|Fleadh||Electronics Power Electronics and Motor Drives Experts|
|The name switched reluctance has now become the popular term for this class of electric machine. The first reference to the term switched-reluctance was made by Nasar in a paper in the IEE Proceedings in 1969. The term became popular from the 1980s onwards, through the efforts of the first commercial exploiters of the technology, Switched Reluctance Drives Ltd. The machines are alternatively known as variable reluctance motors, reflecting the origins of the technology being derived from VR stepper motors. Even so the first recognisable reluctance machines were built over 150 years ago, most famously by Davidson as a traction drive for an electric locomotive in 1838.|
|At low speeds the phase current has to be constrained to protect the electronics because of the high available Volt-seconds. This is typically achieved by hysteresis current chopping as illustrated.|
|At higher speeds the current is naturally constrained, and single-pulse voltage control is normally employed with angle advance prior to the unaligned position to optimise performance.|
|The energy conversion mechanism is illustrated by the co-energy trajectory. The Wcon area represents the energy converted into mechanical energy (or converted from in the case of a generator). Wret is the surplus energy returned to the power supply rails. Minimising Wret by good magnetic design and optimal phase energisation control are the key features of an SR system.|
These are the simplest SR motors with fewest connections between machine and electronics. The disadvantages lie in very high torque ripple and inability to start at all angular positions. Maybe attractive for very high speed applications, but starting problems may preclude their use.
Problems of starting compared with single phase machines can be overcome by stepping the air-gap, or providing asymmetry in the rotor poles. This machine may be of interest where the cost of winding connections is important, but again high torque ripple may be detrimental.
Offers simplest solution to starting and torque ripple without resorting to high numbers of phases. Hence has been the most popular topology in its 6/4 form. Alternative 3-phase machines with doubled-up pole numbers can offer a better solution for lower speed applications. But again watch-out for torque ripple especially in the voltage control single-pulse operating mode.
Maybe popular for reducing torque ripple further, but the large number of power devices and connections will probably limit four phase to a limited application field. Five- and six-phase motors can offer better torque ripple reduction compared with four-phase and three-phase.
|Unipolar current in a conventional SR. (Can be driven bipolar especially if fully pitched wound where torque is produced by the change in mutual reluctance rather than self-reluctance). Single-ended converters can be used but the favourite is the two-transistor forward converter topology (asymmetric half-bridge) which has two power switches connected to either end of the power rails and in series with a winding for fluxing the machine and two diodes forming a return path. In the past there has been some debate about VA ratings when comparing other motor topologies but nowadays with modern majority carrier devices such as IGBTs, this argument is largely irrelevant especially when compromises are made with respect to torque ripple and acoustic noise. Similar rated drive electronics will more than likely have the same size devices. There is a cost disadvantage over a MOSFET inverter since the inherent MOSFET anti-parallel diode cannot be used.|
The animation above was inspired by the excellent presentation on vibration modes and acoustic noise entitled "An investigation into vibration in switched reluctance motors" by Pragasen Pillay & William Cai at the IAS conference in St. Louis 1998.
|Yes SR motors can produce excessive amounts of acoustic noise. The operation of the motor where the salient poles tend to align to minimise reluctance in normal operation leads to high normal forces acting on the stator structure. Harmonics of these normal forces will resonate the natural frequency resonant modes of the stator structure so producing acoustic noise. However, once the mechanism of noise production is understood, then steps can be taken to minimise the noise. The noise can be reduced by careful design on two fronts. Firstly the mechanical design can be optimise to avoid significant resonances at common operating points over the speed range and the structure can generally be 'stiffened-up' to minimise movement. Secondly, the phase energisation can be modulated to reduce the frequency components of the normal forces which cause the most sympathetic vibrations in the motor structure. Control techniques understood to be beneficial are forms of current profiling during the phase energisation. In its simplest form this can be a short period of freewheeling which if selected correctly can reduce the higher harmonics of the normal force when the machine is operating in the single-pulse mode voltage control mode. More complicated control measures entail angular profiling of the individual phase currents to minimise the less desirable force and torque harmonics using power converter switching frequencies above the human ear audible level.|
|MPEG animation of the conceptual workings of an switched reluctance motor drive. Click on the picture to run animation. The file size is 4.9Mb, so please be patient on a slow internet link.|
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