Electrical
• What are those lines above our heads?
Power transmission
 From Ohm’s
 For the same power to be transmitted – a higher
voltage will require a lower current which in turn
will require smaller conductors or wires.
 However higher voltage is more expensive more
hazardous than low voltage.
From Mechanical & electrical Systems in Buildings 4th
edition
By Janis & Tao
 The key is smaller wire less weight on towers
lower cost for the infrastructure
High Voltage
Line
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Ground
Hot
Center Tap
Transformer
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Wires Overhead to the Use of
Electrical
• What’s going on?
– Principals of Electricity Chapter 25
– Electrical Systems and Materials Chapter 26
– Electrical Systems and Materials Chapter 27
– Electrical Wire Design Chapter 28
Use of Electrical
• What’s going on?
– Principals of Electricity Chapter 25
• Circuits
• AC vs DC power
Circuits
A Circuit
Parallel
Series
Use of Electrical
Circuit
Use of Electrical
Parallel
Use of Electrical
Series
AC vs. DC power
Electrical Power
• The electricity that reaches a house or
building from a power company is ….
AC
power
and not
DC
power
Goals
• Understand the difference between DC and
AC power
• Understand the different properties of AC
power
• Be able to calculate the quantities of these AC
properties
• Understand this is not a Heavy Metal rock
band.
Terms
•
•
•
•
•
Direct Current (DC)
Alternating Current (AC)
Power Generation
Ohm’s Law
Impedance
Direct Current (DC)
• An electric current that flows through a circuit
in only one direction, although the rate of
flow may vary
• Most common household items and
appliances operate on DC power
DC Power Generation
• 2 Methods of power
generation:
– Batteries that
convert chemical
energy into electrical
energy
– Generator converts
mechanical energy
into electrical energy
Ohm’s Law for DC
• Ohm’s Law for Direct Current:
–I=E/R
– Where:
• I = Current (Amperes)
• E = Voltage (Volts)
• R = Resistance (Ohms)
Alternating Current (AC)
• An Alternating Current (AC) system is an
electrical system in which voltage and current
are reversed periodically or cyclically in the
circuit
• Nearly all power proved by electrical
companies in the U.S. is through AC systems
Practice Problem
• What is the current flow in a 12-V DC circuit
containing a total resistance of 2 Ohms?
– E = 12, R = 2
–I=E/R
– I = 12 / 2 = 6 Amperes
AC Generation – Basic Concepts
Simple
Generators
(alternators)
Stationary Windings
(conductors)
Voltage generated by a
conductors rotating in a
magnetic flux field
Rotating
Coil
Sine Wave of
Voltage
Using multiple magnets
Alternating Current (AC)
Or looking at it from this view
Alternating Current
 AC generators (large electromagnets) can be built
with much larger and powerful voltage ratings
than DC
 Less expensive than DC to produce due to the
simplicity of the winding and brushes
 Voltages can be stepped down or up efficiently by
the use of simple transformers (induction
principle)
 DC voltages changes are obtained by the use of
resistors as a result have power losses
 Current loss is very low and it can travel long
distances economically
AC Properties
• Because current reverses its direction of flow
rapidly in an AC system, it has unique
properties
– Reactance
– Impedance
– Ohm’s Law
AC vs. DC
• There are two main advantages of AC over DC:
– Lower Generating Cost
– Easier Voltage Transformations
AC Verses DC
 In 1887 direct current (DC) was king.
 121 Edison power stations scattered across the United States delivering
DC electricity
 However DC had a great limitation
 power plants could only send DC electricity about a mile before the
electricity began to lose power.
 So when George Westinghouse introduced his system based on high-voltage
alternating current (AC), which could carry electricity hundreds of miles with
little loss of power, people naturally took notice. A "battle of the currents"
ensued. In the end, Westinghouse's AC prevailed.
 But this special feature isn't about the two electrical systems and how
they worked. Rather, it's a simple explanation that shows the difference
between AC and DC.

AC Verses DC
 George Westinghouse introduced his system
based on high-voltage alternating current
(AC)
 This system could carry electricity hundreds
of miles with little loss of power.
 A "battle of the currents" ensued.
AC Verses DC
 In the end, Westinghouse's AC prevailed.
