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Beyond Bits Motor Control Edition Switched Reluctance Motors Control techniques using Freescale solutions Switched Reluctance Motor Features A switched reluctance (SR) motor is a rotating electric machine where both stator and rotor have salient poles. The stator winding comprises a set of coils, each of which is wound on one pole. The rotor is created from lamination in order to minimize the eddy current losses. SR motors differ in the number of phases wound on the stator. Each has a certain number of suitable combinations of stator and rotor poles. Figure 1 illustrates a typical two-phase SR motor with a 4/2 (stator/rotor) pole configuration and a stepped gap. The stepped gap is used to eliminate dead zones, where motor torque is zero at a symmetrical SR motor and it ensures motor startup in the proper direction. The motor is excited by a sequence of current pulses applied at each phase. The individual phases are consequently excited, forcing the motor to rotate. The current pulses need to be applied to the respective phase at the exact rotor position relative to the excited phase. The inductance profile of SR motors is triangular shaped, with maximum inductance when it is in an aligned position and minimum inductance when unaligned. Figure 2 illustrates the idealized triangular-like inductance profile of both phases of an SR motor with phase A highlighted. The individual phases A and B are shifted electrically by 180 degrees relative to each other. When the respective phase is powered, the interval is called the dwell angle: Qdwell. It is Figure 1: Two-Phase 4/2 Switched Reluctance (SR) Motor Stator (4 Poles) Phase B Phase A Rotor (2 Poles) Aligned Rotor Position defined by the turn-on Qon and the turn-off Qoff angles. When the voltage is applied to the stator phase, the motor creates torque in the direction of increasing inductance. When the phase is energized in its minimum inductance position, the rotor moves to the forthcoming position of maximal inductance. The movement is defined by the magnetization characteristics of the motor. A typical current profile for a constant phase voltage is shown in figure 2. Control of SR Motor The SR motor is driven by voltage strokes coupled with the given rotor position. The profile of the phase current together with the magnetization characteristics defines the generated torque and thus the speed of the motor. Due to this fact, the motor requires electronic control for operation. Several power stage topologies are being implemented, according to the number of motor phases and the desired control algorithm. The particular structure of Unaligned Rotor Position the SR power stage defines the freedom of control for an individual phase. There are a number of control techniques for SR motors. They differ in the structure of the control algorithm and in position evaluation. Three basic techniques for controlling SR motors can be distinguished, according to the motor variables that are being controlled: • Angle control • Voltage control • Current control In angle control techniques, the constant full voltage is applied in the SR motor. The speed of the motor is controlled by changing on/off angles. The speed controller processes the speed error (the difference between the desired speed and the actual speed) and calculates the desired on/off angles. This technique is not suitable for full speed range operation since during low-speed operation the maximal voltage amplitude generates high current peaks in the motor phases. This technique is used to run Beyond Bits Figure 2: Ideal Phase Inductance and Current Profile Stator Phase A Aligned Just in Touch Aligned Rotor iphA LB LA Motor Control Edition switching of individual phases. Also, the motor structure causes noise and torque ripple. The greater the number of poles, the smoother the torque ripple, but motor construction and control electronics become more expensive. Torque ripple can also be reduced by advanced control techniques such as phase current profiling. Freescale Enablement ON the SR motor over nominal speed. At the nominal speed, the full voltage is applied on the motor phases and by properly adjusting on/off angles the motor can achieve operation over the nominal speed. Dwell OFF Position /Time current reaches desired amplitude, the current controller keeps the phase current at the desired level. As is apparent from the description, the SR motor requires position feedback for motor phase commutation. In many cases, this requirement is addressed by using position sensors, such as encoders and Hall sensors. The result is that the implementation of mechanical sensors increases costs and decreases system reliability. Traditionally, developers of motion control products have attempted to lower system costs by reducing the number of sensors. A variety of algorithms for sensorless control have been developed, most of which involve evaluation of the variation of magnetic circuit parameters that are dependent on the rotor position. In voltage control techniques, the speed of the motor is defined by the voltage applied to the motor phases. The voltage applied to the phase is directly controlled by a speed controller. The speed controller processes the speed error (the difference between the desired speed and the actual speed) and generates the desired phase voltage. The desired voltage is generated by the SR inverter using PWM modulation. During PWM modulation, the on/off times are constant. Once the applied voltage has achieved its maximal value, the motor speed can be increased over the nominal speed by changing on/off times. SR Motor Applications In the case of current control, there is one more control loop: inner current control loop employed in the control of the SR motor. In this type of control, the output of the speed controller defines the required current amplitude in the motor phase. Based on the required current amplitude, the new on/off times are calculated. Once the The SR motor itself is a cost-effective machine of simple construction. Since high-speed operation is possible, the motor is suitable for high-speed applications, such as vacuum cleaners, fans and white goods. As discussed above, the disadvantage of the SR motor is the need for shaftposition information for the proper The selection of suitable MCUs for control of SR motors depends on selected algorithms and required speed range. In the case of sensor application and low-speed range, the 8-bit MCU with a PWM module is a sufficient option. The Freescale MC9S08MP16 MCU includes a 6-channel PWM module with commutation support, which is very important for applications with high-speed range where precise commutation is required. The sensorless algorithms are too complex for 8-bit devices without additional external components. The Freescale DSC family provides an optimal solution. This family offers advanced PWM modules, very fast ADC with PWM to ADC synchronization and a powerful DSP core, such as an MC56F8006 or MC56F8013. Both are the smallest representatives of the DSC family and offer the best performance/price ratio. Several other members of the Freescale DSC family are suitable for these applications. Reference designs, application notes and software solutions for SR motor control are available at freescale.com/motorcontrol. 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Headquarters ARCO Tower 15F 1-8-1, Shimo-Meguro, Meguro-ku, Tokyo 153-0064, Japan 0120 191014 +81 3 5437 9125 [email protected] Asia/Pacific: Freescale Semiconductor Hong Kong Ltd. Technical Information Center 2 Dai King Street Tai Po Industrial Estate, Tai Po, N.T., Hong Kong +800 2666 8080 [email protected] For more information, visit freescale.com/motorcontrol Freescale and the Freescale logo are trademarks of Freescale Semiconductor, Inc., Reg. U.S. Pat. & Tm. Off. All other product or service names are the property of their respective owners. © 2012 Freescale Semiconductor, Inc. Document Number: BBSWCHRLCTART REV 0