Modern Mechanix 2, 1938
Now let’s try in one hour…
Voltage
V=Voltage (Volts)
I=Current (Amps)
R=Resistance (Ohms)
V=IR
(Ohm’s Law)
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Voltage
The amount of water depends
on how much pressure is being
applied–– how hard the water is
being pushed. It also depends
on the diameter of the hose.
The harder the pressure and
the larger the diameter of the
hose, the more water passes
each second. The flow of
electrons through a wire
depends on the electrical
pressure pushing the electrons
and on the cross-sectional area
of the wire.
Current
With water, as the diameter of the pipe increases, so does the amount of
water that can flow through it. With electricity, conducting wires take the
place of the pipe. As the cross-sectional area of the wire increases, so
does the amount of electric current (number of electrons) that can flow
through it.
I=V/R
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Current
A nine volt battery supplies
power to a light bulb with a
resistance of 18 ohms. How
much current is flowing through
the light bulb?
I=V/R
Current
I=V/R
I=9/18
I=0.5 Amps
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Current
A CD player with a resistance of
40 ohms has a current of 0.1
amps flowing through it. How
many volts supply the CD
player?
V=IR
Current
V=IR
V=0.1 x 40
V=4 Volts
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Resistance
Resistance is anything that slows
water flow, a smaller pipe or fins on
the inside of a pipe. In electrical
terms, the resistance of a
conducting wire depends on the
metal the wire is made of and its
diameter. Copper, aluminum, and
silver––metals used in conducting
wires––have different resistance.
R=V/I
Resistance
Possible sources of resistance:
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Power
Power is a measure of the
rate of doing work or the rate
at which energy is converted.
Electrical power is the rate at
which electricity is produced
or consumed. Using the
water analogy, electric power
is the combination of the
water pressure (voltage) and
the rate of flow (current) that
results in the ability to do
work.
Power
P=Watts
V=Volts
I=Current
P=VI
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Energy
Electrical energy introduces the concept of time to
electrical power. In the water analogy, it would be the
amount of water falling through the pipe over a period
of time, such as an hour. When we talk about using
power over time, we are talking about using energy.
E=Energy
P=Watts
t=Time
E=Pt
WHhs, kWhs, etc…
Energy
E=Pt
E=100W x8hr x 5 days
E=4000Wh, or 4kWh
At $0.04/kWh,
To run a typical incandescent lamp, it would cost approximately $0.16
for 40 hours
To run a typical fluorescent lamp, it would cost approximately $0.04
for 40 hours
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Alternating Current – AC vs DC
-What is AC current?
-Current switches from positive to negative in cycles.
-60 times per second in North America
-50 times per second in Europe
-This “switching” is produced by generators as the dynamos swing
through magnetic fields, alternating positive and negative charges.
-120V is essentially an average voltage over time, with a more or
less constant flow of current.
Alternating Current – AC vs DC
-Single Phase vs. Three Phase
-Single Phase is one live wire and one neutral. Usually used for
residential
-Three Phase has three live wires and one neutral. Each live wire
carries current that is “120 degrees out of phase”. See diagram.
-This current type is largely used in manufacturing and in
industrial uses due to its more continuous current.
Single Phase
Three Phase
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Alternating Current – AC vs DC
-Edison championed DC current, but
Westinghouse’s AC became the standard.
-Why AC?
-It is easier to switch to DC from AC
than the other way around.
-AC is easy to step up and down,
allowing transport of power
-“Smoother” power for electric motors.
-Problems:
-Phases can get out of synch,
contributing to variations of voltage –
see harmonics and power factor
Alternating Current – Phase Shifts
-The sine waves of an alternating current occasionally get “out of synch”
due to inductance, or noise on the circuit
-“In an alternating current system the voltage and current do not
always reverse at the same instant in time. That is, they are not
always "in phase. “
The current can be considered as being divided into two
components: one in phase with the voltage and one out of phase
with the voltage.
The power factor of a circuit is the ratio of the in phase current to the
total current. Usually expressed as a percentage. Power is the
product of volts, ampere, and power factor.”
Power Factor is the ratio of watts (W) to voltamperes (VA), or real
power to apparent power.
