GENERATORS AND TRANSFORMERS

Induced Current in a Generator

• The effect of inducing a current in a coil by moving a magnet inside it is used for the generation of electricity in power plants • There are two types of generator or dynamo.

Both turn rotational energy into electrical energy.

1.

2.

One type involves rotating a coil inside a magnet.

The other type involves rotating a magnet inside a coil Both types produce ALTERNATING CURRENT.

• gcsescience.com

An electric generator consists of a magnet, which creates a magnetic field, and a loop of wire which rotates in the magnetic field. As the wire rotates in the magnetic field, the changing strength of the magnetic field through the wire produces a force which drives the electric charges around the wire. AS the loop spins, the direction of the force changes, so too then does the direction of the current The changing direction of the force after every 180 degrees of rotation gives the alternating current. http://www.sunblock99.org.uk/sb99/people/DMackay/ac.html

ALTERNATING CURRENT

• If you use an oscilliscope and look at the power found at a normal outlet in your house, you will find is that the power looks like a

sine wave

, and that wave oscillates between -170 volts and 170 volts (the peaks are indeed at 170 volts; it is the effective (rms) voltage that is 120 volts). • The rate of oscillation for the sine wave is 60 cycles per second. Oscillating power like this is generally referred to as AC, or

alternating current

. • AC has at least three advantages over DC in a power distribution grid: – Large electrical generators happen to generate AC , so conversion to DC would involve an extra step. – Transformers must have alternating current to operate, and the power distribution grid depends on transformers. – It is easy to convert AC to DC but expensive to convert DC to AC, so if you were going to pick one or the other AC would be the better choice. • Howstuffworks.com

THREE PHASE POWER

• The power plant produces three different phases of AC power simultaneously, and the three phases are

offset 120 degrees

from each other. If you were to look at the three phases on a graph, they would look like this relative to ground: http://science.howstuffworks.com/power3.htm

TRANSFORMER

• A transformer can change electrical energy of a given voltage into electrical energy at a different voltage level. It consists of two coils arranged in such a way that the magnetic field surrounding one coil cuts through the other coil. When an

alternating voltage

is applied to (across) one coil, the induced

varying magnetic field

set up around that coil induces an alternating voltage in the other coil. Transformers will not work with direct current, since no changing magnetic field is produced, and therefore no current can be

This transformer's job is to reduce the 4160 volts down to the

240 volts

that makes up normal household electrical service. It is a step down transformer http://science.howstuffworks.com/power3.htm

Step Up Transformer at Power Plant http://bigvalleynews.net/BigValleyPower/TourOfBigValleyPowerLLC.html

TRANSFORMER

• The factor which determines whether a transformer is a step up( increasing the voltage) or step down (decreasing the voltage) type is the "turns" ratio. The turns ratio is the ratio of the number of turns in the primary winding to the number of turns in the secondary winding. • http://avstop.com/AC/apgeneral/TRANSF ORMERS.html

http://www.ibiblio.org/kuphaldt/electricCircuits/AC/AC_9.html