Download Motors, Generators , and Transformers

11.2-11.3 Electric Power Distribution,
Generators and Motors
New ideas for today
•Magnetic induction
•Lenz’s law
•Transformers and power transmission
•Motors and Generators
Why such high voltage?
Transformers!
Observations about Power
Distribution
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Household power is AC (alternating current)
Power comes in voltages like 120V & 240V
Power is transmitted at “high voltage”
Power transformers are everywhere
Power Consumption in wires
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Reminder:
power consumption = current × voltage drop
voltage = resistance × current
power consumption = resistance × current2
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So what?
Wires waste power as heat
 Doubling current quadruples wasted power
 Better not transmit high current!
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AC
DC
vs.
Edison
Tesla
Westinghouse
AC = alternating current
Current switches direction 60 times per
second (in N. America)
DC= “direct current”
AC
Power Transmission
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Power delivered to a city is:
power delivered = current × voltage drop
Power wasted in transmission wires is:
power wasted = resistance × current2
For efficient power transmission:
 Use low-resistance wires
(thick, short copper)
 Use low current and high voltage drop
Can accomplish this with AC (alternating current) power
transmission.
160-800 kV
7000 V
120 / 240 V
neutral
ground
hot
Voltage Hierarchy
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High voltage is dangerous
High current is wasteful
Use the following scheme:
low voltage circuits in neighborhoods (120/240 V)
 medium voltage circuits in cities (7000 V)
 high voltage circuits across the countryside
(155,000-765,000 V)
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Use transformers to change voltage
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Electromagnetic Induction
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Changing magnetic field  electric field
Electric field in conductor  current
Current  magnetic field
Induced magnetic field opposes the original
magnetic field change (Lenz’s law)
inductive
charging
B
Lenz’s Law
That “emf” can be useful…
Transformer
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Alternating current in one circuit induces an alternating
current in a second circuit
Transfers power between the two circuits
Doesn’t transfer charge between the two circuits
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Current and Voltage
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Power arriving in the primary circuit must
equal power leaving the secondary circuit
Power = current × voltage
A transformer can change the voltage and
current while keeping the power unchanged!
Secondary voltage = Primary voltage
Secondary turns
Primary turns
Step Down Transformer
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Fewer turns in secondary circuit so charge is
pushed a shorter distance
Smaller voltage rise
A larger current at low voltage flows in the
secondary circuit
Step Up Transformer
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More turns in secondary circuit so charge is pushed a
longer distance
Larger voltage rise
A smaller current at high voltage flows in
the secondary circuit
Transformers are often
filled with nasty stuff
Clicker question
You decide to use a transformer to increase the
voltage from a battery, and hook it up in the circuit
shown below. When you close the switch,
1.5
the voltage across the lightbulb is:
(A) bigger than 1.5 V (B) smaller than 1.5 V (C) zero
Electric Generators and Motors
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A generator provides electric power
A generator requires a mechanical power
A motor provides mechanical power
A motor requires electric power
Alternator
Click me
Electric Generator
Rotating magnet
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makes changing magnetic field
induces AC current in the loop
Converts
mechanical power
into
electrical power
Electric Motor
Input AC power
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AC current makes changing magnetic field
causes magnet to turn
Converts
electrical power into
mechanical power
A motor is a
generator run
backwards !