Transcript AC DC

AC DC
Not the awesome, long-may-theyreign, Australian kings of the power
chord ……..
Technology Wars
 Why is alternating current used to supply
electricity virtually everywhere in the
world??? And not direct current?
 Why iPod vs. Rio vs. Zune?
 Why PC vs. Mac?
Typical reasons why one ‘wins’ over
a competitor…
 Possess key ‘enabling’ technologies
 Better reliability, safety, ease of use, ability to
meets customers’ long term needs
 Public perception about reliability, ‘image’,
safety, ease of use
 More funding available or stronger backing
from scientific community
The War of the Currents
 Thomas Edison (proponent of direct
current) and Nicolai Tesla funded by
Westinghouse (of alternating current)
 Addressed these issues through
extensive research, public
demonstrations and lectures, ‘news’
reports via newspapers
Direct current
 Thought of as supplied by batteries
 Can also have DC powerplants
 Always flows in one direction
Alternating current
 Electrons move back and forth
 Net motion is zero
 BUT the transport of energy between the
electrons is the important thing
 Because it’s cyclic, there is another issue
 Can be transmitted at various
frequencies
 In US, 120V and 60Hz for homes
Your requirements, needs,
expectations for electricity?
 Does voltage, current matter to you?
 Does it matter where the powerplant is?
Residential and commercial
customer requirements
 electricity of various voltages
 Smooth delivery of power for running motors
 People would prefer not to live close to
powerplants, so electricity should be able to be
transmitted over relatively long distances
 Transmission should be efficient, not a lot of
power loss from powerplant to customer
 Might need both DC and AC
View the Tesla video
 http://www.youtube.com/watch?v=ViS4t8j
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Tesla’s contribution
 Didn’t invent DC nor AC BUT….
 Did design
 better AC generators
 An AC transformer (a key technology)
 An AC motor (also key technology)
 The whole AC system would not have
worked without his inventions
Transformer is just wire coiled
around metal
 Magnetic field is generated by
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current in primary coil
Iron core channels magnetic
field through secondary coil
Secondary Voltage is
V2 = (N2/N1) V1
Secondary Current is
I2 = (N1/N2) I1
But Power in = Power out
 negligible power
lost in transformer
 Works only for AC, not DC
AC’s not perfect but it ‘won’
because….
 AC can be transformed from higher to lower
voltage and vice versa
 Enabling technology: the transformer
 AC provides ‘smoother’ flow
 AC can be transmitted over long distances
efficiently
 Easier to convert from AC to DC when needed
than vice versa (stuff that runs on batteries can
be plugged in to an outlet)
Could you still use DC to
power your house?
Let’s look at how it’s done
today….
Electrical Power
Transmission
 When transmitting electric power over long
distances, it is most economical to use high
voltage and low current
 Minimizes I2R power losses
 In practice, voltage is stepped up to about
230 000 V at the generating station and
stepped down to 20 000 V at the distribution
station and finally to 120 V at the customer’s
utility pole
three-phase “live” wires
to house
500,000
230,000
long-distance
138,000 69,000 7–13,000
neighborhood
How does it get to my
house?
Right outside your house,
Voltage is transformed to
120V
120 volt AC outlet
 Lower (rounded) hole is earth
ground
 connected to pipes, usu.
 green wire
 Larger slot is “neutral”
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for current “return”
never far from ground
white wire
if wired correctly
 Smaller slot is “hot”
 swings to +170 and 170
 black wire
 dangerous one
Household Circuits
 The utility company
distributes electric
power to individual
houses with a pair of
wires
 Electrical devices in the
house are connected in
parallel with those wires
 The potential difference
between the wires is
about 120V
So, why the different types of plugs
on different appliances?
 Safety for appliances and wiring
 A way to match the appliance with its
required voltage and frequency
 No way you can mix up a square peg
(120 V plug) into a round hole (240 V)
How do you get 240v?
Breaker panel
 Your house?
Household Circuits,
cont.
 A meter and a circuit breaker are connected
in series with the wire entering the house
 Wires and circuit breakers are selected to
meet the demands of the circuit
 If the current exceeds the rating of the circuit
breaker, the breaker acts as a switch and
opens the circuit
 Household circuits actually use alternating
current and voltage
What’s with that third
prong? Ground Wire
 Electrical equipment
manufacturers use
electrical cords that
have a third wire,
called a case
ground
 Prevents shocks
Generators vs motors
 Generator converts (chemical or wind,
etc.) energy to mechanical energy and
then to electrical energy
 Motor converts electrical energy to
mechanical energy