Download Generators and Alternating Current Maximum emf for a generator

Generators and Alternating
Current
• A generator converts mechanical energy into
electrical energy by induction.
• A generator produced a continuously
changing emf.
• The induced emf alternates in sinusoidal
manner between maxima.
• This variety of current is called alternating
current, or AC
Maximum emf for a generator
emf max = NABω
emfmax = Max. induced emf
N = # of loops
How quickly
loops are
spinning
A = cross-section area of loops
B = magnetic field strength
ω = angular speed of loops
Maximum emf for a generator
Sometimes the frequency of spin is given
instead of angular speed.
2π
f =
Angular distance
1
sec
→ω
Maximum emf for a generator
(equation 2)
emf max = NAB(2πf )
emfmax = Max. induced emf
N = # of loops
A = cross-section area of loops
B = magnetic field strength
Angular speed
f = frequency of spin
1
Mutual Inductance
• Mutual inductance is a measure of the
ability of one circuit carrying a changing
current to induce an emf in a nearby
circuit.
• The coil carrying the current initially is
called the primary coil.
• The coil in which the current is induced
is called the secondary coil.
Transformers
• Transformers are devices that change one
AC potential difference to a diferent AC
potential difference.
• Real transformers are not perfectly efficient.
– Efficiencies of real transformers typically range
from 90% to 99%.
• For this class, we will assume 100%
efficiency. (We like living in an ideal world) :)
Residential Power Poles usually have 3 levels of
wires.
High Voltage
120 –240 Volts
Telephone-Cable
TV
Transformer
Transformers
Transformers
Transformers can
change the
voltage, either up
or down.
primary volts
secondary volts
=
# primary coils # secondary coils
Volts x amps = Volts x amps
in
out
2
Transformer Equation
ΔV2 ΔV1
=
N2
N1
Where
ΔV = potential difference
N = # of turns in coil
Subscript 1 refers to primary coil
Subscript 2 refers to secondary coil
Example: A step up transformer is used on a 120 V
line to provide a potential difference of 2400 V. If the
primary has 75 turns, how many turns must the
secondary have?
ΔV2 ΔV1
=
N2
N1
2400 120
=
75
N2
V1 = 120 V
V2 = 2400 V
N1 = 75 turns
N2 = ?
N2 = 1500 turns
Power for a transformer
• The power into a transformer equal the
power out of a transformer
P1 = P2
• To determine the power, use the current:
P = IV
3