Switched Reluctance Motor Drives Information

Switched Reluctance Motor Drives

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.

Key Features Of SR Technology

Principle of Operation
The motor is doubly salient with phase coils mounted around diametrically opposite stator poles. Energisation of a phase will lead to the rotor moving into alignment with the stator poles, so minimising the reluctance of the magnetic path. This is the same principle of operation as the VR stepper motor. As a high performance variable speed drive, the motor's magnetics are optimised for closed-loop operation. Rotor position information is used to control phase energisation in an optimal way to achieve smooth, continuous torque and high efficency. The theoretical equations governing the torque production mechanism have been published countless times in the literature, so below is a simple graphical explanation. The current waveforms are superimposed on the angular unsaturated phase inductance. The maximum inductance corresponds to the minimum reluctance pole-aligned position. Positive torque is only produced at angles when the inductance gradient is positive.

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 W_con area represents the energy converted into mechanical energy (or converted from in the case of a generator). W_ret is the surplus energy returned to the power supply rails. Minimising W_ret by good magnetic design and optimal phase energisation control are the key features of an SR system.

Machine Topologies
SR machines can offer a wide variety of aspect ratios and salient pole topologies. This means that each application is likely to be better suited to a specific SR topology. Therefore it is difficult to give an overview of which topology offers what advantage or disadvantage without resorting to sweeping statements. However, here goes.

	Single-Phase Motor 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.
	Two-Phase Motor 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.
	Three-Phase Motor 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.
	Four-Phase Motor 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.

Power Electronics

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.

Commutation Control
Voltage or current control. Chopping or PWM. Depends on speed (available Volt-seconds) and any servo performance criteria. The control electronics will now be performed digitally and cost versus performance will be the main factor in selecting the most suitable circuit. ASICs have been mooted as the best option for very high volume drives, but microcontrollers, and more recently DSPs are proving to be the most popular control devices because of their flexibility. Current feedback is required for low speed operation and PWM is now finding favour because of acoustic noise issues. Rotor position feedback is required in some form and traditionally this is done with a rotor mounted sensor. This is a major cost and reliability issue and so a large amount of R&D effort has been placed in eliminating this sensor.

Major SR Issues

Rotor Position Sensing
As mentioned above, rotor-mounted position sensors are a liability. Not only do they introduce cost to the motor, but they can also be a major source of poor performance and unreliability. Work world-wide has now produced a number of viable schemes for sensorless operation. They all require monitoring of the phase current and applied voltage (flux observation), then by using knowledge of the magnetic characteristics, the rotor position is determined . This information is then used to optimise perfomance. It is interesting that the most often touted negative issues of SR performance may all be reduced by the merging of control means around real-time monitoring of the phase energisation in terms of flux-linkage and current coupled with knowledge of the machine being controlled (or possibly a self-learning control mechanism).

Torque Ripple
SR machines have a significant torque ripple, especially when operated in single-pulse voltage control mode. This is the price to pay for high efficiency. For many applications where the machine is operating at fairly high speeds, this is not a problem since the mechanical time-constant is far longer than the fast rates of change of instantaneous torque produced by the motor. There are applications where the torque ripple is a major concern and a well publicised application by way of example here is automotive power assisted steering (EPAS). The human being can sense very low levels of torque purturbation and so minimising not only the peak-to-peak levels, but also angular rates of change are a high priority. Effort can be put into both the machine design and the control strategies to help. Optimising the individual phase torque-angle characteristic by salient pole shape profiling, longditudinal skewing of the rotor and angular phase current profiling can all help to minimise the inherent torque ripple. Significant work has already been carried-out by a number of institutions, noteably Arkon Unversity into ways of improving torque ripple performance. Judging by the number of patents filed by TRW Inc on a electric power assisted steering system utilising a variable reluctance machine, then healthy interest in using SR for torque ripple sensistive applications is still prevalent.

Acoustic Noise
Motor throwing a wobbly * after William Cai & Pragasen Pillay

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.

* 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.

NB. Peter Rasmussen at Aalborg University has made available demonstrations of some of his research work into acoustic noise for downloading.

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.