
What are those lines above our heads?

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
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Ground
Hot
Center Tap
Transformer
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
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


What’s going on?

Principals of Electricity Chapter 25
 Circuits
 AC vs DC power
A Circuit
Parallel
Series
Circuit
Parallel
Series

The electricity that reaches a house or
building from a power company is ….
AC
power
and not
DC
power




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.





Direct Current (DC)
Alternating Current
(AC)
Power Generation
Ohm’s Law
Impedance


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

2 Methods of
power generation:


Batteries that
convert chemical
energy into
electrical energy
Generator converts
mechanical energy
into electrical
energy

Ohm’s Law for Direct Current:


I=E/R
Where:
 I = Current (Amperes)
 E = Voltage (Volts)
 R = Resistance (Ohms)


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

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

Simple
Generators
(alternators)
Stationary Windings
(conductors)
Voltage generated by a
conductors rotating in a
magnetic flux field
Rotating
Coil
Sine Wave of
Voltage
Or looking at it from this view



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

Because current reverses its direction of flow
rapidly in an AC system, it has unique
properties
Reactance
 Impedance
 Ohm’s Law


There are two main advantages of AC over DC:


Lower Generating Cost
Easier Voltage Transformations



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.



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.

In the end, Westinghouse's AC prevailed.
You earthlings
…….
Westinghouse huh!

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
Kick it up or kick it down
Change it from AC to DC
No, not this
type of
transformer
Now we’re talk’n!
Real transformers
This is exciting!
We could talk about actors
that work with
transformers

But that would be just to boring.
This has to be the cutest transformer
ever!
What a
piece of
beauty
Transformers
Residential
Residential
Mikaela ..is that a Delta transformer
-or-
a Wye transformer?

3-Phase Power

Primarily found in Commercial Buildings
 Delta
 Wye



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

Residential uses single phase power


Commercial buildings use many panels to
subdivide current and protect individual
circuits
Separate panels are used for:




Lighting
Power
Emergency
Miscellaneous

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



Commercial buildings all use 3-phase power
Voltages are usually 480/277
Difference is the setup


Delta
Wye


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.

Neutral conductor is
centered between
two-phase conductors

High leg serves only
3-phase loads and
cannot be used with
the neutral

Neutral conductor is
connected between all
3-phase conductors

Allows each phase to
be used for single
phase loads


Delta is usually used in industrial
applications
Wye is typical for office buildings
and shopping centers
Now we need to discuss
reactance, inductive
reactance, and
impedance
The opposition to alternating current due to
capacitance (capacitive reactance) or inductance
(inductive reactance).
What does that mean?


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.

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

Alternating current induces an alternating
magnetic field in the coil that increases the
opposition to the flow of current.

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.


Can you think of an example of Inductive
Reactance in your apartment or home?
A dimmer switch
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


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. *

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.*

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


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.

Ohm's Law states: In an electrical circuit,
current flow is directly proportional to voltage
and inversely proportional to impedance
So now you know
Basic Electricity ?
Who Cares