 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 4 th edition By Janis & Tao  The key is smaller wire less weight on towers lower cost for the infrastructure

High Voltage Line Ground Hot Center Tap Transformer INTRODUCTIONINTRODUCTION

INTRODUCTIONINTRODUCTION

INTRODUCTIONINTRODUCTION

INTRODUCTIONINTRODUCTION

 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) Sine Wave of Voltage Stationary Windings (conductors) Rotating Coil Voltage generated by a conductors rotating in a magnetic flux field

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

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 120 o 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: I nductive 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