Appliance Design - April 2008 - (Page 42)

MOTORS capacitor to shift the phase-angle of its current relative to the other winding. This fixedspeed arrangement is simple and involves no active electronics, but cannot provide any effective speed or torque control. The motor is run at full magnetization current at all times. As a result, its energy efficiency at lower torque levels is compromised, because the AD01074Rosen:_ 12/12/06 motor flux-level is always set at the12:15 PM highest level necessary for full-power operation. So the overall reason that ACIM motor efficiency is not high is that the stator must carry the burden of higher circulating Volt-Amperes to induce rotor magnetization. to induce the rotor current. For applications that do not require precise velocity control, a simple open-loop variable-frequency inverter can power the ACIM with variable speed. Because of the transformer action within the ACIM, the open-loop frequency control method must keep the ratio of frequency and voltage constant to avoid magnetic saturation and the resultant high currents. This control method is often called “volts per Hertz” control because of the fixed ratio. An example of such a system is shown in Fig. 2. These variable-speed methods apply to 3-phase ACIM, not to the fixed-speed split capacitor types mentioned previously. Better control of ACIM speed and torque is possible by applying vector control methods, but these do not provide good lowspeed performance unless an expensive shaft encoder is used for rotor position feedback. Moreover, the overall efficiency of the ACIM does not measure-up to what a permanent magnet machine can achieve. ACIM drive and control The ACIM is an asynchronous machine, meaning that the mechanical rotation is not Page 1 synchronous with the electrical rotation: there is some slip (a few percent) necessary EC does it Permanent magnet motors In contrast to the ACIM, permanent magnet machines do not rely on induced AC currents to provide rotor magnetization. Fixed permanent magnets on the rotor take care of that task. As a result, permanent magnet motors use less overall current for a given torque, and provide superior performance down to zero speed, unlike the ACIM. This also means that permanent magnet motors have higher electrical efficiency than induction motors. Permanent magnet motors are synchronous, so there is no slip between the electrical and mechanical rotation speed. One of the most commonly used permanent magnet motors is the brushless DC (BLDC) motor. Its name is a bit of a misnomer, as it is not truly a DC machine. The name was applied more than 30 years ago when it was compared to the brush-type DC motor for its similar speed-torque characteristic. At issue was mechanical commutation (brushes and commutator) versus electrical commutation. So the BLDC motor behaves like a DC (brush) motor as opposed to an AC (induction) motor, but the applied winding voltages are really AC for both. Rosenberg’s electronically commutated (EC) fans dramatically outperform other AC and DC designs. Compact and low-noise, they offer exceptional variable speed capability, greater efficiency, higher speeds and wider operating range. With no carbon brushes to wear out, you also get longer service life. They’re ideal for pressure control, master-slave designs and RS485 networks. Call today to see what Rosenberg EC fans can do for you. Call: (704) 893-0883 Fax: (704) 882-0755 e-mail sales@rosenbergusa.com www.rosenbergusa.com Representatives wanted See us at AHR Booth #3162 Trapezoidal vs. sinusoidal commutation Both trapezoidal six-step (BLDC) and sinusoidally commutated motors (permawww.applianceDESIGN.com For more Information Enter 112 42 applianceDESIGN April 2008 http://www.rosenbergusa.com http://www.rosenbergusa.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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