AC vs. DC
• Review
– AC dominant power system for buildings
– DC needed for electronic equipment, TV’s
computers etc. and special building equipment
such as elevators and industrial equipment
• Conversion
– AC to DC and DC to AC
– AC converted to DC is by a rectifier
– DC converted to AC is by an inverter
Transformers
Kick it up or kick it down
Change it from AC to DC
Transformers
Now we’re talk’n!
Transformers
Real transformers
This is exciting!
But that would be just to boring.
This has to be the cutest transformer
ever!
What a
piece of
beauty
Transformers
Residential
Transformers
Residential
Transformers
• 3-Phase Power
– Primarily found in Commercial Buildings
• Delta
• Wye
Power Circuits
• Commercial buildings use 3-phase power
• 3 hot wires carry motor loads, and grounding
is provided by the conduit system
• Large motors are equipped with starters to
control starting current and protect from
overloads
Power Circuits
• Residential uses single phase power
Panels
• Commercial buildings use many panels to
subdivide current and protect individual
circuits
• Separate panels are used for:
– Lighting
– Power
– Emergency
– Miscellaneous
Panels
Lighting
• Commercial lighting circuits differ from their
residential counterparts as follows:
– Shared Neutral
– More Amperes
– Higher Voltage
Duplex outlets are NOT connected to lighting in
commercial
Transformers
Electrical Service
• Commercial buildings all use 3-phase power
• Voltages are usually 480/277
• Difference is the setup
– Delta
– Wye
3 phase power
3 phase power
• A single phase generator is an alternator with
a single set armature coil producing a single
voltage waveform.
• A three –phase alternator has three sets of
coils spaced at 120o apart and generates three
sets of voltage waveforms.
Delta
• Neutral conductor is
centered between twophase conductors
• High leg serves only
3-phase loads and
cannot be used with
the neutral
Wye
• Neutral conductor is
connected between all
3-phase conductors
• Allows each phase to
be used for single
phase loads
Transformers
• Delta is usually used in industrial
applications
• Wye is typical for office buildings and
shopping centers
Transformers
Transformers
Transformers
The opposition to alternating current due to
capacitance (capacitive reactance) or inductance
(inductive reactance).
Reactance
What does that mean?
Reactance
• Reactance is of two types: Inductive and
Capacitive.
• Inductive reactance is associated with the
magnetic field that surrounds a wire or a coil
carrying a current.
Reactance
• An alternating current in such a conductor, or
inductor, sets up an alternating magnetic field
that in turn affects the current in, and the
voltage (potential difference) across, that part
of the circuit
Inductive Reactance
 Alternating current induces an alternating
magnetic field in the coil that increases
the opposition to the flow of current.
Inductive Reactance
• This magnetic field, and the inductive
reactance that arises from it, is amplified by
the presence of the iron core in the coil, so
that the electrical reactance (combined with
the resistance) becomes so great that an
inadequate current supply reaches the lamps.
Inductive Reactance
 Can you think of an example of Inductive
Reactance in your apartment or home?
 A dimmer switch
Impedance
Loudspeakers have impedances of 8 ohms, 6
ohms or 4 ohms (those are "nominal" or
approximate values, because the impedance of
a speaker changes all the time with the different
frequencies of music)*
Ok so what …this still does not explain
impedance
Impedance
• First impedance has nothing to do with sound
quality*
• In a loudspeaker, current does all the work;
voltage is the "push" behind the current, kind
of similar to the way water pressure (voltage)
forces the water (current) through a hose. *
Impedance
 If you have a narrow hose (a high
impedance), not as much water (current)
flows. Use a larger diameter hose (lower
resistance) and more water (current) flows.*
Impedance
• If speaker impedance is too low, too much
current will run through the AV receiver's
output transistors, causing the receiver to
overheat and shut down*.
* Speaker Impedance and Ohms Explained by Alan Lofft
Impedance
• you do not want your speaker cables to raise
impedance or resistance and waste your AV
receiver's power on its way to your speakers.*
• Use 12-gauge speaker cables between the
receiver and speakers to eliminate problems
of increased resistance.
Impedance
 Ohm's Law states: In an electrical circuit,
current flow is directly proportional to
voltage and inversely proportional to
impedance
So now you know