TAILIEUCHUNG - HVAC Systems Design Handbook part 12

While most HVAC designers will have the support of a competent electrical design staff, it is important to understand certain fundamentals of electricity, power distribution, and utilization, because so many HVAC system devices are mechanically driven and controlled. This book cannot present electrical topics in great detail, but it can address several common topics and refer to more definitive works. | Source HVAC Systems Design Handbook Chapter 12 Electrical Features of HVAC Systems Introduction While most HVAC designers will have the support of a competent electrical design staff it is important to understand certain fundamentals of electricity power distribution and utilization because so many HVAC system devices are mechanically driven and controlled. This book cannot present electrical topics in great detail but it can address several common topics and refer to more definitive works. Fundamentals of Electric Power Electricity is basically electrons in motion. Electromotive forces cause free or loosely bound electrons to move along or through a medium. Materials such as aluminum copper silver and gold allow electrons to move freely and are called conductors. Materials such as porcelain glass rubber plastics and oils resist electron movement and are called insulators. Forces that move electrons are magnetic. Moving a conductive wire in a way that cuts across a magnetic field induces a force or voltage in the wire. If there is a path for the electrons to follow a flow will be established. The strength of the motive force is defined in volts and the magnitude of the current is measured in amperes. The resistance to current flow is analogous to the friction loss of water flowing through a pipe. Voltage current and resistance are related to each other in the Ohm s law equation E IR 397 Downloaded from Digital Engineering Library @ McGraw-Hill Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Electrical Features of HVAC Systems 398 Chapter Twelve where E voltage V I current A R resistance O Power defined as a force moving through a distance per unit time is defined electrically by the equation P EI where P power in watts. In direct-current dc systems the voltage is applied in one direction only. In alternating-current ac systems the voltage

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