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Switched reluctance motors promise higher efficiency in exchange for more sophisticated control electronics. 

Energy efficiency and high performance are indispensable requirements for the home appliance market. Consumers want sophisticated electronics with lots of features but, with rising energy costs, they also want electronics that will conserve energy. To achieve this, these electronics require motors that deliver high efficiency at various speeds as well as compactness.

The industry continues to transition from induction motors that are comparatively inefficient and cumbersome from a design perspective to switched reluctance motors that offer high efficiency and smaller size to accommodate the size and weight needs of today's electronics.

A switched reluctance motor is an electric motor in which torque is produced by the tendency of the rotor to move to a position where the inductance of the excited winding is maximised. During motor operation, each stator phase is excited when its inductance is increasing, and unexcited when its inductance is decreasing. The air gap is at a minimum at the aligned position and the magnetic reluctance of the flux flow is at its lowest.

An easy way to make the rotor turn is to sequentially switch the current from one phase to the next and to synchronise each phase's excitation as a function of the rotor position. The direction of rotation is independent of the direction of the current flowing through the phase. Rather, it only depends on the sequence of the stator winding excitation. This unipolar principle requires only one switch to be in series with a phase winding. This phase independence and unipolar principle have encouraged various converter topologies.

Switched reluctance motors have several distinct advantages over most motors, and that includes induction motors. Because a switched reluctance motor has a salient rotor without rotor windings, the material costs are reduced. Furthermore, independent windings make it possible to build in fault tolerance and provide a robust structure.

As windings are energised and de-energised only when needed, this decreases the actual power consumption. Also, it has high torque-to-inertia ratio and high starting torque without exhibiting the problem of in-rush current. With other motor applications, this in-rush current during startup might cause the line voltage to dip momentarily, which adversely affects the power quality and can pose a problem in meeting government regulations. However, there can be some drawbacks to using switched reluctance motor technology.

Noise reduction

Switched reluctance motor operation requires knowledge of the rotor position. Therefore, switched reluctance motors usually must include sensors, which increases cost. But, with the advent of high-speed digital signal processors specialised for motion control applications, it has become possible to control switched reluctance motors with a sensor-less algorithm. Another drawback of switched reluctance motors is the need for sophisticated acoustic noise control due to the vibrations inherent with operation and application needs to be unaffected by torque ripple or control.

In the home appliance industry, switched reluctance motors are used mainly in vacuum cleaners because they operate at high speeds - tens of thousands of revolutions per minute - and provide high torque to generate strong suction. Because of the high-speed, the momentum of the rotator also acts as a low pass filter to the torque ripple, which mitigates the drawbacks of the switched reluctance motor. As the main source of noise in vacuum cleaners is its fan, the noise caused by switched reluctance motors is less prominent. Based on this, using switched reluctance motors for vacuum cleaners is the optimal choice.

Longer life

Many low-cost vacuum cleaners use a simple on-off switch with a universal motor because of the high speed operation up to 30,000rpm. The motor speed is controlled by using a triac chopper with a synchronising circuit with position sensor information. This concept of phase angle control is to apply only a portion of the AC line voltage to the load. The phase angle is varied continuously and results in a variety of voltage waveforms. The drawback of this simple electronic controller is that switching the AC waveform can produce undesirable electromagnetic interference.

Care must be taken to prevent the EMI from radiating back onto the line or affecting the triac circuit. Moreover, the high peak-to-peak current gives poor motor efficiency and the subsequent high brush temperature leads to limited motor lifetime. In particular, controversy over the harmful effects of carbon dust generated by the brush is spurring the development of switched reluctance motor solutions for vacuum cleaners.

A switched reluctance motor driver is an asymmetric converter with a pulsewidth modulation control circuit for delivering the power and commutation. Because of the circuit complexity, this solution is limited to the high-end market. There are many kinds of converters being used in the industry for the switched reluctance motor. Selection criterion depends on the cost, control scheme, and performance.

The diagram shows an equivalent circuit for one phase. This converter consists of two insulated-gate bipolar transistors (IGBTs) and two diodes. When Q1 and Q2 are turned on, the stator windings are excited and the rotor will start to move to align with the excited stator pole by the reluctance torque. Soon before the stator and rotor pole are aligned, Q1 and Q2 are turned off and D1 and D2 are turned on. This makes a negative voltage to phase winding and a fast decrease of current, which suppresses the generation of the negative torque. The magnitude and the waveform of the current are regulated in order to meet the torque and speed requirements.

When a designer chooses the circuit topology for switched reluctance motor, this can be implemented on a circuit board with either a discrete or modular solution. Although the discrete solution offers a lot of design flexibility in the layout, gate resistors and devices selection, a solution with power modules can offer space efficiency and high reliability, enhanced productivity and cost-effectiveness in mass production. In the market, there are modules available for switched reluctance motor drives, single-phase and two-phase switched reluctance motor converters. Increasing the number of switched reluctance motor phases reduces the torque ripple, but at the expense of requiring more electronics with which to operate the switched reluctance motor. At least two phases are required to guarantee starting, and at least three phases are required to insure the starting direction.

With the global concern for energy, more improvements in switched reluctance motor technology need to take place such as minimising noise and torque ripple through the improvement of the motor itself, increased accuracy of the sensor-less algorithm and the control algorithm. The more innovations that semiconductor suppliers and switched reluctance motor manufacturers can implement, the more we can collectively contribute to a greener world.

Byoungchul Cho is a senior engineer and Sung-Il Yong is a senior manager at Fairchild Semiconductor

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