BEX 100 – Basic Electricity

Alternating Current (AC)Unit 15

• To understand the differences between (DC) direct current & (AC) alternating current

• To be able to identify different types of AC current “wave forms”

• To understand the different “values” associated with AC current

• Understanding “skin effects” of AC current

Unit Objectives:

Introduction

• Most of the electrical power produced in the world is AC / Alternating Current

• AC current is “bi-directional”

• AC current can be ”transformed” by means of :

A Step up transformer Step down transformer

INTRODUCTION

TRANSMISSION VOLTAGES RUN 69KV TO 345KV. WHY????

AC VOLTAGE CANNOT BE STORED: SO CAN’T BE USED IN AUTOS

1940 WORLD’S FAIR: EDISON LOW VOLTAGE DC: $1 PER LAMP

WESTINGHOUSE: HIGHER VOLTAGE AC: $.25 PER LAMP: SET PRECEDENT

AC Wave Forms

AC current flow “reverses direction” at periodic intervals

AC Wave Forms

• “Triangle / Linear Waves”

• Voltage rises/falls at a constant (linear) rate per time cycle

AC Wave FormsThe “Sine Wave”• The most common• Produced by rotating

machines• Has a total of 360 degrees

in each “cycle”• “Frequency” – the number

of cycles per second• Frequency is measured in

“Hertz”

AC Wave FormsThe “Sine Wave”

Alternating Current

Sine Wave Values

Sine Wave Values

“Peak to Peak” value• Maximum positive to

negative voltage “Peak” value• Measured from zero to

either the maximum positive or negative voltage

Sine Wave Values

Sine Wave Values

• “Average” values• Actually a Direct Current (DC) value• Used when AC voltage is “rectified” to DC voltage• The amount of voltage indicated by a DC voltmeter

measured across the load

Sine Wave Values

Resistive Loads

Loads that contain “pure resistance” such as:

• A Heating element• Incandescent lights

Current “copies” voltageCurrent is in “phase” with

the voltage

REVIEW

1. WHAT DOES AC STAND FOR?

2. WHAT IS THE MOST COMMON AC WAVE FORM?

3. NAME A BIG ADVANTAGE OF AC CURRENT.

4. WHY RUN HIGH VOLTAGES?

5. WHY ISN’’T AC USED IN AUTOS?

6. WHAT TERM DESCRIBES STEPPING VOLTAGE UP OR DOWN?

7. DEFINE THE TERM HERTZ.

8. WHAT DOES PEAK TO PEAK STAND FOR ?

Elizabethtown Technical CollegeBasic Electricity BEXS100/101

Text: Delmar’s Standard Textbook of ElectricityUnit 15 – Alternating Currents