1.1 Introduction
The environmental arguments for electric propulsion become more compelling when they can be supported by an economic case that will appeal to the vehicle buyer. Here the current technology of electric and hybrid drive is reviewed in a way that shows the technical imperatives alongside the economic ones. After an analytical study of drive system comparisons for different vehicle categories, 'clean-sheet-design' integrated vehicle electric-drive systems are reviewed for small and medium cars and a concluding section encapsulates a procedure for optimizing motor, drive and batteries in the form of a power-pack solution.
A section on electric-drive fundamentals, establishing basic terminology, appears in the Introduction.
In the preface to the case study sections (5 and 6), contained in the second half of Section 4, the whole macro-economics of electric vehicles is discussed, with the wider aspects of the fuel infra-structure, as is a full analysis of competing electric-drive and energy-storage systems, for EVs.
1.2 Case for electric vehicles
1.2.1 ENVIRONMENTAL IMPERATIVE
Fig. 1.1 Life expectancy related to energy usage, as seen by the World Bank. ENERGY USAGE
The current world population of motor vehicles stands at 700 million, of which over 600 million are owned in G7 economies 1 . This number is set to increase to around 1000 million in the next ten years. The bulk of this growth is expected to occur in Second World countries where per capita income is reaching levels where car ownership is known to commence. This has two serious implications (Fig. 1.1): a large increase in the usage of hydrocarbon fuels and an increase in pollution to globally unsustainable levels. Much has been heard of the so-called Greenhouse Effect.
If carbon dioxide is on a scale of 1 as a greenhouse gas, methane is 25 and CFCs are 30 000- 50 000. Clearly the release of hydrocarbons and CFCs by man must be curtailed as soon as possible;
CO 2 is a different matter. If the quantity in the atmosphere was doubled from 20 to 40%, the temperature would increase by 5 o C and the sea level would rise by 1 meter. However, the additional plant activity would eliminate famine for millions in Africa , the Middle East and Asia . In scientific circles, the 'jury is still out' on carbon dioxide.
The problem emissions are those of carbon monoxide, sculpture dioxide, nitrous oxide and lead, not to mention solid particles from the exhausts of diesels. In all of these, man is competing with nature. The problem is that man's emissions are now set to reach levels which history shows have had dramatic consequences in nature. For example, in 1815, a volcano emitted 200 million tons of sculpture dioxide into the atmosphere. In 18l6 there was a cloud of sulphuric acid in the sky which blocked out the sun in the northern hemisphere for the whole of the summer. The temperature fell by 7°C and there were no crops. Every 2000 megawatt power station which runs on coal emits 150 000 tons of sulphur dioxide per annum. Acid rain destroys our forests and buildings in the northern hemisphere. Pollution on this scale in the southern hemisphere is unsustainable. Nitrous Oxide is emitted when nitrogen burns at 1500° C or above. This gas reaches high concentrations in cities and is converted by sunlight into photosynthesis smog, which is becoming a major health hazard worldwide. A change in the technology of motor transport could have the fastest impact on this problem as most vehicles are replaced every ten years.
1.2.2 ELECTRIC VEHICLES AS PRIMARY TRANSPORT
Consumers vote with their wallets! Electric vehicles will only have a healthy market based on a primary transport role using technology that achieves the performance of internal combustion engines. This means sources of energy other than batteries (Fig. 1.2). In reality we have a choice of IC engine, gas turbine and fuel cell, but how can we maintain performance whilst reducing pollution? The secret is to stop wasting the 72% of energy that currently goes out of the exhaust pipe or up from the radiator. The IC engine is currently operated with a fuel/air ratio of 14:1. This can be increased to 34:1 but the engine can no longer accelerate rapidly. Fortunately, this can be overcome by other means. The gas turbine is an efficient solution for large engines over 100 kW in commercial vehicles. Its performance is not as good as an IC engine's at lower powers, however, and fuel-cell electrics offer the best promise. Fuel cells are the technology of the future. There are many sorts but only one type of any immediate relevance to vehicles and this is the proton exchange membrane (PEM) cell. Using the Carnot cycle, this has a conversion efficiency limit of 83%.
