International Appliance Manufacturing 2008 - (Page 25) Fig. 2. A 3-phase inverter drives PMSM windings, and also features current-sensing and fault-generation circuitry. FOC control makes practical and economic sense, because modifying the hardware for implementing motor control is minimal. These appliances feature a three-phase inverter, which is used as the power stage to drive the motor windings. By adding a DSC (see Fig. 1) and currentsensing circuitry (see Fig. 2) through the help of two low-cost resistors, software on the DSC improves the motor efficiency through FOC control. The sensorless control technique implements the FOC algorithm by estimating the position of the motor without using position sensors (as shown in Fig. 1). The FOC algorithm — which is executed at the same rate as the PWM — is configured by ensuring that the PWM triggers ADC conversions for the two windings, using two shunt resistors. Then, a potentiometer is used to set the reference speed of the motor. The ADC Interrupts are enabled, to execute the algorithm. The position estimator (see Fig. 1) is based on the currents and voltages of the motor, and uses a motor model to measure the motor position indirectly, via an observer. The motor position can be estimated by assuming that the PMSM model is the same as that of a DC motor. A current-observer model aids in the measurement of the back EMF, indirectly, by feeding the motor and its model with the same input. Since the motor model has a closed-loop observer, it works to ensure that the estimated value matches the measured value. FOC is accomplished by keeping the stator magnetic field ninety degrees ahead of the rotor, at all times. This requires constant rotor-position information. In FOC, designers need to deploy a different algorithm to detect or estimate rotor position. FOC results in better torque production and less torque-ripple generation by the motor. Using the 3-phase voltage, the FOC algorithm generates a vector to control the 3-phase stator current. Converting the physical current into a rotational vector using transforms makes the torque and flux components timeinvariant. This time invariance enables control with conventional Proportional and Integral (PI) controllers, as with a DC motor. International Appliance Manufacturing 2008 25
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