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Topic : Motors and Generators

Notes for Motors and Generators

Below are the dot points of Motors and Generators. Click on the dot point to expand relvant information. These notes were written by; Steven Zhang Click here to donate him

The Motor Effect


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

Charged particles moving in an external magnetic field will experience a force. If the moving charged particles are flowing through, and confined within, a conductor that is in an external magnetic field, the conductor will also experience a force. This effect is known as the motor effect. F = qvB Use left hand “FBI gun” An example: Van Allen Radiation Belts The Earth’s magnetic field captures charged particles from the solar wind (low energy) and cosmic rays (high energy). The charges are force to spiral along the field lines accumulating into two doughnut-shaped belts of “radiation” called the upper and lower Van Allen radiation belts.


  • Describe qualitatively and quantitatively the force on long parallel currentcarrying conductors:


Ampere’s Law Two parallel wires, each carrying a current, will exert a force on the other. This happens because each current produces a magnetic field (as in Oersted’s experiment). Therefore each wire find itself carrying a current across the magnetic field produced by the other wire and hence experiences a force.

itself carrying a current across the magnetic field produced by the other wire and hence experiences a force. Determining the magnitude of the force between two parallel conductors The magnetic field strength at a distance, d, from a long straight conductor carrying a current, I, can be found using the formula:


  • Define torque as the turning moment of a force using:
    τ!= Fd

Torque is turning force. Its’ units are Newton-metres (Nm).

AB


  • 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

The motor effect Recall that charged particles moving in an external magnetic field will experience a force. If the moving charged particles are flowing through, and confined within, a conductor that is in an external magnetic field, the conductor will also experience a force. An electric motor is a device that transforms electrical potential energy into rotational kinetic energy.


  • 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

Anatomy of a DC motor

  • Permanent magnets: provide an external magnetic field in which the coil rotates. As the magnets are fixed, they are known as the stator.
  • Rotating coil: carries a direct current that interacts with the magnetic field, producing torque.
  • Armature: is made of ferromagnetic material and allows the coil to rotate freely on an axle. The armature and coil together are known as the rotor. The armature protrudes from the motor casing, enabling the movement of the coil to be used to do work.
  • Commutators: reverse the current of the coil every half turn to maintain consistent direction and torque. It is a mechanical switch that automatically changes the direction of the current flowing through the coil when the torque falls to zero.
  • Brushes: maintain electrical contact of coils with the rest of the circuit.

The development of DC motors outstripped that of AC motors and generators for two reasons:

  • Voltaic batteries could supply power
  • They could use powerful electromagnets that were far stronger than permanent magnets

The development of the commutator was important because it led to the development of modern electric motors and generators. It enabled motors to provide steady circular motion of a drive shaft.


  • Describe how the required magnetic fields can be produced either by currentcarrying coils or permanent magnets

The magnetic field of a DC motor can be provided either by permanent magnets or by electromagnets

Electromagnetic Induction


  • Outline Michael Faraday’s discovery of the generation of an electric current by a moving magnet


Faraday had found that 3 things are necessary to generate (or “induce”) an EMF (voltage supply):

  • A magnetic field (from some magnets or electromagnet)
  • A conductor (eg. wire or coil of wire)
  • Relative motion / change between the field and the conductor

If the conductor formed a closed loop then an induced current would also flow.

If this wire is dropped so that it cuts flux lines, then a voltage appears between the ends because electrons are forced to the right. They eventually stop moving because they create an electric field pushing them back. As long as the magnet is moving, an emf and current is induced.


  • Define magnetic field strength B as magnetic flux density

Magnetic flux density is the magnetic flux per unit area and is a measure of the magnetic field strength.


  • 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

The induced emf is proportional to the rate of change of flux through the circuit. See Faraday’s Law (above).


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

This is a supplementary law to Faraday’s Law. It says that any induced emf or current will have a direction that opposes the change that caused it. This is really just a restatement of the law of conservation of energy because the induced electrical energy has come from the thing that causes the original motion. Eg. In a hydroelectric power station, the kinetic energy of flowing water is converted into electrical energy.


  • Explain that, in electric motors, back emf opposes the supply emf
The coil becomes an electromagnet and generates an alternating B field BUT it also experiences the changing B field and generates its own emf that opposes the applied emf.

Back emf Back emf is generated in any coil that experiences changing B fields, even though it is producing them.

Note that back emf is frequency dependent – the higher the frequency of the changing field, the greater the back emf produced.

Back emf is also produced in the rotating coil of a motor:

  • When the motor is spinning at its operating speed, back emf will have its max value, but…
  • When the motor is just turned on it isn’t spinning yet so there is no back emf.
  • This can lead to excessive current so the motor may be protected by using a “starting resistance” that limits current. When up to speed the resistor is taken out of the circuit.

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


Eddy Currents – are induced currents (usually unwanted or unintended) in two-dimensional conductors (eg. sheet metal) or three-dimensional conductors (eg. a block of steel) . Sometimes it is necessary to design against them. Eg. the core of a motor is made of soft iron, and is made of thin layers (laminated) to prevent eddy currents.

