M6-S6: Generators

 

Generator is defined as a device which converts mechanical (kinetic) energy into electrical energy. Simple generators utilise electromagnetic induction through creating relative movement of conductor within an external magnetic field. 

AC Generator

  • Usage of Faraday’s electromagnetic induction naturally generates current that periodically alternates in direction.
    • Since each turn of coil experiences the same number of change in flux, the total induced EMF is equal to the magnitude of EMF in one turn of coil multiplied by the number of turns.

  • The frequency of AC depends on rotation speed of coil or external magnetic field
  • Since there is no need to prevent current reversal, a slip-ring commutator is used with no physical splits within the device.
    • Slip-ring commutators are in contact with the brush and external AC supply throughout the 360º rotation of the armature  

 

DC Generator

  • DC generators produce unidirectional EMF through the use of split-ring commutators. Similar to ones present in DC motors, split ring commutators ‘reverse’ the current direction every 180º. Consequently, the net currently direction is not changed.

  • The direction of current in one side of the coil is reversed but the net direction of the current around the coil (either clockwise or anti-clockwise) is kept constant.
  • This effect is not possible with a slip-ring commutator as current flow is not interfered and thus kept continuous.

 

AC vs DC EMF Production

 

  • Emf versus time graphs entails positive and negative values which indicate direction of emf or induced current.
    • DC generators produce unidirectional current so their emf are all positive
    • AC generators produce bidirectional current. The direction alternates twice every revolution. Emf graph for AC generators is similar to a sine graph.

 

EMF vs Rotational Speed

 

Increased rotation of armature has two effects:

  • Increases frequency of rotation. The period of one revolution becomes shorter
  • Amplitude of emf graphs becomes greater. Increased rotation also increases the magnitude of induced emf due to Faraday’s Law. In other words, faster the change in flux, greater the induced emf. 

 

AC vs DC Generators

 

AC Generator

DC Generator

Similarity

·       Both devices use Faraday’s law of electromagnetic induction. As a result, the rate of electrical energy production depends on rate of change in magnetic flux experienced by the coil i.e. rotational speed of coil/armature

·       Fundamental components are the same e.g. external magnetic field, coil and external mechanical energy

Difference

·       Bidirectional

·       Uses slip-ring commutator

·       Can be used for transformers

 

·       Unidirectional

·       Uses split-ring commutator

·       Brushes are more important as they reverse current direction periodically (however, this means DC generators require more frequent maintenance

Applications

·       Common in household appliances and devices e.g. vacuum cleaner, toaster, transformers for laptops

·       Common for larger devices that require more efficient energy supply

·       Charging batteries

·       Electroplating and refining of metals e.g. coating coins with copper

 

 

Practice Question 1

Explain the function of generators by including important components and fundamental physics principles.  (3 marks) 

 

Practice Question 2

Generators, when not connected to an electric device, are easy to turn. However, when they are connected, the armature becomes quite difficult to turn. Explain why this is the case. (2 marks) 

 

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