Scientists can achieve 58% now and are predicting 70% within ten years. Fuel cells have many excellent qualities. Small units are efficient - especially at light load. New construction techniques are reducing costs all the time and £200/kW was already achievable in 1992 using a hydrogen/air mixture. The real problem is providing the fuel.
1.2.3 THE FUEL INFRASTRUCTURE
Current engines obtain their energy by burning hydrocarbons such as propane, methane, petrol, diesel and so on. However, hydrogen is the fuel of the future. What powers a Saturn 5 Moon Rocket? Coincidence, or sheer necessity? Liquid hydrogen has an energy density of 55 000 BTUs per pound compared to 19 000 BTUs per pound for petrol and 17 000 BTUs per pound for propane. The problem is obtaining large amounts of hydrogen efficiently from hydrocarbon fuels. The percentage of hydrogen directly contained in these fuels is small in energy terms. For example, methane (CH 4 ) has 17.5% of its energy in carbon and 25% in hydrogen. However, l. Petrol car: A journey of 68 miles each day consumes 2.5 gallons of fuel and takes 2 hours.
Amount of energy in fuel = 5.14 x 10 8 joules Thermal power = 71.3 kW Mechanical power = 20 kW average Efficiency = 28%
2. Battery electric car as secondary transport.
Power station efficiency 40% Electric car efficiency 80% OVERALL 32% CONCLUSION: Pollution is moved from car to power station. There is only an environmental return if the car’s performance is sacrificed or the power station is non-thermal and range/ performance is limited.
3. Hybrid car as primary transport.
Hydrocarbon to electricity Via lean burn petrol engine 45% Electricity to mechanical power 90% OVERALL 40.5% CONCLUSION: Pollution reduced by 55% and fuel consumption is 70% of petrol vehicle with performance/range as the petrol vehicle.
4. Fuel-cell electric car as primary transport.
Hydrocarbon to hydrogen conversion 80% Fuel-cell hydrogen to electricity 60% Electricity to mechanical power 90% OVERALL 43% (potential for 48% in 10 years) CONCLUSION: Pollution reduced by 90%; fuel consumption is 66% of petrol vehicle and performance/range is as petrol vehicle.
Fig. 1. 2 Some crude comparisons for fuel related to pollution.
there is now a solution to this problem, with a reforming process developed by Hydrogen Power Corporation/Engelhard called Thermal Catalytic Reforming. Put simply, it is the chemical process:
3Fe + 4H 2 O = Fe 3 O 4 + 4H 2 and Fe 3 O 4 + 2C = 3Fe + 2CO 2
The first process takes place with a catalyst at 130° C. The hydrogen is stored in a hydride tank until required. The iron is returned to a central facility for reduction by the second process. The main points about this cycle are that a high proportion of hydrocarbon heat energy is converted into hydrogen and that 1 kg of iron provides enough hydrogen for a small car to travel 6 km on a fuel cell.
IC engines and gas turbines run well on most hydrocarbons and hydrogen. Fuel cells need hydrogen. Hydrogen has to be used and stored safely. This could be achieved by reforming it on demand at fuel stations - the waste heat would be used to generate electricity to be pumped back into the national grid. The primary fuel could be any hydrocarbon such as petrol, diesel, methanol, propane or methane. The only constraint is that the fuel source must have low sulphur content so as not to poison the catalyst. In the UK , we have a head start called the Natural Gas Grid. This is likely to become of critical importance for energy distribution, removing the need to distribute petrol and diesel by road. To satisfy future transport needs, we retain our 'fuel' stations as the means of distribution. This brings us to the problem of on-board hydrogen storage.
