Appliance Design - April 2008 - (Page 45)

MOTORS sion of which motor and control type is best for an application. For BLDC motors, a straightforward PWM current loop can control winding current, while commutation is handled by position sensors (Hall-effect or shaft encoder) or position estimate from winding voltage (from zero-crossing of unused winding). The current loop (which equals torque loop) is then controlled by a velocity control loop, and even an outer position control loop. Top speed of BLDC motor drive is reached when the motor back-EMF reaches the drive bus voltage. At that operating point, the drive cannot deliver enough current to increase motor speed, even at 100 percent duty-cycle. Sinusoidal commutation offers some additional benefits. Simple open-loop variable-frequency sinusoidal drive control is used for ACIM, but not for permanent magnet synchronous motors. PMAC motors are driven with closed-loop sinusoidal current control. Maximum performance and efficiency is obtained with field-oriented control. Field orientation requires rotor position information, either through direct measurement (encoder) or estimation (sensorless). Motor currents are modeled in a two-axis format with orthogonal direct (D) and quadrature (Q) axis components. The D-axis vector controls the magnetization of the motor, independently of the Q-axis (torque-producing) vector. In normal operation of a PMAC motor, the D-axis command is zero, since the permanent magnets provide the necessary flux-levels. The Q-axis current is then the controlled parameter used to close the torque-loop. For extended speed-range of a PMAC motor, the D-axis command can be set to reduce the motor flux (essentially opposing the flux generated by the permanent magnets). Reduced flux results in a reduced motor constant (both the speed constant Ke, and the torque constant Kt). So the net effect is to enable a higher motor top-speed from the same bus voltage. This method, known as Field Weakening, can practically extend the motor top speed to 3 times higher than without field weakening – at the direct expense of torque. In other words, more speed but lower torque – just like a mechanical gear-ratio. Field weakening can be particularly useful in a clothes washer drive for example: high torque at low-speed Fig. 6. Sensorless PMAC controller. is needed for the washing cycle, but highspeed at low torque is needed for best water extraction during the spin-cycle. An example of a complete sensorless PMAC motor drive control system is shown in Fig. 6. This controller uses motor current and voltage feedback to accurately estimate rotor position, eliminating the need for any expensive position sensors or shaft encod- ers. This complete control system is implemented as a dedicated hardware engine using a low-cost digital process.  For more information, enter . . . . . . . . . . . . . . . . . . . .311 Or email: epersso1@irf.com Acknowledgements: The author wishes to thank Nicholas Nagel at Illinois Institute of Technology for providing some of the figures. For more Information Enter 116 www.applianceDESIGN.com AD01084Elco.indd 1 applianceDESIGN April 2008 45 12/4/07 10:47:09 AM http://www.elcomotors.com http://www.elcomotors.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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