Transcript Generators and Alternating Current

Alternating Current, Generators, and
Motors
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Generator – a device that uses induction to
convert mechanical energy to electrical energy
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Commonly uses rotational energy by having steam
or running water turn a turbine
 Steam may be generated by a coal or natural gas fire or
from geothermal heat sources
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The rotation of the turbine causes a wire loop to
rotate in a magnetic field
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When a loop is parallel to a magnetic field, the charges
are perpendicular to the magnetic field
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When a loop is perpendicular to a magnetic field, the
charges are parallel to the magnetic field
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Current is maximized, induced emf is maximized
Current is zero, induced emf is zero
Induced emf versus time graphs as a sine curve
Maximum emf for a generator = number of loops *
cross sectional area of the loops * magnetic field
strength * angular frequency of rotation of loops
emfmax = NAB
Angular frequency = 2*pi*frequency
 = 2f
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Sample Problem: A generator consists of exactly eight turns
of wire, each with an area A = 0.095m2 and a total resistance
of 12. The loop rotates in a magnetic field of 0.55T at a
constant frequency of 60.0Hz. Find the maximum induced
emf and maximum current in the loop.
f = 60.0Hz A = 0.095m2 R = 12
B = 0.55T = 0.55V*s/m2 N = 8
emfmax = ?
Imax = ?
 = 2f
emfmax = NAB
I=emf / R
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Alternating current (ac) – an electric current
that changes direction at regular intervals
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Typically produced in generators
Is reflected in the sinusoidal nature of the graph
In the US, Canada, and Central America the
current reverses itself at a frequency of 60Hz or
60 reversals/second
In Europe and most of Asia and Africa, the
frequency is 50Hz
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Since alternating current is constantly reversing,
maximum current and emf values are not as useful
as they are in direct current
Of more importance are instantaneous and rootmean-square (rms) values
Rms current – the amount of direct current that
dissipates as much energy in a resistor as an
instantaneous alternating current does during a
complete cycle
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An equivalent value allowing for accurate comparisons
between alternating and direct current
Power can be calculated by using the appropriate
rms values in the equations given previously
Potential Difference
Current
Instantaneous values
v
i
Maximum values
Vmax
Imax
rms values
Vrms=Vmax/√2 =
0.707*Vmax
Irms = Imax/√2 =
0.707*Imax
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Power = rms current squared * resistance
Power = one-half * maximum current squared *
resistance
P = (Irms)2R = ½(Imax)2R
Ohm’s law still applies in ac circuits
Rms potential difference = rms current *
resistance
Vrms = Irms*R
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Sample problem: A generator with a maximum
output emf of 205V is connected to a 115
resistor. Calculate the rms potential difference.
Find the rms current through the resistor. Find
the maximum ac current in the circuit.
Vmax = 205V
R = 115
Vrms = ?
Irms = ?
Imax = ?
Vrms = .707*Vmax
Irms = Vrms / R
Irms = .707*Imax
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Alternating current can be converted in to direct current
The conducting loop in an ac generator must be free to rotate
while remaining part of the circuit at all times
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The ends of the conducting loop are connected to conducting rings called
slip rings that rotate with the loop
Connections to the external circuit are made by stationary graphite strips
called brushes that stay in contact with the slip rings
Both the loop current and the output current are continuously changing
direction
By replacing the two slip rings with a single split slip ring called a
commutator, the generator can produce direct current
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The brushes change halves of the commutator at the same instant the
current reverses so there is a double reversal which cancels out leaving the
current flowing in a single direction
By using multiple loops and commutators, the fluctuations from the
individual loops are canceled out resulting in an almost constant output
current
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Motors – convert electrical energy into mechanical energy
Reverse of a generator
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The coil of wire is mounted on a rotating shaft and is positioned between
the poles of a magnet.
Brushes make contact with a commutator, which alternates the current in
the coil.
The alternation of current causes the magnetic field produced by the
current to regularly reverse and thus always be repelled by the fixed
magnetic field.
The coil and shaft are therefore kept in continuous rotational motion
Back emf – the emf induced in a motor’s coil that tends to reduce the
current powering the motor
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Looks much like a dc generator
The induced emf
If this did not occur, Lenz’s law would be violated
The faster the coil rotates, the greater the back emf
The potential difference available to supply current to the motor equals
the difference between the applied potential difference and the back emf