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Digital Control of Sensorless High-Speed SR Motor Drives 1/2

Digital Control of Sensorless High-Speed SR Motor Drives

Recent progressive variable speed drives have been designed to increase product performance and system efficiency. One such motor type, which can benefit from digital control, is the switched reluctance (SR) motor. It offers cost and reliability advantages over other types of adjustable speed drives due to its simple mechanical construction, high efficiency and high power density among other features. On the other hand, high torque ripple, due to double saliency construction, limits SR motor use in many applications.

Another SR motor advantage is high speed operation (> 50,000 rpm). This means a smaller motor can be used for a given output power, which reduces the size and weight of the target application. A vacuum cleaner is a typical application that can benefit from this feature. The high-speed SR motor makes the vacuum cleaner smaller and lighter, and the noise generated by torque ripple is comparable to other types of motors.

The SR motor is a rotating electric machine where both stator and rotor have salient poles. The stator winding is comprised of a set of coils, each of which is wound on one pole. The rotor is laminated in order to minimize the eddy-current losses. SR motors differ in the number of phases wound on the stator. Each of them has a certain number of suitable combinations of stator and rotor poles. Figure 1 illustrates a typical 2-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 in a symmetrical SR motor, and it ensures a motor start up in the proper direction.

2-Phase_4_2_SR_Motor
Figure 1 2-Phase 4/2 SR Motor

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. When any pair of rotor poles is exactly in line with the stator poles of the selected phase, the phase is said to be in an aligned position, i.e., the rotor is in the position of maximal stator inductance (see Figure 1). If the axis of the rotor is in line with the interpolar axis of the stator poles, the rotor is said to be in an unaligned position, i.e., the rotor is in a position of minimal stator inductance.

The inductance profile of SR motors is triangular shaped, with maximum inductance when aligned 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° relative to each other. When the respective phase is powered, the interval is called the dwell angle (θdwell). It is defined by the turn-on (θon) and the turn-off (θoff) 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.

Ideal_Phase_Inductance_and_Current_Profile
Figure 2 Ideal Phase Inductance and Current Profile

For a constant phase voltage the phase current is at its maximum in the position where the inductance starts to increase. This corresponds to the position where the rotor and the stator poles start to overlap. When the phase is turned off, the phase current falls to zero. The phase current present in the region of decreasing inductance generates negative torque. The torque generated by the motor is controlled by the applied phase voltage and by the appropriate definition of switching turn-on and turn-off angles.

The SR motor requires position feedback for motor phase commutation. In many cases, this requirement is addressed by using position sensors, such as encoders, hall sensors, etc. However, implementing mechanical sensors increases costs and decreases system reliability. Traditionally, motion control product developers have attempted to lower system costs by reducing the number of sensors. A variety of algorithms for sensorless control have been developed, most involving flux linkage estimation. These methods calculate the actual phase flux linkage and use its relation to the reference flux linkage for position estimation. The main disadvantage of all these methods is that the estimation of the flux linkage is based on precise knowledge of the phase resistance. However, phase resistance varies significantly with temperature, which yields unwanted integration errors, especially at low speed. These integration errors create a significant position estimation error.

Another method for sensorless position estimation is based on phase current peak detection. The principle of this method can be seen in Figure 2. The phase starts to be excited at the moment corresponding to a desired current amplitude. The current begins to rise until the stator and rotor poles begin to overlap. At this moment, the phase current reaches its maximum. Since we know the exact position of the rotor, we can estimate rotor position based on this current peak. If the current peak is detected, the peak time is saved. Knowing the time of two consecutive current peaks, we can calculate the commutation period and corresponding on/off times. The current peak can be detected by external circuitry, or, by using a powerful digital signal controller, it can be evaluated directly by software.

The current peak method has the advantage of being independent of the motor parameters. All we need to know is the rotor position at the current peak. Another advantage is that the current peak detection algorithm is very simple compared to the flux linkage estimation method. The current peak method can be used at very high speeds, whereas the low number of current samples for flux calculation limits the precision of the flux linkage estimation method. Due to the operating principle, the current peak method can be used with voltage control only, since in current control we lose information on the current peak.

Even though the control technique is quite simple if fully implemented digitally without any external components, it requires a powerful microcontroller (MCU). This MCU has to be capable of very fast phase current sampling and current peak evaluation. For example, in a 2-phase SR motor running at 60,000 rpm, the commutation period is only 250 s. To gain sufficient precision in current peak detection, the phase current has to be evaluated at least every 5 s.

The MC56F8006 digital signal controller (DSC) is a good choice for this application. The 56F8006 is a member of the 56800E core-based family of DSCs, which combines on a single chip the processing power of a DSP and the functionality of an MCU with a flexible set of peripherals to create an extremely cost-effective solution. These hybrid controllers offer many dedicated peripherals, such as pulse width modulation (PWM) modules, analog-to-digital converters (ADC), timers, communication peripherals (SCI, SPI, I2C) and on-chip flash and RAM.

In Figure 3, digital implementation of the current peak detection algorithm is demonstrated in a high-speed sensorless SR motor control application for vacuum cleaners. The application meets the following performance specifications:

    — High-speed 2-phase SR motor sensorless control based on current peak detection
    — Direct current sensing by integrated ADC
    — Software current peak evaluation
    — Designed to fit vacuum cleaner applications
    — Tested with a 2-phase SR motor designed for 60,000 rpm
    — Single direction of rotation due to asymmetric construction of the 2-phase SR motor
    — Speed open loop control
    — Start up from any position using alignment and a patented algorithm (Patent No. US6448736 B1)

Start up time and maximal speed depends on the SR motor parameters
Figure 3 illustrates the system concept, which incorporates a 2-phase SR high voltage power stage, a 2-phase SR motor and an MC56F8006 controller board that executes the control algorithm. In response to the user interface and feedback signals, the system generates PWM signals for the 2-phase SR high-voltage power stage. High-voltage waveforms generated by the DC to AC inverter are applied to the motor.

System_Concept
Figure 3 System Concept

The overall state of the application is controlled by an application state machine, which is executed in a background loop. The application state machine consists of init, stop, alignment, start up, run and error states.

by Pavel Grasblum, Ph.D., Freescale Semiconductor, Inc.

Read the Italian version: Controllo digitale del motore SR (motore a riluttanza variabile) 1/2

Read also: Digital Control of Sensorless High-Speed SR Motor Drives 2/2

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