Alternating Current Generator

How it works The coil is rotated (eg by a turbine) in a magnetic field Therefore a current is induced in the coil. As one side of the coil rotates the current changes direction (as the movement through the field has changed from going up, to going down)

0 current Maximum current Maximum current (other directions) 0 Current

Increasing the size of the induced current How can the size of an induced current be increased? increase the speed at which the coil rotates increase the strength of the magnetic field increase the number of turns in the coil increase the total area of the coil. In a power station generator, an electromagnet is often used as this can provide a stronger magnetic field than is possible with a permanent magnet. Photo credit: David Parsons / National Renewable Energy Laboratory The generator of Niagara Mohawk’s Dunkirk steam station in New York. This coal-fired power plant produces 600,000 kilowatts of 60 cycle power.

What is a Generator? A generator is a device that converts mechanical energy into electrical energy. It is the opposite of an electric motor. Power stations use generators to produce electricity on a large scale. Mechanical energy is provided by rotating turbines that can be powered by: high-pressure steam – in coal, oil, gas and nuclear power stations wind – in wind turbines falling water – in hydroelectric power stations Photo credit: Clipper Windpower / National Renewable Energy Laboratory

Generating or Motoring Electrical Current + Magnetic Field = Motion (motor) Motion + Magnetic Field = Electrical Current (generator)

How does a Generator Work? Teacher notes This activity extends the idea of simple electromagnetic induction from slides 16 and 19, and explains how slip rings enable a generator to produce alternating current.

How does a Generator Work? Teacher notes This simulation takes the concepts from the previous slide to create an animated generator simulation. The current, forces and magnetic field can be toggled on and off to aid visualization and understanding.

What is alternating current (a.c.)? Alternating current (a.c.) is an electric current that is constantly changing direction. Alternating current is produced by most generators and is used in mains electricity. Motors often work using alternating current. The voltage of alternating current is easily changed with a transformer. Alternating current can be transferred efficiently over large distances.

A.C or D.C. The difference between alternating current (a.c.) and direct current (d.c.) can be seen using an oscilloscope. For each current, the oscilloscope trace is a graph showing how the voltage of an electricity supply varies with time. d.c. time voltage a.c. time voltage peak forward peak reverse

A.C or D.C. d.c. time voltage The voltage of a d.c. supply is steady and always in the same direction. The voltage of an a.c. supply follows a repeated pattern: it rises to a peak, returns to zero changes direction and so on. a.c. time voltage peak forward peak reverse

Frequency of A.C. The frequency of a.c. electricity is the number of complete cycles per second, which is measured in hertz (Hz). 1 complete cycle The a.c. frequency can be determined from an oscilloscope by counting the number of complete waves per unit time. a.c. time voltage peak forward peak reverse If the frequency is increased, the number of complete waves shown on the screen increases. For example, if the frequency is doubled, the number of waves doubles.

Inducing Currents in a Coil Teacher notes This virtual experiment can be used as an alternative introduction to electromagnetic induction or as a revision exercise.

Lenz’s Law If a magnet is moved in to or out of a coil, the direction of the current induced can be predicted using Lenz’s Law: “An induced current always flows in a direction such that it opposes the change which produced it.” http://phet.colorado.edu/en/simulation/faraday

So when moving a magnet away from a coil the current in the coil will create a magnetic field that tries to attract the magnet towards it. When moving a magnet towards a coil, the current will create a magnetic field that repels the magnet.

Generators Stick in the diagram of the generator & complete the following… Write down the main difference between a motor and an a.c. generator. If you wanted to make a d.c. generator what would you have to change? List 4 ways that you can increase the current induced. Copy the diagram of the current induced and show the position of the coil in the magnetic field at key points on the graph. (on whiteboard) In which position is the coil when the maximum current is induced? Photo credit: David Parsons / National Renewable Energy Laboratory The generator of Niagara Mohawk’s Dunkirk steam station in New York. This coal-fired power plant produces 600,000 kilowatts of 60 cycle power.

Factors affecting induced current Teacher notes Appropriately coloured voting cards could be used with this classification activity to increase class participation.