Controllability key to SR motors

The basic technology behind switched reluctance (SR) motors has been around for well over 100 years. Their use is pretty widespread throughout industry, although generally it has to be said, in more niche applications. The Dyson-developed 'world's fastest electric motor' for a hand-held vacuum cleaner was based on SR technology. It is also used in the air conditioning system of European trains as well as in washing machines in laundrettes. Its inherent robustness means that many industries have sought these motors where reliability is the main design driver and the motor is required to keep running for years on end. SR technology competes with permanent magnet motors and sometimes induction motors for various applications and it is up to engineers to make the trade-off and design decision about which is the most appropriate technology. For some time, brushed and permanent magnet motors have been the accepted electric motor of choice in the automotive industry. However, as the cost of rare earth elements (RREs) has rocketed, both car manufacturers and designers are looking at alternatives to what has almost become the de facto industry standard. Nick Pascoe, chief executive of Controlled Power Technologies (CPT), says: "A significant element of the bill of materials cost of a high output electric motors is the cost of its rare earth permanent magnets. The cost of rare earth materials, driven by the limited Chinese supply, is now five or six times its level of one year ago, which is driving major price increases for automotive users. This price volatility is a big concern to the automotive industry." Dunton-based Control Power Technologies has been developing SR motor technology for the past 10 years and is now taking its technology to market. The company which has its roots in Visteon and Ford has developed quite a niche product using an SR motor at its core; an electric super charger. Although it may sound pretty exotic, with the current evolutionary trend from 'micro hybrid' toward 'mild hybrid' cars – including downsized engines with no compromise to driveability - it begins to make perfect sense. SR motors basically comprise of a stator and a rotor. The stator consists of coiled sections on the outside of the motor which do not move. The rotor is the movable part of the motor which is continually pulled around as current is simultaneously passed through groups of coils. SR motors have no permanent magnets and the magnetic field is entirely driven by electricity passing through a coil winding. The stator coils are switched in sequence and that essentially pulls the rotor around. The switching is synchronised with rotor position and, being brushless, it results in inherent robustness. Each group of stator coils is known as a phase and generally CPT opts for a three-phase set up within the machine. Automotive OEMs are downsizing engines in an effort to reduce fuel consumption and ultimately CO2. To make up the power to weight deficit of a smaller engine, however, they are tending to add turbochargers. One problem this creates is the lag in the system during the initial drive phase. Fuel is controlled by injectors and can be scaled up or down with ease. However, it is the air entering the system that limits engine output. If the turbo has not had a chance to spin up, the lag in the system means there is a significant lack of air in the engine intake charge, resulting in less power, even if it is just for a few seconds. As a result, CPT's electric supercharger will kick in during the low revving lag period to pump in extra air, matching the fuel and giving the engine the power output the vehicle needs. So why use switched reluctance motors? "The automotive industry has commonly shied away from SR motors," says Guy Morris, chief technology officer at CPT. "Although the fundamentals of SR have been understood since people began to understand electromagnetism, it has been easier to develop alternatives as in the early years there was little or no capability to precisely control the SR motor. "Control is everything and that is why electronics and fast microprocessors have been able to change what we can do dramatically. We've developed very sophisticated control algorithms that actually look at what is going on and make tiny adjustments down in the micro-second range. It is quite subtle control." A big advantage of SR technology is no doubt the lack of rare earth elements inside, but another key strength of the motors is their efficiency over a wide range, which is now coupled with the controllability developed by CPT. Being able to manage the torque ripple, essentially the drop off in rotational force (torque) as the motor is switched between phases during rotation, has been one of the key enabling innovations for SR. The precise control algorithms of CPT have almost negated the torque ripple effect altogether. They can also be quite flexible in terms of their packaging requirement. They can be very flat or very thin, the 'pancake' and the 'sausage' are two extremes of possible lay outs, CPT has opted to sit somewhere in the middle and the turbocharger looks and feels much like a small alternator. Following on from the success of the electric supercharger, CPT has also used SR technology to commercialise an Integrated Starter Generator. Essentially, this is a stop-start system that uses an SR motor in combination with advanced lead acid batteries to achieve micro-mild hybridisation. Critically, however, it also allows energy to be captured and generated, enabling regenerative braking. "This is what a lot of hybridisation is about," says Morris, "being able to deploy one machine to do several functions. Controllability starts to become the big issue for the engineers. On these SR-based systems, you can control almost anything you want." As part of its commercialisation process, CPT has been able to miniaturise and fully integrate the electronics and control elements so that the unit is almost completely standalone, and bolt-on. Additionally, as the machines have all the control elements integrated inside the casing, they have no issues with electromagnetic energy causing interference, which can completely wipe out the AM/FM radio signal. "Electromagnetic interference and also acoustic noise are text book concerns," says Morris. "In truth, however, they are very quiet machines. But, like all electric machines, they do have resonant frequencies and we have to manage that through a combination of mechanical design and electronics optimisation. "Another is air gap. As with most motors the closer you can run, the smaller the gap that the magnetic flux has to act across, the more efficient the machine. But principally, we run the same air gaps as induction motors and our efficiencies are very competitive. So that means we can use standard manufacturing techniques." The simple fact is that the internal combustion engine will be the drivetrain of choice for many years to come. But the outcome needs to be that less CO2 being produced. As a result, most of the large automotive OEMs are looking at downsizing engines, as this is one of the most cost effective ways to reduce CO2 emissions and fuel consumption. But this needs to be coupled will other 'micro-mild hybridisation' technologies to fill any performance gap. Pascoe adds: "You have to tackle the issue of torque deficit, which these devices can do. We need to maintain or slightly improve performance, since customers will not pay more for less. For the OEM offering an affordable value driven solution is key. It is this evolutionary 'micro-mild' hybridisation that we think is an area that is often overlooked, but could potentially bring the most benefits."