9

9.3 Motors and Generators

Contextual Outline

Electricity is a convenient and flexible form of energy. It can be generated and distributed with comparative ease, and most importantly, it can be readily converted into other forms of energy such as heat, light, sound or mechanical energy. Electricity is a key element in the development of industrialised nations.

There are many examples of electric motors both around the home: refrigerators; washing machines; vacuum cleaners; and in the community: industrial motors and trains. The design of a motor for an electrical appliance requires consideration of whether it will run at a set speed, how much power it must supply, whether it will be powered by AC or DC and what reliability is required. The essentials of an electric motor are the supply of electrical energy to a coil in a magnetic field causing it to rotate.

The generation of large quantities of electrical power requires relative motion between a magnetic field and a coil. In the generator, mechanical energy is being converted into electrical energy while the opposite occurs in the electric motor. Once generated, electricity must be distributed over long distances from the power station to cities and towns. Transmission lines carry the electrical energy at a high voltage from the generator and transformers eventually reduce the voltage to that required by the consumer.

The electricity produced by most generators is in the form of alternating current.
In general AC generators, motors and other electrical equipment are simpler, cheaper and more reliable than their DC counterparts. AC electricity can be easily transformed into higher or lower voltages making it more versatile than DC electricity. Since the frequency of AC electricity can be precisely controlled, it is
used in motors that require accurate speed, such as clocks and tape recorders.

Outcomes

This module contributes to the following course outcomes.

A student:

H3 assesses the impact of particular advances in physics on the development of technologies

H4 assesses the impact of applications of physics on society and the environment

H7 explains the effect of energy transfers and transformation

H8 analyses wave interactions and explains the effects of those interactions

H9 explains the effects of electric, magnetic and gravitational fields

H11 justifies the appropriateness of a particular investigation plan

H12 evaluates ways in which accuracy and reliability could be improved in investigations

H13 uses terminology and reporting styles appropriately and successfully to communicate information and understanding

H14 assesses the validity of conclusions drawn from gathered data and information

H15 explains why an investigation is best undertaken individually or by a team

H16 justifies positive values about and attitudes towards both the living and non-living components of the environment, ethical behaviour and a desire for critical evaluation of the consequences of the applications of science.

1. Motors use the effect of forces on current-carrying conductors in magnetic fields

Students learn to:

• identify that moving charged particles in a magnetic field experience a force

• 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

• describe qualitatively and quantitatively the force on long parallel current-carrying conductors:

• define torque as the turning moment of a force using:

• identify the forces experienced by a current-carrying loop in a magnetic field and describe the net result of the forces

• account for the motor effect due to the force acting on a current-carrying conductor in a magnetic field

• describe the main features of a DC electric motor

• discuss the importance of the invention of the commutator for developing electric motors

• describe the role of the metal split ring and the brushes in the operation of the commutator

• describe how the required magnetic fields can be produced either by current-carrying coils or permanent magnets

Students:

• identify data sources, gather, analyse and present information to discuss the Van Allen radiation belts as examples of motion of charged particles in a field

• perform a first-hand investigation to demonstrate the motor effect

• solve problems and analyse information about the force on current-carrying conductors in magnetic fields using and

• solve problems and analyse information about simple motors using:

and

• gather and process secondary information to analyse the function of the parts of a commutator

• identify data sources, gather and process information to qualitatively describe the application of the motor effect in:

– the galvanometer

– the loudspeaker

2. The relative motion between a conductor and magnetic field is used to generate an electrical voltage

Students learn to:

• outline Michael Faraday‘s discovery of the generation of an electric current by a moving magnet

• define magnetic field strength B as magnetic flux density

• explain the concept of magnetic flux in terms of magnetic flux density and surface area

• explain generated potential difference as the rate of change of magnetic flux through a circuit

• account for Lenz’s Law in terms of conservation of energy and relate it to the production of back emf in motors

• explain that, in electric
motors, back emf opposes the supply emf

• apply Lenz’s Law to the production of eddy currents

Students:

• perform an investigation to model the generation of an electric current by moving a magnet in a coil or a coil near a magnet

• plan, chose equipment or resources for, and perform a first-hand investigation to predict and verify by gathering and analysing information about the change in strength and direction of the magnetic field if:

– the distance from the
conductor is increased

– the amount of current
is increased

– the current is reversed

– the relative speed
is increased

• gather, analyse and present information to explain how the principle of induction applies to cooktops in electric ranges

• gather secondary information to identify how eddy currents have been utilised in switching devices and electromagnetic braking

3. Generators are used to provide large scale power production in isolated areas and as a backup in emergency situations

• identify the main components of a generator

• compare the structure and function of a generator to an electric motor

• describe the operation of an AC and a DC generator

• discuss the energy losses that occur as energy is fed through transmission lines from the generator to the consumer

• analyse the effects of the development of AC and DC generators on society and the environment

• assess evidence about the physiological effects on humans living near high voltage power lines

• perform first-hand investigations to produce direct current using voltaic cells

• plan, chose equipment or resources for, and perform a first-hand investigation to demonstrate the production of an alternating current

• gather secondary information to compare advantages and disadvantages of AC and DC generators and relate these to their use

• gather and analyse information to identify how transmission lines are:

– insulated from supporting
structures

– protected from lightning
strikes

4. Transformers allow generated power to be either increased or decreased before it is used

Students learn to:

• explain the purpose and principles of transformers in electrical circuits

• compare step-up and step-down transformers

• determine the relationship between the ratio of the number of turns in the primary and secondary coils and the ratio of primary to secondary voltage

• explain why current transformations are related to conservation of energy

• explain the role of transformers in electricity sub-stations

• discuss why some electrical appliances in the home that are connected to the mains domestic power supply use
a transformer

• analyse the impact of the development of transformers on society

Students:

• perform an investigation
to model the structure of a transformer to demonstrate how secondary voltage is produced

• solve problems and
analyse information about transformers using:

• gather, analyse and use available evidence to discuss how difficulties of heating caused by eddy currents
in transformers may be overcome

• gather and analyse information and use
available evidence to
assess the need for transformers in the transfer
of electrical energy from
a power station to its point
of use

5. Motors are used in industries and the home usually to convert electrical energy into more useful forms of energy

• describe the main features
of an AC electric motor

• explain that AC motors usually produce low power, relate this to their use in power tools and explain why they would therefore be unsuitable for use in heavy industry

• explain the advantages of induction motors

• perform an investigation to demonstrate the principle of an AC induction motor and discuss why the majority of motors are AC induction motors

• gather, process and analyse information to identify some of the energy transfers and transformations involving the conversion of electrical energy into more useful forms in the home and industry