Appliance Design - April 2008 - (Page 43)

MOTORS nent magnet AC or PMAC) most commonly have three stator windings arranged in a 3-wire WYE configuration as shown in Fig. 3. Both motor types are also driven from the same 3-phase Voltage-Source Inverter (VSI) topology also shown in Fig. 3. Phase-currents for both types of motors are controlled by pulse-width modulation of the applied voltage on each phase. Both BLDC and PMAC motors clearly have a lot in common. A casual observer might not even see any difference in the overall construction of these two types of motors. The difference is in the “torque function” of each motor type, and how current is driven into the windings. If each motor type is spun, and the resulting back EMF examined on an oscilloscope, it would be seen that the BLDC motor has a flattopped trapezoidal voltage, and the PMAC motor has a sinusoidal shape. In order to derive smooth torque from these motors, the applied current should be the same shape as the back EMF. In the case of the BLDC motor, current is applied to only two of the three wind- Fig. 3. Permanent magnet motor driven by voltage source inverter. ings at a time. During one commutation interval, a steady current is injected into one motor lead, returning through a second lead – and the third lead is open. (It can be used for position estimate by measuring zero-cross). As the motor shaft rotates, the next set of windings is energized, sequencing through all six possible combinations (thus the name six-step) – at which point the motor has gone through one electrical revolution (fractional mechanical revolution depending on how many winding “poles” the motor has). An important implication of this commutation method is that only two of the six inverter switches are conducting at any time, and only two of the three windings carry current at any time. These two factors affect distribution Convenience Matters A. O. Smith Cares Are you looking for an electric motor supplier that can reduce assembly costs for your convection ovens? A. O. Smith provides convenience and added value by furnishing air-moving sub assemblies for convection fans and cooling blowers. A. O. Smith offers customized fan assemblies with mounting plates, heating elements and wiring harnesses to ﬁt your oven applications. Multiple bearing options are available for the best combination of cost and longevity of the motor Cross ﬂow blowers available in 60mm and 45mm wheel diameters Motors and fans designed to meet your convection oven’s airﬂow and noise speciﬁcations Contact A. O. Smith to see how we can meet your unique requirements. 531 N. Fourth St. • Tipp City, OH 45371 • Tel: (937) 667-2431 • Fax: (937) 667-5030 www.aosmithmotors.com AD04084AOSmith.indd 1 For more Information Enter 113 applianceDESIGN 3/12/08 2:55:59 PM www.applianceDESIGN.com April 2008 43 http://www.aosmithmotors.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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