Some devices rely on eddy currents to work:

  • Electromagnetic braking – a moving conductor near magnets will slow down because the eddy currents oppose its motion.
  • Electromagnetic switching – security ‘gates’ that are really coils with AC generate a high frequency B field. Metal in this field develops eddy currents that work against the field, slowing it down. A detector circuit picks up on this and sets off an alarm.
  • Induction Cooktops- are an application of Faraday’s Law. Instead of a heating element, this cooktop contains a set of coils with alternating current passing through them. This produces a changing B field above the cooktop. A metal saucepan placed on the cooktop is a conductor in the changing B field and therefore an electric current is induced in the base of the pan. The current heats the pan, and this heat cooks the food. Induction cooktops are approximately twice as efficient as a gas cooktop, but are expensive to purchase.

Electric Generators


  • Identify the main components of a generator

An electric generator (dynamo) is a device that includes all of the elements necessary to transform mechanical kinetic energy to electricity according to Faraday’s Law:

  • A magnetic field (provided by a set of permanent magnets);
  • A conductor (a coil mounted on an axle, so it can spin);
  • Relative motion (the coil is made to spin by some other form of energy).

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

In fact, most generators are constructed just like a motor, however the flow of energy through them is different. Motor: electrical energy > kinetic energy Generator: kinetic energy > electrical energy


  •  Describe the operation of an AC and a DC generator

Doubling the frequency of rotation doubles the maximum induced emf

EMF is generated in the coil and a circuit is completed to the outside world through ring connectors, just like motors. If standard slip rings are used then a dynamo naturally produces alternating current AC.


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

Even good electrical conductors like copper used to supply electricity, sometimes through considerable cable lengths to towns and cities, generate substantial resistances. It follows that to minimise energy loss in the wires, the current needs to be kept low (heating losses vary as the square of the current). This is achieved by transmitting the energy at high voltages.


  • 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

1979 study found children living near high voltage power lines appeared to develop a particular form of cancer. 1997 study showed no evidence of an increase risk of childhood cancer at residential magnetic field levels. 1998 panel stated that EM fields should be considered “possible human carcinogens” and that there is “no conclusive and consistent evidence that EM fields cause any human disease.”

Transformers


  •  Explain the purpose and principles of transformers in electrical circuits

A transformer is a device that alters the voltage and current of an electricity supply. The AC voltage source produces an alternating current in the primary coil. This produces an alternating B field that threads through the secondary coil. The secondary coil now has:

  • Conductor
  • B field
  • Change

and therefore generates its own voltage. If there is a closed loop then an alternating current will flow as well.


  • Compare step-up and step-  down transformers

Step-up transformers: increase voltage and decrease current Step-down transformers: decrease voltage and increase current


  • 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 voltage transformations are related to the conservation of energy

The Principle of Conservation of Energy states that energy cannot be created or destroyed but that it can be transformed from one form to another. This means that if a step-up transformer gives a greater voltage at the output, its current must be decreased: i.e. power in = power out.


  • Explain the role of transformers in electricity sub-stations

NSW power stations produce electricity with a voltage of about 23,000 V and a current of about 30,000 A. Unfortunately, this amount is too high to be sent through a cable. This is because it heats the cable causing energy loss. This is called joule heating and happens because: P = I2R So to reduce joule heating, the current must be reduced as much as possible with a stepup transformer. Additional transformers between the power station and consumer (in sub-stations) gradually step down the voltage, to 240 V by the time it gets to household users. This is because at high voltages, electricity can conduct through air, making it dangerous for use in the home.


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

Most electronic circuits are designed to operate at low DC voltages of between 3 V and 12 V. Therefore, household appliances that have electronic circuits in them will have either a plug-in transformer or an inbuilt transformer to step down the domestic 240 V supply. These transformers also have a rectifier circuit built into them that converts AC to DC. TVs also contain a step-up transformer for producing the high voltages needed for the CRT.


  • Analyse the impact of the development of transformers on society

The development of the generator and transformer has allowed for the setting up of national power grids in almost every country, making that most convenient and flexible form of energy, electricity, accessible from many miles away. The transformer’s role is to step voltage up and down to make efficient transportation and distribution possible.

Electric Motors


  • Describe the main features of an AC electric motor

AC induction motor:

  • The rotor – end rings short circuit non-ferrous rotor bars, that is sealed i.e. no external connections at all (usually a “squirrel cage”). Encased in a laminated iron armature.
  • The stator – surrounding electromagnet.
  • Connection to stator – the surrounding electromagnet receives the AC.

  • AC (synchronous) motor:
    • A rotating coil
    • Surrounding magnets
    • Connection to coil via slip rings (commutator for DC motor)

In an AC induction motor, the principle of operation is:

  1. AC to surrounding electromagnet, which…
  2. Produces an oscillating (rotating) B field, which…
  3. Induces a current in the rotor, which…
  4. Turns the rotor into an electromagnet that…
  5. Tries to oppose the field being generated by the stator.
  6. The stator and the rotor push against each other (using their B fields), which…
  7. Causes the rotor to turn! Brilliant!

  • Explain that AC motors usually produce low power and relate this to their use in power tools

Power is the rate of work. Work is done when energy is transformed from one type to another. Induction motors are considered to produce low power because the amount of mechanical work they achieve is low compared with the electrical energy consumed. The ‘lost power’ of induction motors is consumed in magnetising the working parts of the motor and in creating induction currents in the rotor. AC induction motors are considered to be unsuitable for use in heavy industry because their low power rating would make them too expensive to run when performing a specific task. However, they are used extensively in power tools and electric domestic appliances where the loss of power is not economically significant


  • Explain the advantages of induction motor

Advantages of AC induction motors:

  1. Simplicity of design;
  2. High efficiency (hence low maintenance – there are no brushes or commutators to wear out);
  3. Relatively low cost

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