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Alternating Current – Power Factor
-What to remember:
-Electrical equipment such as ballasts or motors on a circuit may
cause inductance.
- This causes a power loss in the circuit, termed “reactive” or
“negative” power
-This can be remedied by:
-A using equipment with a high power factor
-B “cleaning” the circuit with a capacitor
Series Circuits
V=IR
where
R = R1 + R2 + R3 +
-Components in series have the same
current flowing through them.
-If one component fails, the circuit will
not close. All other components will not
work.
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Series Circuits
Given Es = 12 volt, I3 = 2 amp, and that R3=R4, Find the value of each
of R3 and R4.
Given that R3 = 10 ohm, R4 = 20 ohm, and I3 = 2 amp, find Es
Parallel Circuits
V=IR
where
1 / R = 1 / R1 + 1 /
R2 + 1 / R3 +...
-Components in parallel have the
same voltage drop across them.
-If one component fails, the circuit will
continue to function.
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Parallel Circuits
Given I1= 2 amp, and I2 = 4 amp, find I3 in terms of Es .
Given Es = 12 volt, I3 = 2 amp, and that R1=R2, Find the value
of each of R1 and R2.
Given that R1 = 10 ohm, R2 = 10 ohm, and I3 = 2 amp, find Es
Symbols
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Wiring
-Usually made of ductile (flexible and easily drawn) and
conductive metal:
-Silver = best, but too expensive
-Copper = good
-Aluminum = major problems with overheating and
arcing. Used in housing 1960-1975.
Wiring
-Most wire used for lighting is
insulated to protect from short
circuiting or arcing. Depending
upon the current carried,
distance traveled, and load,
different sheathing will be used:
-Romex: flexible, nonmetallic sheathed cable
Romex
BX
Greenfield
-BX: flexible, metallic
sheathed (armored) cable
-Greenfield: flexible metal
conduit
EMT
-EMT: rigid metal conduit
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Switches and Dimmers
Switches and Dimmers
Lamp Life:
Dimming an incandescent lamp will increase the lamp life up to 20 times
depending upon wattage and dimming levels.
However, the color temperature of the lamp will change…
Dimming a fluorescent or HID source will NOT increase the lamp life. A
special ballast is required for both lamp types.
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Switches and Dimmers
Toggle Switch
-single pole on/off
-Three and four way switches
Dimmer Switches
Switches and Dimmers
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Switches and Dimmers
Switches and Dimmers
Wall box dimmer
Dimmer rack
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Switches and Dimmers
Perceived Brightness: As lamps are dimmed,
light level decreases but the human eye may
perceive a higher light level than is actually
recorded by a light meter. This yields the “square
law” curve, the theoretical relationship between
measured light level and perceived brightness:
Perceived Light (%) = 100 x square root
(Measured Light (%)/100)
Consider this example (courtesy Lutron):
At full brightness, the measured light level is 60fc.
At the lowest dimmed level, 10% perceived light is
desired:
•1% measured light (0.6fcd) is perceived as 10%
(desired result)
•5% measured light (3fcd) is perceived as 22%
(2x brighter than desired)
•10% measured light (6fcd) is perceived as 32%
(3x brighter than desired)
Transformers and Low Voltage Sources
-12V and 24V are most common
-Tighter, more precise beam
-Less “noise” and smaller filament
-Smaller lamps = smaller fixtures
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Transformers
Toroidal Low Voltage
Transformer
Electronic Low Voltage
Transformer
-Size
-Noise - buzzing
-Communication
Transformers – Voltage Drop
Voltage drop will dim the light source.
Voltage drop occurs when transformer is too far from light source and is a
result of power loss due to heat over the wire.
It can be fixed/avoided by
A Using larger (smaller gauge) wire
B Using a higher input voltage
C Limiting distances
http://www.electrician.com/vd_calculator.html
*This is a similar problem to Hydro Quebec’s power transport and explains
their high voltage approach.
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Ballasts
-Regulate current for these light sources:
-Fluorescent
-HID
-Induction
Ballasts
-Things to consider:
-Size of ballast
-Operating temperature
-Power Factor
-Magnetic vs. Electronic
-Control/Dimming
-Voltage of operation
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