E and B fields in the
wires causes self-inductance and slows the
movement of electrons at the outside of the wire.
This effectively makes the wire thinner.
● Transformer losses

14. Power Transmission
● Electricity is generated in a power station
at around 10kV by rotating a coil in a
magnetic field.
● All generators in all power stations on the
same grid are synchronised such that they
all produce electricity that oscillates
together.
● Three windings are used on the generator
to produce 3-phase electricity. This allows
for more efficient generation
●

15. Power Transmission
● The main substation
steps up the voltage
(and the current
down) to around
400kV or higher for
transmission.
● Each pylon carries
wires for all three
phases.
●

16. Power Transmission
● The transmission
substation reduces
the voltage to around
11kV for localised
transmission.
● This increases the
current in the wires
but reduces the costs
of power poles and
increases safety.
●

17. Power Transmission
● A distribution substation steps
the voltage down to 240V for
local transmission to houses.
● Some businesses and houses
require 2 or three phase
power whilst others need
single phase.
● If power is accessed between
phases then 415V is available
for use.

18. Transmission Lines
● Power lines are usually carried
above ground for economic
reasons and for ease of installation
and maintenance.
● However, the power lines, which
are almost always bare, must be
insulated from the metal pylons to
prevent the pylons becoming live or
the wires short circuiting.
● The wires are therefore suspended
from porcelain, glass or ceramic
discs which are strong and very
good insulators even under high
voltages.

19. Transmission Lines
● In order to try to prevent
surges and damage due to
lightning strikes, the pylons
usually have another wire at
their highest point.
● This wire is connected
directly to the Earth at
regular intervals and provides
a very low resistance path for
the lightning surge to travel
down if it is struck, thereby
saving the transmission lines.

20. The Battle of the Currents
● Edison was the first person to set up an electricity company in the
1880s.
● He had selected DC for his system and produced light bulbs for homes
and streets and developed DC motors and appliances.
● Edison built power stations in the individual suburbs of New York and
ran cables directly to his customers within a 1 mile (1.6km) radius.
● Edison's DC system was transmitted at a fixed voltage of 100V as this
was deemed safe enough for general use and because his Edison
appliances were designed to run at this voltage.
● His cables were limited by length as if they were any longer, the
resistance of the wire itself was enough to reduce the voltage too far
below 100V to be useful.

21. The Battle of the Currents
● Westinghouse bought patents from Nicola Tesla and built AC
motors and appliances to run on his AC electricity supply.
● Westinghouse used transformers to increase the voltage up to
very high levels for transmission over long distances.
● He was convinced that his system of AC with transformers was
far more efficient.
● He was also able to use transformers to increase and decrease
the voltage at will to supply different appliances.
● Westinghouse won a competition to build a power station at
Niagara falls in 1886 and his AC system proved to be so efficient
that it eventually marked the downfall of mains DC.