Iron titanium hydride has long been known as a storage medium but one would need 500 kg to store 10 liters of hydrogen, at a cost of £3000 in 1992. The gas is stored in a standard propane tank filled with this material. If the tank is ruptured, the gas is given off slowly because of its absorption in the hydride. In the USA experiments are also taking place with cryogenic storage which is potentially cheaper and lighter. The overall distribution scheme is illustrated in Fig. 1.3. To summarize, the benefits of a change to hybrid/fuel-cell electric vehicles are: (i) engineering is practical; (ii) performance is acceptable to the consumer; (iii) it reduces fuel consumption; (iv) it reduces pollution, especially Nitrous oxide; (v) it reduces dependence on imported oil; (vi) it can be achieved quickly; (vii) it can be achieved at sensible cost; (viii) it prevents increased demand for oil; (ix) it fits in with the existing fuel infrastructure and (x) it solves the pollution problem in relation to projected pollution levels, not existing ones - the prime cause of the catalytic converter being ineffective.
1.2.4 FUEL-CELL ELECTRIC VEHICLE
This vehicle category, Fig. 1.4, will use a fuel cell to provide the motive power for the average power requirement and utilize a booster battery to provide the peak power for acceleration. Hydrogen would be stored in a tank full of metal hydride powder, or cryogenically. This system provides enough waste heat for cabin heating purposes. The fuel cell can recharge the battery when the vehicle is not in use. If the vehicle has an AC drive, it is possible for it to generate electricity for supply to portable tools, a house, or injection into the national grid. Fuel cells should reduce emission levels by a factor of 10, compared with IC engines on 14:1 air: fuel mixture.
1.2.5 CHARACTERISTICS OF FUEL CELLS
What is a fuel cell? It is an electrochemical cell which converts fuel gas and oxidant into electricity and water plus waste heat (see Section 4). The PEM cell has graphite electrodes with a layer of membrane sandwiched in between, plus gas-tight seals. Each cell is about 6 mm thick and produces 1 V off-load and 0.7 V on-load, at a current of around 250 amps. Consequently a fuel cell for a 15
Fig. 1.3 Hydrogen distribution system.
Fig. 1.4 Fuel-cell electric vehicle.
kW average power would produce about 60-70 V DC at 250 amps. In size it would be about 200 mm square and about 600 mm long. The cell operates at a temperature of 80 ° C. When cold, it can give 50% power instantly and full power after about 3 minutes. The units exhibit very long life.
The problem until recently has been seal life when operated on air as opposed to oxygen. New materials have solved this problem. Output doubles when pure oxygen is used. Fuel cells do not like pollutants such as carbon monoxide in the source gases. Gas is normally injected at 0.66 atmospheres into the stack. The main challenge now is to refine the design so as to optimize the cost relative to performance. This will take time because the effort deployed at this time is small in relation to the effort put into batteries or other fuel-cell types. There is a very real case for a major multinational effort to train scientists and engineers in this technology in the short term, and to reduce the time to introduction on a large scale.
1.2.6 THE ROLE OF BATTERIES
Batteries have been with us for at least 150 years and have two main problems: they are heavy and they do not like repeated deep discharge. Batteries which are deep cycled, irrespective of the technology, deteriorate in performance with age. So the question must be asked 'what can batteries do well?'. The answer is to provide limited performance in deep discharge, or alternatively, much better performance as a provider of peak power for hybrid and fuel-cell vehicles.
Much work is under way on high temperature cells. These are unlikely to meet cost or weight constraints of primary transport applications. The best high temperature batteries can offer 100 Wh/kg. Overall, fuel cells already give 300 Wh/kg and this can be improved with development.
What is needed is a battery with different capabilities to normal car starter batteries, namely very low internal resistance, long life, excellent gas recombination, room temperature operation, totally sealed, compact construction, reasonable deep discharge life as well as being physically robust.
The battery which satisfies the above criteria is the lead-acid foil battery, as manufactured by Hawker Siddeley. This type of construction has replaced nickel-cadmium pocket batteries on many aircraft. In particular the lead-acid foil battery retains far more charge from regeneration than conventional designs and can be charged and discharged rapidly. However, there is a trick to achieving this. Most batteries are made up of 'rectangular' arrays of cells so it is no wonder that the temperature of the cells varies with position in the stack. To charge a battery quickly it is vital to keep the cells at an even temperature. Consequently it is necessary to liquid cool the cells so as to obtain best performance and long life. Other points worthy of note are that batteries work best when hot; 40°C is ideal for lead-acid. The battery electrolyte is just the place to dump waste heat from the motor/engine/fuel cell.
