PRODUCTION OF ALTERNATING CURRENT
chose equipment or resources for and perform a first-hand investigation to
demonstrate the production of an alternating current.
9.3.3, column 3, dot point 1)
hand-cranked generator or dynamo set to AC output so that the slip rings are in
use. Attach CRO probe from input A
of CRO to the generator brushes to measure the AC output voltage when the
armature is rotated by hand. (Note - Do not attach the CRO probe to any
output terminals on the generator if these exist. You will almost
certainly not get a very good AC signal if you do so. The signal will be
mixed with a lot of noise. For a good strong, clean signal you must attach
the CRO probe directly to the brushes of the generator!)
CRO to AC signal input & start with 1 V/div and 5 ms/div.
Note the nature of the output waveform.
It is a typical AC signal – sinusoidal and extending in both the
positive & negative voltage directions (vertical scale).
you have the facility to do so, it is also very useful to demonstrate a DC
output. Set the generator to DC
output so that the split ring commutator is now in use. Attach CRO probe to the brushes in contact with the
commutator. Compare the voltage
output this time with the AC one. The
output is a typical DC voltage signal – sinusoidal but all positive.
diagram below shows the basic set-up. Most
laboratory generator models consist of two coils each with many turns of wire
and use permanent magnets rather than the electromagnet shown.
The diagram is not drawn to scale.
DEMONSTRATION - PRODUCTION
OF SECONDARY VOLTAGE IN A TRANSFORMER
an investigation to model the structure of a transformer to demonstrate how
secondary voltage is produced.
9.3.4, column 3, dot point 1)
up the transformer coils as shown below. Use
CRO. Attach primary coil to AC
power and to input A of CRO. Attach
secondary coil to input B of CRO. Settings
on CRO – start with 5 V/div and 5 ms/div.
Observe output voltage waveform from secondary coil and compare this with
voltage waveform input to primary coil. Both
voltages are clearly AC in nature and for a step-up transformer (more turns of
wire on secondary than on primary), the secondary voltage will be larger than
that the diagram below is not drawn to scale.
to use a soft iron core in the centre of the primary coil to intensify the
magnetic field threading through the secondary. See following diagram.
PRINCIPLE OF AC INDUCTION MOTOR
an investigation to demonstrate the principle of an AC induction motor.
9.3.5, column 3, dot point 1)
that the basic principle of operation of an AC induction motor is that a
changing magnetic field in the stator induces a current in the rotor and then
drags the resultant induced magnetic field of the rotor around with it as it
(the stator field) rotates. This,
in turn drags the rotor around as well. I’m
not altogether convinced that the Syllabus writers fully appreciated what they
were asking when they asked for this to be demonstrated to students in a school
think it will suffice to show that a moving magnetic field in the vicinity of a
solid metal (non-magnetic) conductor can cause the conductor to move.
this, hang a hollow copper cylinder from a retort stand using fishing line, as
shown in the following diagram. Mark
a clearly visible coloured dot on the cylinder to make movement obvious.
Hold two neodymium (rare earth) magnets (very powerful) on opposite sides
of the cylinder with the north pole of one facing the south pole of the other.
Show that the copper cylinder is not attracted by the magnets.
Hold one magnet still. Rotate
the other magnet in a circular motion in the vertical plane always keeping the
same pole facing the copper. You
should observe the copper cylinder start to rotate about its vertical axis.
Try both directions of rotation of the magnet.
demonstrate that the rotation of the cylinder is not caused by moving air
currents due to your hand. Put the
magnets down and repeat the circular motion movements with your hand in the same
vicinity of the cylinder as previous. Cylinder
should not rotate (we hope).
that in this demonstration the moving magnetic field induces eddy currents in
the copper cylinder, which produce magnetic fields, which then interact with the
moving field of the magnets to produce a net rotation of the cylinder.
Explain also the differences between this demonstration and what happens
in a real AC induction motor.
that in order to satisfy the Syllabus statement it is NOT sufficient to simply
show that relative movement between a conductor and a magnetic field can induce
a current. Yes, that is a
demonstration of induction. However,
if that is what the Syllabus Committee required for this dot-point, that is what
they should have asked for. Instead
the Syllabus specifically asks for a demonstration of the principle of an AC
induction motor not a simple demonstration of induction.
The two are distinctly different in level of complexity.