Appliance Design - April 2008 - (Page 40)

MOTORS Decisions Drive Fig. 1. ACIM torque-versus-speed curve. Fig. 2. Volts/Hz induction motor control. Appliance designers must choose among several motor drive options. by eric persson Eric Persson is executive director, Global Field Applications Engineering, International Rectifier, El Segundo, Calif. 40 applianceDESIGN April 2008 he AC induction motor has dominated appliance applications as the prime-mover for many decades. Most of these applications used relays for on-off control of split-capacitor AC induction motors. Mechanical transmissions provided changes in speed and direction. However, many modern appliances, including clothes washers, HVAC, refrigeration appliances, and others now take advantage of variable-speed motor drives to improve their functionality and energy efficiency – all while improving performance, reliability, and cost. An additional benefit of electronically controlled, variable-speed motion control is that it generally enables improved energy efficiency compared to previous mechanical methods. This combination of better control and improved energy efficiency is driving a rapidly increasing adoption rate of variablespeed motor drives in appliance applications. Appliance designers have a number of commonly used motors, drives, and control systems from which to choose, so it is essen- T tial to understand their differences before specifying a system for a particular appliance application. AC induction motor The AC induction motor (ACIM) has been used for more than a century, and continues to survive today because of its simple, rugged construction and low-cost. An outer cylinder of steel laminations wound with copper wire comprises the stator of the ACIM. The rotor is also a cylindrical steel lamination stack, typically with cast aluminum rotor bars and end caps that form the one-turn windings of the rotor. Transformer action between the stator and rotor induces current in the rotor that creates its magnetic poles. The interaction between the rotating stator field and the rotor field result in the motor’s torque-versus-speed characteristic shown in Fig 1. In order to provide the effect of a rotating magnetic field, the winding is arranged into two- or three-phased windings in the stator. For traditional fixed-frequency line-powered ACIM, one winding is connected through a www.applianceDESIGN.com http://www.appliancedesign.com

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Appliance Design - April 2008 Contents Editorial Shipments/Forecasts News Watch A Variant of Thermoacoustic Refrigeration Known as Acoustic-Stirling is Expanding Its Range Outside Cryocooling Into More Conventional Applications Such As Refrigeration and Air Conditioning Free-Piston, Stirling Coolers Are Now Showing Up in Mainstream Applications As Costs Come Down and Heat Transport Solutions Are Devised Heat Exchanger Innovations Are Helping To Improve the Overall Efficiency of Vapor Compression Systems Environmental Concerns Have Once Again Ignited a Debate Over Refrigerants A New Glass Ceramic Material for Induction Cooktops Can Be Decorated In a Variety of Glimmering Metallic, 3-D Looks That Can Employ a Number of Shades, Patterns, Styles, Colors, Intensities, and Textures Colorful, Reflective Coatings for Glass Create a Variety of Design Options To Help Designers Establish a Unique Look for Their Products Appliance Designers Faced With Different Motor and Drive Options Need To Understand the Differences and Tradeoffs Before Choosing A New MEMS Valve Offers a Number of Advantages Over Solenoids In Certain Applications, Including the Opportunity To Cut Energy Use The AHR Expo Set Attendance Records and Showcased Thousands of Innovative New Products Classifieds Design Marts Advertiser’s Index Association Report: CEA

Appliance Design - April 2008

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