Nickel-cadmium batteries offer better performance than lead-acid but are double the cost per Wh of storage at present and sealed versions are limited to 10 Ah but larger units are under development. The best nickel-cadmium units available at present are the SAFT STM/STH series.
Sealed lead-acid and aqueous nickel-cadmium cells have peak power in W/kg of 90 and 180, with Wh/kg values being 35 and 55 respectively.
In terms of safety, long series strings of aqueous batteries are not a good idea. The leakage from tracking is high and they are very dangerous to work on. Consequently batteries should be of sealed construction with no more than 110 V in a single string. Ideally, the maximum voltage should be 220 V DC, that is +/ - 110 V to ground arranged as two separate strings with a centre tap, so that no more than 110 V appears on a connector, with respect to ground, Fig. 1.5.
Fig. 1.5 Battery connections and earthing. There is an opening in the market place for a low cost 2 pole, 220 V, 300 A remote-control circuit breaker to act as battery isolator with 5 kA short-circuit capacity. However, there is a problem with earthing the centre tap of the battery as one may need an isolating transformer in the battery charger. Consequently, in many of the new schemes proposed in the USA , a different route is implemented which is used in trolley buses - the all-insulated system. In most of these schemes, large capacity batteries are used (15-30 kWh) at a typical nominal voltage of 300 V. This will vary from 250 V fully discharged to 375 V at the end of charging.
The electrical system is fully insulated from earth. During charging, the mains supply can be either centre tap ground or one-end ground. In the centre tap ground (typical USA situation, with 110/0/110) the potential of the vehicle electrics is balanced to earth. When one end is earth (typical European situation) the potential of the vehicle electrics will move up and down at the supply frequency with respect to ground and there is the prospect of earth leakage current through any capacitance to earth of the vehicle electrical system. However, this is very small, usually because the tyres isolate the vehicle. However, when charging it would be desirable to ground the vehicle body to prevent any shocks from people touching the vehicle and standing on a grounded surface.
1.2.7 ELECTRIC VEHICLE SPECIFICATIONS
From the previous considerations one can now start the task of specifying EV capability/ performance trade-offs. Polaron believe EVs will be partitioned as shown in Fig. 1.6. This does not pretend to be an exhaustive list but to show the range and scale of requirements to be provided for. The most interesting observation is that in the mass market, 30-150 kW, a solution is possible using just two sizes of drive, 45 and 75 kW. To complement the drives, motors are required of two speed ratings for each size, say 5000 rpm where compatibility with a prime mover is required, and 12 000 rpm for the direct drive series hybrid/pure electric case.
1.2.8 HYBRID VEHICLE EXAMPLES
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Power GVW Engine Motor; Motor Turbo Application Rating type rating alternator
Below Less than None Brush DC Up to None Straight battery 40 kW 2 tons 40 KW electric van or car 40 kW 2 ton IC Brushless DC 1 x 45 kW None Parallel hybrid
-150 kW 5000 rpm family car 2 ton GT Brushless DC 1 x 75 kW 1 x 100 kW Parallel hybrid 12€000 rpm 60€000 rpm performance saloon 3 ton IC Brushless DC 1 x 75 kW None Parallel hybrid 5000 rpm 1 ton truck 5 ton GT Brushless DC 2 x 45 kW 1 x 100 kW Series hybrid 12€000 rpm 60€000 rpm 2 ton truck 7 ton GT Brushless DC 2 x 75 kW 1 x 150 KW Series hybrid 12€000 rpm 50€000 rpm single deck bus 150 kW 10 ton GT Switched 1 x rating 1 x rating Heavy traction to reluctance 5000 rpm 50€000 rpm and road haulage motor at 150 kW 1 MW 25€000 rpm Series hybrid 40 tons at 1 MW configuration
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Fig. 1.6 Short-term battery electric and hybrid vehicles.
