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Motors for electric vehicles There are many options when it comes to choosing motors for electric vehicles. Here's a guide to the various motor types, their advantages & their drawbacks. By GERRY NOLAN For petrol-heads, it's what's under the bonnet that counts - the grunt to dollar ratio. It's the horsepower that lays the rubber on the road - right? And for more horsepower, you want more cubic centimetres of engine, fuel injection, turbocharging and to hell with efficiency. Electric vehicle owners will talk 10 SILICON CHIP about "watts" under the bonnet, motor efficiency (usually in the high nineties), AC or DC, rare earth magnets, power to weight ratios, and commutated or brushless motors. But, regardless, the talk will still be about the driving force - the motor. While we're at it, the conversion from horsepower to watts is lHP = 746 watts. So your 190HP family car engine is now rated at 142 kilowatts. How electric motors work As you may know, it was Andre Marie Ampere who said that a current carrying conductor placed in a magnetic field will be deflected. The calculation of this deflecting force (F) is expressed by the "Bli" rule: F = B(li) where a conductor of length 1, carrying a current i, is located in a magnetic field B (assuming that the con~ ductor is perpendicular to BJ. The direction of the force is obtained by the left-hand rule, which is illustrated in Fig.1. The converse is also true - that is, if we move a conductor so that it "cuts" Left: the Australian-designed "Solar Star II" sports car. The kevlar front body is in place but the curved section behind the driver has yet to receive the solar panels. a magnetic field, an electric current will be generated. Faraday first defined this law and the right-hand rule that goes with it. It is this principle that we employ when using our electric motor for regenerative braking to recharge the energy source. If we make a large number of conducting loops, wrap them around an armature and connect them to a commutator so that the current is "switched" to remain in the same direction (with respect to the magnetic field), we can produce a constant deflecting force that will be converted into rotary motion of the armature and be usable as a driving force - a motor, in fact . A single coil is shown diagrammatically in Fig.3 and a cross-section of a DC commutator motor is shown in Fig.2, with all the parts labelled. From this simple discussion it will be apparent that the motor deflecting force or "power" will be increased by increasing the magnetic field strength, the number of turns on the armature and the armature current. Physical limits to these increases will be the size of the motor and the ability to cool it. Many other factors such as the number of field windings, number of poles, speed of armature (rpm), inductive losses, FR losses and so on will also help determine the motor's power output. A modified Pope motor replaces the engine and gearbox under the centre console of the Sydney University Susuki Carry Van. Force F Flux 8 -'----Current i Fig.1: the left hand rule will indicate the direction of the force when a conductor carrying current I is perpendicular to a magnetic field B. Parts of a DC motor Evolution of motors Recently, I had the somewhat bemusing experience of being told by an electrical engineer that he "did not know" if the electric motor he was using was AC or DC. What would you call a motor that ran with a DC input pulse rate of up to 12kHz? Earlier "classical" motors, millions of which are still in service, satisfy these criteria: they operate from pure DC or AC sinewave and can start and run without electronic controllers. However, it is unrealistic to talk about motors for vehicles without controllers. Much of the development on con- trollers to provide adjustable speed and power was carried out on the "classical " motors. As you can see from Fig.4, the three classical motors all have wound fields. The DC commutator and AC synchronous motors have wound armatures and require brushes, while the AC induction motor has a wound field only. The next stage of evolution is shown in Fig.5, where the field windings of the DC motor and the armature windings of the AC motor are replaced by permanent magnets (PM). This resulted in the brushless AC motor but the DC motor had to have the rotor and stator transposed, as shown in Fig.6, before it could be brushless. Why the strong push to get rid of Left-hand rule Armature core Annatuce w;,a;,g //r~··.~1~ L•:::g::(: r ( fo ,_ Field Poles .. - ~ - -- i I ·, /✓~-~I·- " \ I · ' Shaft 1 .. Rotation .._ ,' I ~ Pole core / ',·- Pole face / '• - __ , ___ . · · "'-· wlodlogs) · > / 7"-···· z------=1--------"') Trailing pole tip Field yoke (also makes stator) Fig.2: cross section of a DC motor showing the DC poles & armature. MAY 1991 11 Simple AC motor Armature winding s \ Field poles )('-~~=-.--- Brushes / To external circuit Fig.3: this diagram shows the basic principle of a simple motor. Note that that the armature winding rotates in a magnetic field. The brushes & commutator ensure current reversal in the armature winding as the armature rotates past the poles. commutators, slip-rings and brushes? Although commutator/brush systems are reliable, well-proven and resilient, they are also dirty, noisy and require regular maintenance. In addition, commutator speed is limited, they produce radio frequency interference (RFI), the brush gear takes up a lot of space and they can be difficult to cool. Anyone who has had to "bed-in" the brushes or clean the commutators on large electric motors will really appreciate the advent of brushless motors. Commutator motors are also out of DC DC This is the Swiss Brusa AC induction motor that powers the "Solar Star 11" sports runabout (see opposite page). There are many types of motors and hybrids, from tiny robot motors to the giant motors used in coal loaders. Virtually all of them can be found in one of the categories listed below and we'll look at each one briefly: • DC commutator (series, shunt and compound); • AC induction; • AC synchronous; • Brushless PM (permanent magnet) DC; • Brushless PM AC synchronous motor. The series DC commutator motor is widely used for traction applications (eg, trains, trams, forklifts) because of its high torque at low revolutions. However, because it uses brushes and a commutator and has fairly low efficiency (84% average), it is not a good 3 phase AC 3 phase AC the question when volatile gases are present as, for example, in the ventilation of fuel tank evaporation spaces, where the slightest spark will cause a disaster. Using permanent magnets instead of a wound rotor also cuts out the FR losses that inevitably occur in a conventional field winding. Types of motor LL~ . ·--, .. ~ ' Wound field DC commutator AC synchronous AC induction Fig.4: the three classical motors all have wound fields. The DC commutator and AC synchronous motors have wound armatures and require brushes, while the AC induction motor has a wound field only. 12 SILICON CHIP Solar sports car takes shape The Australian designed "Solar Star II" is the answer to critics who say solar cars are not practical. It not only doesn't look like a solar powered car but it should have performance rivalling that of conventional cars. By GERRY NOLAN My first impression of this car was one of shock. This couldn't be a solar car. Where was the familiar cockroach look and the spindly, narrow bicycle wheels? At first glance, the "Solar Star II" looks like an expensive, low-slung sports car. Then, when you look closely, you find that it really is a sports car. And it has the look of an instant classic, like the MG TF and Austin Healy sports cars. Using 10 Alco 12V, 75Ah deep cycle batteries, each weighing 19.2kg, the car will weigh about 400kg. A roomy 2-seater with hightech suspension, low profile tyres and streamlined, lightweight kevlar body, it will have a top speed of 130- This view shows the battery mounting position which is forward of the rear wheels (solar panel removed). 140km/h and acceleration equal to or better than more mundane vehicles. Designer and builder Les Pukloswski, of Huntington Enterprises, built the car in only a few months using the Ford GT40 as a body mould. This has enabled him to fit a stock windscreen and other parts, saving many thousands of dollars and months of development time. The "Solar Star II" has literally been built from a sketch Les did in few minutes when Leon Howe of Star Micronics asked him to come up with some ideas late last year. The 18kW Brusa-controlled, 3phase AC induction motor, rotating at up to 11,000 rpm will power the rear wheels through two stages of toothed belt drive, giving an 8:1 reduction to a 1:1 lightweight differential. The car has disc brakes front and rear and a fully adjustable, hightech suspension with magnesium uprights that can be raised and lowered to suit different surfaces and running regimes. At present, the car is using 24 panels of the 14% efficient ShowaArco monocrystalline solar panels from Dimitri Lajovic's 1990 World Les Puklowski demonstrates how the solar panels will fit the curve of the mounting board. At the top of the page is a computer generated artist's impression of the "Solar Star 11". Solar Challenge "Alarus", but Leon Howe is trying to obtain the 18% efficient panels developed by the University of NSW. This would save the necessity of attaching an additional solar array for long distance cruising. After being displayed at PC 91 and racing at the Gold Coast Indy Grand Prix, the car is presently undergoing registration procedures and you·r correspondent is waiting impatiently to get behind the wheel for a day. "Solar Star II" is shaping to be a really "practical" vehicle and an extremely attractive one at that. The question is, will Star Micronics go into mass production? MAY 1991 13 DC 3 phase AC ( ·. . ) ··- .. __ __ --:>"' PM DC commutator AC PM/reluctance hybrid Fig.5: these 'diagrams show how the field windings in a DC motor and the armature windings in an AC motor can be replaced by permanent magnets (PM). This gives rise to the brushless AC motor, as shown at right, but the DC motor still requires brushes. option for electric vehicles. With a fixed AC supply, induction motors run at an essentially constant speed which means that their use in traction applications has been limited. However, by using solid state controllers to produce a variable frequency 3-phase AC supply, they become a practical option. The same comments can be made about AC synchronous motors which have a DC-energised field or a permanent magnet rotor. Brushless DC motors have always been a misnomer. Originally developed by the Japanese for use in turntables and tape decks (where conventional brush motors created audio interference), they really are AC motors with the field commutation and speed control performed by switching transistors. They have always been very good for applications requiring low power. However, with the availability of new materials for use in the construction of motors , particularly rare earth magnets, and more effective electronic control systems, brushless motors are now available at much higher powers. Only a few years ago, engineers believed that the upper power limit of the brushless DC PM motor was around lOkW but now ZOkW motors are available. Brushless PM motors , both AC and DC , now use rare earth magnets such as samarium-cobalt and neodymiumiron-boron (Nd-Fe-BJ and will soon 14 SILICON CHIP 3 phase squarewave or sinewave PM brushless DC Fig.6: the brushless DC motor is obtained by going one step further and transposing the rotor and stator. be using an improved grade ofNd-FeB called " UGISTAB", marketed by Aimants Ugimag in the UK. When compared with Nd-Fe -B magnets , which are in themselves much stronger than anything else, the UGISTAB magnet has increased coercivity, greater resistance to corrosion and better magnetic stability over a wider temperature range. Brushless PM motors with peak efficiencies of up to 97% and power to weight ratios of up to 3kW /kg are now available. Of the 36 vehicles that started in the 1990 World Solar Challenge, 28 used brushless DC PM motors and three used brushless AC PM motors. UNIQ motors, mentioned in the January 1991 issue of SILICON CHIP, are produced by Unique Mobility in Colorado, USA. They achieve a high power-to-weight ratio and efficiency by a unique arrangement in which radially positioned pe,manent magnets are mounted on either or both of two hollow cylindrical rotor elements which coaxially "sandwich" a thin hollow stator. In this way, the stator windings are exposed to the entire magnetic flux. UNIQ motors were used in seven of the vehicles in the World Solar Challenge, in sizes varying from 1.5kW to 15kW. What type of motor? As indicated by our discussion so far in this article, there _is no "best" motor. The motor selection for an electric vehicle is made on the basis of efficiency, weight and cost. You could clean up that old electric motor that's been lying around in your workshop for years and build something around it, or you could decide on the type of vehicle you want, what you want to do with it, what you're going to use for energy and then decide on the motor that will best suit your requirements, for the price. Putting all this in a formal way, the vehicle factors which determine the size of motor are: the typical driving cycle; vehicle limits (type, weight and payload); and type and weight of the energy source. Typical driving cycles will vary enormously. As mentioned in a previous article, surveys show that around 90% of all daily 1-way car trips are less than 35km long with over 50% being less than 10km long. So let's look at what you would need to consider in designing a pr;i.ct'ical electric vehicle. Although we are not designing a sports car, for safety reasons, we will want to be able to stay with most other vehicles when accelerating away from the lights. This will require a motor with high torque at low speeds. On the other hand, we need to stay with traffic on the freeway too, so we don't want the power dropping off as the motor revs increase. Let's just say we want a range of 7080km with fairly snappy acceleration and a top speed of at least 80km/h. Typically, the urban vehicle carries an average of only 1.2 people but, as there aren't many 0.2 people around, In fact, Leon Howe of Star Micronics, sponsor of the "Solar Star", is so determined to prove that solar/electric vehicles are a viable alternative that he has already commissioned Les Puklowski of Huntington Enterprises in Sydney to build Report by GERRY NOLAN "Solar Star 11 ", a 2-seater solar assisted city sports runabout. was a resounding success in that it Twenty-six vehicles participated "We wuz robbed!" is the cry heard has shown clearly which questions in the event and were powered by from the solar car people. to ask and which problems must be energy sources that included elecThe 1991 Energy Challenge, solved for the next event, in January tric, solar, hydrogen , ethanol, steam, which was sponsored by the NSW 1992. compressed natural gas (CNG), Department of Minerals and Energy, No doubt the emphasis on practiliquid petroleum gas (LPG), human took place over the weekend of 18cality will ~ncourage the CNG enpower and hybrid power. 20th January. Ultimately, the award The National Roads and Motorwas to go to "the entrant which prothusiasts but the solar car people are asking why they were encourists Association (NRMA) Technical vided the most personal mobility (that is, practical and economical) aged to compete if there was never Panel, headed by John Ward, Manfor the least environmental impact." any chance that they could win. ager of Technical Services, awarded On this reasoning, the first prize Certainly, their frustration and diseach vehicle a GGI number, which appointment is a spur to prove that indicated the environmental impact was awarded to the Alsco Linen laundry truck, using compressed natural solar vehicles can be practical as the vehicle had in moving a payload well as economical and non-pollutover a distance. The perfect score gas (CNG), primarily because of its relatively large payload which put it ing. was zero, which only the solar and at the top of the class for practihuman powered vehicles were able to achieve. cality. Notwithstanding this, the so"Solar Star", the fastest solar lar cars claimed to be the clear car on earth, covered the total winners on the basis of the race distance of 373.1 km in a Greenhouse Gas Index (GGI) cumulative time of 7hrs 23min, and fuel/energy efficiency. Only giving it an average speed of one car, a CNG fuelled Toyota 50.5km/h. On several occaCamry, finished in front of the sions, the speed limit of 11 0km/ "Solar Star". h was attained on the freeway sections of the race. Figures However, to put the event and the results into perspective, recently released by the NRMA Hans Tholstrup has asked that show that the average speed people remember his statement attained on the Princes Highway, for all vehicles, is 64km/h. prior to the event: "the 1991 Energy Challenge is a world first Although the "Solar Star" only and will be a demonstration run carried one person (as did all gathering knowledge for a scithe solar vehicles, the human powered vehicles and most of entific event in 1992". In other words, neither the formula for the other vehicles), it certainly gave the driver, Manfred calculating the winner, or the methods of measuring the variHerman, "a high degree of perThe winner of the 1991 Energy Challenge, the ous parameters have yet been sonal mobility" - the basis for ALSCO Linen Mistubishi Canter, leaves the finalised. "practicality" written into the starting gate at Newcastle. It was powered by compressed natural gas (CNG). regulations. Tholstrup feels that the event Controversial results in the 1991 Energy Challenge we'll need to allow for the vehicle payload to be two people with briefcases and sportsgear or overnight bags, say about 175-Z00kg all-up. The vehicle weight itself is a major consideration. By using the lightweight, high strength fibres that are now available, vehicle weight can be kept to a mm1mum. A reasonable empty weight for a 2-seater would be about 180-220kg. With a range of 70-80km, using about 70-80Wh/km, we'll need 4.96.4kWh of storage capacity. Assuming lead-acid batteries and an energy density of 40Wh/kg (conservative but realistic), we'll require 122.5-160kg of batteries. This gives us a total vehicle gross weight of 480-5 70kg, say around 525kg. A vehicle of this weight will use about 75Wh/km at an average 60km/ h; ie, at a rate of 4.5kW. Taking into MAY 1991 15 account inefficiencies and the need for extra power to accelerate and pass, we will need a ·motor of around 56kW power output. Using these figures as a rough ruleof-thumb, you can calculate the size of motor you will require if you change any of the factors involved. But beware the snowballing effect! If you increase the range required you'll increase the battery capacity required and the weight will go up, which will require a more powerful motor, which will require a larger battery capacity, and so on. These parameters lend themselves very well to computer modelling and a lot ofresearch time and effort can be saved by optimising the motor requirements for a particular vehicle application before investing time and money in hardware. Other performance factors that will affect the choice of a motor are: regenerative braking capability, rapid and smooth motor control and braking, high torque at all operating speeds, high propulsion efficiency over the typical driving cycle, high power to weight ratio , reliability and cost. 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