Download Motors and Generators

Motors and Generators
A topic (in two parts) about the
interaction between magnetic
fields
Assumed Knowledge from the Preliminary
• Demonstration 1: Electric
currents produce magnetic
fields
• The recognition that electric
currents produce magnetic
fields is essential.
• Use of hand rules should be
well practised
Images from Andriessen et. al. Physics 2 Jacaranda p109, 110
The Motor Effect
• A current carrying conductor in an external magnetic field
experiences a force.
• F=BIlsin calculates the magnitude of the force on a current
carrying conductor in a magnetic field, where
F = the force on the conductor (N – newtons)
B = the magnetic flux density of the external field (T – tesla)
I = current in the conductor (A – amperes)
l = the length of conductor in the field (m – metres)
 = angle between the conductor and the field (o)
• The right hand palm rule gives the direction of the force.
Image from Andriessen et. al. Physics 2 Jacaranda p113
Demonstration 2
Modelling the Motor Effect
Focus area 1
-Column 3 dot point 2
-Column 2 dot point 1
Features of the DC electric motor
• Demonstration 3
Aspects of the DC Motor
• Knowing the right equation
Image from Andriessen et. al. Physics 2 Jacaranda p118
Electromagnetic induction
• Electromagnetic induction is producing an emf between the
ends of a conductor when there is a magnetic flux change
around it.
[emf is electromotive force – a measure of the energy supplied
to each coulomb of charge by the source (battery, generator
etc)]

  n
t
Where  = induced emf
n = no. of turns in the coil
 = change in magnetic flux
t = change in time (or time taken for
the change in flux to occur)
Demonstration 4
Modelling generation of electric current
Focus area 2
- Column 3 dot points 1 and 2
Demonstration 5
Lenz’s Law
Lenz’s Law
• Lenz’s Law: The direction of an induced emf is such that it
opposes the change that causes it.
Image from Andriessen et. al. Physics 2 Jacaranda p138
Demonstration 6
Demonstrating production of AC current
Focus area 3
Column 3 dot point 1
•
•
Transformers
The purpose of a transformer is to increase or decrease AC
voltages.
Transformers consist of two coils of insulated wire wound
onto a soft iron core.
Image from Andriessen et. al. Physics 2 Jacaranda p157
Demonstration 7
Modelling structure of a transformer
Focus area 4
Column 3 dot point 1
AC Induction Motors
• An AC motor has
- a stator, consisting of field coils (electromagnets) which
provide the rotating external magnetic field
- a rotor, consisting of conductors on the motor’s shaft.
- a cooling fan
Demonstration 8 – Demonstrating AC induction motor principle
A bench pulley is placed between opposite poles of two strong
magnets.
Disc
on
shaft
magnets
When a rotating magnetic field spins around a stationary disc,
a. eddy currents are induced in the disc in the region of the field
b. the induced magnetic field will start the disc moving.
c. the change causing the induced current was the movement of
the field relative to the disc, so the opposition to the change will
be the moving of the disc to reduce the relative motion between
the field and disc.