It is now proposed to have a look at two cases (a) 45 kW parallel hybrid vehicle; (b) 90 kW series hybrid vehicle, as in (Fig. 1.7). The 45 kW parallel hybrid vehicle consists of, typically, a small engine driving through a motor directly into the differential gear and hence to the road wheels. Minimization of weight is the key issue on such a design along with low rolling resistance and low drag. At 60 mph a good design can expect to draw 8 kW to keep going on a flat level road. The vehicle would be fitted with an engine rated to supply about one-third of the peak requirement, that is 15 kW plus an allowance for air conditioning if relevant. The motor has to deliver up to 45 kW using energy stored in batteries. This can be done either by a constant torque motor operating via a gearbox or a constant power motor operating with only two gears or without a gearbox. The latter is rapidly becoming the standard for EVs using front wheel drive.
The vehicle uses the battery to provide peak acceleration power for overtaking, hill climbing and so on. On the flat a 0-60 mph acceleration time of around 12 seconds would be typical for this class of vehicle and a top speed of perhaps 80 mph, where permitted; the engine is started when the road speed exceeds 20 mph and then clutched into the motor. The engine then charges the batteries as well as satisfying the average demand of the car. During acceleration the electric drive and the engine work together to provide peak acceleration. It is in the cruise condition that optimum efficiency is required. Consequently more sophisticated designs use 3 way clutch units so that the motor can be mechanically disconnected when the battery is fully charged and only switched back in for acceleration. In this condition attention must also be paid to the minimization of rolling resistance and windage losses (Figs 1.6 and 1.8(a)).
The series hybrid vehicle corresponds to a high performance sports saloon. A 0-60 mph time of 7 seconds and a top speed of 120 mph could be expected (Figs 1.7 and 1.8(b)). The main power source would be a gas turbine which would operate through a PWM inverter stage to feed 300- 500 V DC into the main bus. There are two separate drives, each driving a rear wheel of the vehicle. To reduce weight, the motors would be designed for 12 000 rpm and gearboxes employed to reduce the speed to the road wheels - about 1800 rpm at 120 mph. The gas turbine may operate over a 2:1 speed range to give good efficiency. Specific fuel consumption is doubled at 15 kW compared to 100 kW. However, overall consumption would still be that of a 'Mini', with emissions to match. The peak power for acceleration would come from batteries - probably nickel-cadmium in this case, where cost pressures are not so demanding.
Fig. 1.7 A 45 kW parallel hybrid and 90 kW series hybrid.
Fig. 1.8 Torque-speed curves for 45 kW vehicle (a) and each motor.
Fig. 1.9 Fuel-cell power conversion.
1.2.9 ELECTRICAL SYSTEM DESIGN CHALLENGE
What are the design problems for the electrical system? The first one is cost. Unless the final product is attractive to the consumer, we do not have a market. Where are we now? For 1000 off systems at 45 kW, a brushless DC motor would cost £1000, a controller £2000, and a battery £2000 (lead-acid). These 1992 prices will reduce with mass production. The second design challenge is one of methodology. Electric vehicles have been traditionally built by placing motor and batteries then spreading the electrical system over the vehicle. This needs to change. Polaron would like to suggest a modular approach to the problem whereby sealed batteries and controller power electronics are in one unit and the motor is in fact the second. The third design challenge is one of compatibility. Low performance vehicles can be built with 110 V electrical systems. However, as the power increases this is not practical. But both fuel cells and batteries are low voltage heavy current devices - how can this conflict be addressed? The solution is to use power conversion. In Fig. 1.9 a 100 stage fuel cell is integrated with a 216 V battery to give a stabilized 300 V DC rail. The motor and controller are then built at 300 V where the currents are significantly reduced on the 100 V system. As the power level rises, voltages up to 500 V DC can be anticipated. However, when the power conversion is switched off the highest voltage will be the battery voltage. This additional power conversion will be needed for another reason. If vehicles are equipped with small booster batteries for acceleration, the DC link voltage will change significantly according to load conditions. The power conversion provides a means of stabilizing for this variation.
