Consumers are increasingly choosing more sophisticated and highly-efficient electronics. To achieve this, these electronics require motors that deliver high efficiency, wide variable speed and compactness. The industry continues to transition from induction motors that are less efficient and more cumbersome from a design perspective, to switched reluctance motors (SRMs). The latter offers efficiency and an ultra-compact size to accommodate the size and weight needs of today's electronics.

The switched reluctance motor is an electric motor in which the torque is produced by the tendency of the rotor to move to a position where the inductance of the excited winding is maximized. During motor operation, each stator phase is excited while its inductance increases, and unexcited as its inductance decreases. 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 phase and to synchronize 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 uni-polar principle requires only one switch to be in series with a phase winding. This phase independence and uni-polar principle have encouraged the various converter topologies.

SRMs have several distinct advantages over most motors, including, but not limited to, induction motors. Because a switched reluctance motor has a salient rotor without rotor windings, the material costs are reduced. Furthermore, independent windings make the fault tolerant operation possible and provide a robust structure. This robust structure decreases the actual power consumption as the windings are energized and de-energized only when required. It also has high torque-to-inertia ratio and high starting torque without the problem of in-rush current.

With other motor applications, this in-rush current during start up might cause the line voltage to dip momentarily, which adversely affects the power quality and can pose a problem in meeting government regulations. There can be some drawbacks to using SRM technology which need to be examined. For example, SRM operation requires knowledge of rotor position. Therefore, SRMs usually must include sensors, which can increase the overall cost of the system. Another drawback of SRMs 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.

One of those potential drawbacks has been addressed through design. With the advent of high-speed digital signal processors specialized for motion control applications, it has become possible to control SRMs with a sensor-less algorithm.

In the home appliance industry, SRMs are used mainly in vacuum cleaners because they operate at high speeds of -- tens of thousands of RPMs -- and require high torque to generate strong suction. Because of this high speed, the momentum of the rotator also acts as a low pass filter to the torque ripple, which mitigates the SRM's drawbacks.

Further, since the main source of audible noise in vacuum cleaners is its fan, the noise caused by SRMs are less prominent. Based on the above considerations, choosing SRMs for vacuum cleaner use is an optimal choice.

Many low-cost vacuum cleaners use a simple on-off switch with a universal motor because of the high speed operation up to 30,000 RPMs. The motor speed is controlled by using a triac chopper with a synchronizing 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 and inexpensive electronic controller is that switching the AC waveform can produce an undesirable electromagnetic interference. Therefore, care must be taken to prevent this EMI from radiating back onto the line or affecting the triac circuit itself. Moreover, the high peak-to-peak current results in poor motor efficiency and the subsequent high brush temperature leads to a limited motor lifetime. In particular, controversy over the harmful effects of carbon dust generated by the brush is spurring the development of SRM solutions for vacuum cleaners.

An SRM driver is an asymmetric converter with a PWM control circuit for delivering the power and commutation. Because of the circuit complexity, this solution is limited to the high-end market. Actually, there are many kinds of converters being used in the industry for the SRM. Selection criterion can depend on the cost, control scheme, and the performance. The figure shown below illustrates an equivalent circuit for one phase.

Equivalent circuit for one phase SRM

This converter consists of two 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 motors, it can be implemented on a PCB with either a discrete or modular solution. Although the discrete solution offers a lot of design flexibility in the layout, gate resistors and device 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 two modules available for SRM drives, single-phase and two-phase SRM converters. Increasing the number of SRM phases reduces the torque ripple, but at the expense of requiring more electronics with which to operate the SRM. At least two phases are required to guarantee starting, and at least three phases are required to ensure the starting direction.

In the case of the two-phase SRM module, four NPT-IGBTs, four FRDs, three drive ICs and discrete bootstrap diodes are included in the module. The SRM module has all of these components and an additional thermistor. By placing the thermistor inside the module rather than on the heat sink, the actual silicon temperature can be tracked with a smaller time constant and verified with a lower margin of error. The final advantage of the module is the optimization of the silicon by the adjustment of the power loss based on the real operation characteristics of vacuum cleaners.

Energy efficiency and high performance are indispensable requirements for the home appliance market. Consumers want sophisticated electronics with plenty of functionality, but with the rising costs of energy, they also want electronics that will conserve energy over the entire lifetime of the product's use. This is the reason that SRM technology is being adopted in many common applications such as vacuum cleaners.

The adoption of power module use for SRM drives are expected to be viewed as a solution for many motor drive applications. And with the rising global concern for energy, more improvements in SRM technology need to take place; such as minimizing noise and torque ripple through the improvement of the motor itself, increased accuracy of the sensor-less algorithm and the control algorithm itself.

The more innovations that semiconductor suppliers and SRM manufacturers can implement, the more we can collectively contribute to a greener world.

Byoungchul Cho and Sungil Yong work for the Motion Control System Group of Fairchild Semiconductor, Korea