EGH166: Hands on Lab

Lab 6: AC Electricity

 

 

Introduction

Background

Virtually everything in our world uses electricity in some way, and so anything that an engineer may work on will in some way tie into electricity. Every day of your life you use electricity to power some useful device. Recall the last power outage you may have experienced, think about all of the items you wanted to use but could not, the microwave, a light, the television, almost everything in a home.

 

Electrical power allows us to convert from a standard power source to a wide variety of means for doing work. A blender performs mechanical work to move and mix dough. A light produces heat and radiant energy so we can see. A stereo system converts electricity into sound. A refrigerator uses electricity to control temperature. There are very few things in the world that do not use electricity to run. Even a car uses a battery to get it started.

Purpose

The purpose of this lab is to familiarize you with some fundamentals of electrical power, as well as a hands-on experience with building a model power system.

Basic Principles

In this lab write-up will cover the following basic principles:

 

1.      Fundamental Electric Units,

2.      The Stages of an Electrical System, and

3.      Electrical Wiring Standards.

Lab Experience

In the lab experience you will build a model power system.


Fundamental Electric Units

Introduction

In this unit we will describe the following electrical parameters:

 

1.      Voltage,

2.      Resistance, and

3.      Current.

Voltage

Voltage indicates the strength of the electricity, meaning how hard it is able to push. When a voltage is present electricity wants to flow (electrons want to move to balance out the voltage). A very high voltage can cause electrons to jump through the air, or through insulating materials; we see this as a spark. Lightning is a huge spark; when a very large voltage develops between the clouds and the ground an electrical balance is achieved by a lightning bolt.

Resistance

The electrical resistance of a material is a measure of how much the material fights the flow of electricity. Electrical insulators such as rubber and ceramics have an extremely high resistance: Electrons do not move easily through the material.

Current

A measure of how much electricity is flowing. It is derived from an exact count of electrons traveling past a point in a certain amount of time.

Physical Relationships

Resistance and Current are related to each other in a given electrical circuit. More voltage (more push) creates more current. Greater resistance decreases the current.

 

Example: Think of a garden hose. City water pressure is like voltage, current is the amount of water coming out of the hose and resistance is like the size of the opening at the end of the hose. A very wide opening on the hose (low resistance) would allow a great deal of flow (current).

Mathematical Relationships

The relationship of these units can be easily computed:

Voltage = Current x Resistance

V [Volts] = i [Amps] x R [Ohms] V = i R

When using electricity to drive other devices a unit more commonly used is power, which is measured in Watts.

Power = Current x Voltage

P [Watts] = i [Amps] x V [Volts] === P = i V (Poison Ivy)

Also, combining the formulas produces:

Power = Current2 x Resistance P = i2 R

 


 

The Stages of an Electrical System

Introduction

Power is transferred from a power plant to all of the homes, businesses and buildings throughout a town. The transmission of power from its generation point to the residential loads goes through the following stages:

 

1.      Generation,

2.      Substations,

3.      Transmission and Distribution Lines, and

4.      Residential Loads.

 

Generation

A few methods exist to produce electrical power. Some of the common power sources are nuclear, hydraulic, coal or wind. Each of these methods in some way causes a shaft to rotate which turns a generator.

 

A generator is simply a motor used in reverse mode. It is known that when electricity is applied properly to a motor, the motor spins. A generator produces electrical power when its shaft is spun. You may have seen a gas powered generator. This type of generator has a combustion engine (similar to a lawnmower), which spins a shaft of a generator and produces electrical current. It is often necessary to condition the electrical signal for use on standard AC devices because the combustion engine may not produce a reliable rotational speed.

 

The fundamental principal of a generator is that when a magnet moves past a conducting material (coil of wire), it pushes the electrons in the material and produces current. When a generator spins it is moving a strong magnetic field past a long coil of wire.

 

The electrical signal available at standard USA outlets is a 120 Volt 60 Hz AC signal. If a graph of voltage vs. time was displayed it would appear as shown.

 

Power plants typically produce a voltage of 2300 Volts, which must be transmitted at high voltage (i.e. 800 kv) and converted efficiently to 110, 220 or 440 volts for homes and industry. Efficiency is achieved by minimizing the loss of power between the power plant and the end use.

Substations

The transmission of power from the generation to the loads passes through several substations, which convert the voltage levels using transformers. Transformers are electrical machines that convert electrical energy of a certain voltage level on electrical energy of a different voltage level. A transformer uses a magnetic field as a middle stage of the conversion. The conversion factor is determined by the ratio of the number of turns on each coil.

 

 

N1 (primary) N2 (secondary)

 

A transformer takes the voltage level coming in, divides it by the number of turns (or coils) at the primary (input exchange rate) to find the magnetic field strength and then multiplies it by the number of turns at

the secondary coil to get the output voltage.

Turn ratio = Vin / Vout = N1/ N2 N = Number of turns of coils

Transmission and Distribution Lines

Very high voltage is used on power lines to travel long distances to the substations. A somewhat lower voltage is used to route the electricity around town (called distribution system). The lowest voltage of

110 volts is used to travel from the pole near the home into all of the sockets inside the house. Transmitting electrical power is more efficient at higher voltages. The reason 230,000 volts are not routed all the way up to the end use is for safety and cost reasons. Extreme voltages can easily produce deadly current levels, and considering the frequency that power lines are broken or damaged that would be a dangerous approach. Another problem is that wires that can insulate and carry such high voltage levels are very costly, so minimizing the length of expensive cable used is an important consideration.

Residential Loads

Electrical energy travels to homes and powers the devices found there. Wire up the necessary switches and electrical outlets to power up four light bulbs and a toaster. Install a circuit breaker to protect the system against short circuits.

 


 

Electrical Wiring Standards

Standards

There are standards used in household wiring to facilitate learning, understanding and maintenance. Consistency with the standards is very important if you need the wiring certified and if you want outsiders to easily understand what you did, and so others will know which wires are hot (dangerous) or safe. The following illustrates the standards for installing a basic household AC power outlet.

 

 

q          Black: Hot, 120V.

q          White: Neutral, common.

q          Green: Electrical ground.

The White and Green wires are connected to each other back at the main fuse box, however they should not be used interchangeably. The ground is meant to be a direct, fast path for the current to flow in case of short circuits. It is used for safety purposes.

 


Lab Experience

 

Teams and Leaders

The class will be divided into two groups and each group will be divided into five teams. A leader will be chosen for each team. Each team leader must explain how the project was completed to his/her group and must provide notes for everyone's notebooks.

 

q       Chief Engineer - Overall responsibility for installation, testing, and running of the electrical power system. Coordinates activities with the team leaders.

 

q       Safety Engineer - Overall responsibility for safety. Checks on ground connections, makes sure danger areas are clearly defined with yellow caution tape.

 

Team

Function

Measurement Team

Determines where measurements are to be made and arrange with the other teams to get the measurements when the system is on. Fill out the measurement table so that it can be used on the lab report.

Generation Team

Wires the generator to a step up transformer for a 480V power transmission line. Tests both, the generator and the transformer T1.

Transmission Team -

 

 

Takes the 480V power fed from generation, connects it to a long transmission line, and steps it down to 240V using a power transformer. Coordinates connection with the distribution team.

Distribution Team

Takes the 240V from the transmission line and then steps it down to 120V. Wires up the circuit breakers and coordinates connection with the room wiring teams.

Room Wiring Team

Takes the voltage from the circuit breaker and wires two receptacles, each controlled by a switch. Wires up light bulb sockets so they can be plugged in to the 120V receptacles.

Power System Description

The following figure shows a single line diagram of the scale model power system to be built in the lab.

 

 

Generator: It will be represented by an electrical outlet. A power cord will be used to connect the generator to T1.

T1: 120V/480V step-up transformer.

TL: 480V transmission line.

T2: 480V/240V step-down transformer.

DL: 240V distribution line.

T3: 240V/120V step-down transformer.

Load: Up to four 60 watt incandescent light bulbs and a 800 watt toaster

 

Build a Small Scale Electrical Power System

Connect all the given components and meters to build a small-scale power system as shown in the figure given in class. Make sure to connect ground throughout the system.

 

In Lab Measurements:

1.      Use an ohmmeter to measure the "Cold" resistance of a light bulb and that of a toaster (out of circuit). Record the power ratings on the light bulbs, and also on the toaster.

2.      Energize the circuit. Measure voltage and current levels at each stage of the power system for each of the four cases listed (Note that power losses are negligible since this is just a scale model). Tabulate results.

 

Table 1. Voltages and Currents Measured Across the Power System.

 

 

Case #

Load Description

Generation

Transmission

Distribution

Load

Volts

 Amps

Volts

Amps

Volts

Amps

Volts

Amps

1

1 light bulb

 

 

 

 

 

 

 

 

2

2 light bulbs

 

 

 

 

 

 

 

 

3

3 light bulbs

 

 

 

  

 

 

 

 

4

 1 Toaster

 

 

 

 

 

 

 

 

 

Post Lab Analysis

For each of the cases listed on the given table:

 

1.      Calculate the equivalent "Cold" resistance of the load and then determine the current levels for each stage of the power system using the measured load resistance, rated voltages, and turn ratios. Compare to the experimental results. Calculate the errors and tabulate results.

 

2.      Generate a table, which compares power at each stage to rating power of load at each load case.

 

3.      Back-calculate the equivalent "Hot" resistance of the total load using measured voltage and current values. Compare "Hot" vs. "Cold" resistances. Discuss the difference.

4.      Draw an electric circuit showing the voltage and current values for case #2 only. Neglect power losses on the transmission lines and transformers.

 

Questions:

 

a) Did the voltage and current levels match what you would expect?

 

b) What types of meters were used? What could happen if an ammeter is connected in parallel to the load to measure current? What if a voltmeter is connected in series to the load to measure voltage? Are the results accurate?

 

c) Is it possible to measure "Hot" resistance directly with an ohmmeter? Why or why not?

 

Using an AC Generator

Replace the power plant by a portable AC generator powered by a bicycle.

 

1.      Ride the bike to provide power to the circuit with only one light bulb connected. What do you observe?

 

2.      Add one more light bulb to the load and energize the circuit. Reflect on the difference between the electrical power generation capacity of a person, and that of the built power source.

 

Questions:

 

d) What problems did you encounter with the human powered system?

 

e) Explain why it became more difficult to turn the generator when an additional light bulb was added to the load?

 

LAB REPORT

Format

   Lab reports must be done TEAM

   Follow given lab report format.

   Maximum 4-5 pages (including figures and tables)

 

General Guidelines

   Cover page

   Description of Experimental Apparatus

   Introduction/Background

   Include a description of the system and the appropriate tables showing the experimental and analytical results.

   Provide solutions to questions using the same order presented in write-up.

   Analysis of results/Summary.