1.2.10 MOTOR TYPES AND LOCATIONS (FIG. 1.10)
Which is the best type of motor? Answer - the cheapest. Which is the cheapest motor? Answer - the lightest. Which is the lightest motor? Answer - the most efficient. On this criteria, there is no doubt that a permanent magnet brushless DC motor would sweep the board. However, our enthusiasm must be tempered by two other considerations, cost of materials and controller costs.
The factors affecting selection are covered in Section 1.3.
1.2.11 CHOPPER CONTROLLER FOR A 45 kW MOTOR
Figure 1.11 illustrates a typical pure battery electric vehicle scheme which could also be used in hybrid mode with an engine if required. The motor is a shunt field unit such as the Nelco Nexus 2 unit used in many industrial EVs. This machine is a 4 pole motor with interpoles and operates at a maximum voltage of 200 V DC. The field supply is typically 30 amps for maximum torque.
The controller consists of a 2 quadrant chopper with a switch capacity of 400 amps. An electromechanical contactor shorts out the positive chopper switch in cruise mode for maximum efficiency. The chopper is fitted with input RF filtering and pre-charge to extend contactor life.
The chopper switches at 16 kHz and the output contains a small L/C filter to remove the dv/dt from the machine armature. A Hall effect DCCT measures the armature current for the control system.
Fig. 1.10 Motor specifications. Nelco electric 34 kW brush motor specification
In the power supply area, there are four components: first is the battery charger, in this case a CUK converter, or a boost/buck chopper is also a possibility to make the mains current look like a sine wave for ensuring IEC555 compliance. Control of battery charging conditions is one of the most important considerations in extending battery life in deep discharge. For lead-acid batteries the level of float voltage is critical as well as maintaining cell temperature. The battery charger could incorporate a 20 kHz isolating transformer if costs permit. Experiments are under way with inductive power transfer which isolates the car and makes it necessary to plug in for charging. Another possibility is an automatic self-aligning connector which the car drives into when parking. The next consideration is the auxiliary 13.6 V battery supply. The vehicle seems likely to retain a separate 12 V battery for lighting and control functions. A 300 W DC/DC converter will satisfy this requirement. The third consideration is the control system power.
This is a small (20 W) DC/DC converter which provides the control power for the chopper. It is likely to be incorporated with the main control PCB and could also be supplied from the 13.6 V battery. The final factor is the field controller. This is a 4 quadrant chopper which provides the motor field supply. It has to be able to reverse the current so that the motor can reverse without contactors in the armature circuit. If the motor has a tachometer fitted, this may be used for braking control and blending with electromechanical brakes. The important issue with this controller is that the power switching is contained in a single unit so that all the DC components are kept in one place. This is important for another reason to meet IEC555 RF interference legislation. Therefore all insulated systems will require an isolated conductive casing which can be connected to vehicle chassis.
1.2.12 CONTROLLER FOR A 45 kW AC MOTOR (BRUSHLESS DC OR INDUCTION)
This is illustrated in Fig. 1.12. The drive consists of a 3 phase PWM Drive which feeds the 3 phase motor. The beauty of this arrangement is that the motor can be disconnected and the mains fed to the inverter arms to give a high power battery charger, by phase locking the PWM to the mains.
Fig. 1.11 Controller for 34 kW shunt field DC motor.
An alternative to this arrangement is for the inverter to put power back into the mains. In case of fault, three alternistors provide current limit protection. In the brushless DC case, the motor permanent magnets provide 50% of the flux and the remainder comes from a 50 amp circulating current Id at right angles to the torque producing component Iq.
The inverter is constructed using 300 amp IBGT phase leg packages which minimize the inductance between transistors and associated bypass diodes. The inverter output is filtered by 6 x 10 mH capacitors plus 3 x 5 mH inductors. This reduces the 18 kHz carrier ripple current in the motor to about 20 AP/P. There is a real time digital signal processor (DSP) which performs vector control using state space techniques and this includes 3rd harmonic injection to maximize the inverter output voltage. