Switched Reluctance Motor Features A switched reluctance SR motor is a rotating electric machine where both stator and rotor have salient poles
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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 o

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Switched Reluctance Motor Features A switched reluctance SR motor is a rotating electric machine where both stator and rotor have salient poles

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Presentation on theme: "Switched Reluctance Motor Features A switched reluctance SR motor is a rotating electric machine where both stator and rotor have salient poles"— Presentation transcript:

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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: dwell . It is defined by the turn-on on and the turn-off off 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 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: Ř$QJ Q Ř 9J Q Ř &Q Q In angle contr ol 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 Figure 1: Two-Phase 4/2 Switched Reluctance (SR) Motor Aligned Rotor Position Unaligned Rotor Position Stator (4 Poles) Phase A Phase B Rotor (2 Poles) Switched Reluctance Motors Control techniques using Freescale solutions Beyond Bits Motor Control Edition Motor Control Edition

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. 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. 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 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. SR Motor Applications 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 shaft- position information for the proper 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 K LQ I L & I control of SR motors depends on selected algorithms and r equired speed range. In the case of sensor application and low-speed range, K L & ZLK PG is a sufficient option. The Fr eescale &6 & LQG KQQ PG ZLK commutation support, which is very important for applications with high-speed range where pr ecise commutation is required. The sensorless algorithms are too complex for 8-bit devices

without additional external components. The Freescale '6& IPL\ SLG Q SLP solution. This family offers advanced PG \ I $'& ZLK $'& \QKQL]LQ QG a powerful DSP core, such as an &) &) %K are the smallest r epresentatives of K '6& IPL\ QG II K performance/price ratio. Several other PP I K ) '6& IPL\ are suitable for these applications. Refer ence designs, application notes and software solutions for SR motor control are available at freescale.com/motorcontrol Figure 2: Ideal Phase Inductance and Current Profile JustinTouch atorPhase Rotor Aligned Aligned Position/Time phA ON Dwell OFF

Beyond Bits Motor Control Edition
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