generators and transformers

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Transcript 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
• There are two types of generator or dynamo.
Both turn rotational energy into electrical energy.
1. One type involves rotating a coil inside a
2. The other type involves rotating a magnet
inside a coil
Both types produce ALTERNATING CURRENT.
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.
• 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
– 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.
• 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:
• 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 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
Step Up Transformer at Power Plant
• 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.