Appliance Design - October 2007 - (Page 24)

MOTORS Fig. 2. Sensorless field-oriented control algorithm. The Analog Signal Engine extracts the motor winding current from the DC link current signal by synchronizing the A/D converter sampling with the power-inverter device switching. The Motion Control Engine executes the sensorless FOC algorithm described by the control schematic in Fig. 2. The algorithm includes a reverse-rotation function that transforms the measured stator currents into a reference frame synchronized with the angle of the rotor-magnet flux. The transformed currents have two quasiDC components: a direct-axis current is aligned with the rotor flux, and a quadrature-axis component that generates motor torque due to interaction with the rotor magnet. The d and q axis current-loop compensators calculate the stator voltages to force the currents to track the set point values. The forward-rotation function transforms these voltages to sinusoidal AC voltages in the stator reference frame. The space-vector PWM generator uses these signals to derive transistor-switching signals for the 3-phase inverter. When driving a classical, permanent-magnet synchronous motor with surface-mounted rotor magnets, the d axis reference current is set to zero to maximize the torque per amp. However, when driving an interior, permanent-magnet motor, the d axis current will generate a reluctance-torque component to augment the torque produced by the rotor magnets. The IPM control function is the key control element that enables the operation at a higher efficiency when driving the IPM motor. The other key feature of the sensorless FOC algorithm is that it does not require high-resolution rotor-angle sensors typically found in industrial-drive systems. In appliance drives, where-low speed performance is not important, the rotor angle can be derived from the winding back-EMF signal. In brushless DC drives with a 6-step commutation sequence, the back EMF is available directly by sampling the voltage on the unconnected winding. However, when driving the motor with sinusoidal currents, the back EMF has to be calculated indirectly from the motor circuit model. The equations below describe the 2-phase equivalent circuit for the permanent-magnet, synchronous motor. The 2phase currents are derived using the Clarke transform that calculates the currents in two quadrature windings that will produce the same field as the currents in the 3-phase windings. The 2-phase winding currents are the outputs of the forward rotation function that drive the space-vector PWM generator. v v RS .i RS .i LS . di dt L. di S dt d dt d dt r . cos r r r . sin The important aspect of the 2-phase circuit model is that the back EMF terms are time Fig. 3. Rotor angle and speed estimator. 24 applianceDESIGN October 2007 www.applianceDESIGN.com http://www.appliancedesign.com

Table of Contents Feed for the Digital Edition of Appliance Design - October 2007

Contents Editorial Shipments/Forecasts News Watch Applying Powder Coatings to Plastic Parts is No Longer a Pipe Dream New Technology Platform Provides the Ability for a single IC to Control Two- Motors in a Single Appliance, Simplifying Motor Control Design Advanced Motor-Control Techniques have Become More Accessible to a Wider Array of Appliances, Helping to Improve Efficiency and Reduce Noise Adding USB Host Capability to Appliances Permits In-Field Programming and Acquisition of Test Data ZigBee Modules Make it Easier to Design Wireless Connectivity into Applications New Concepts such as Rapid Tooling and Rapid Injection Molding Provide Stepping Stones Between Prototyping and Full Scale Production A Number of Factors and New Developments Affect the Decision on Whether to Build Prototypes In-House or Outsource to a Service Bureau DesignMart Advertiser’s Index Association Report: CEA

Appliance Design - October 2007

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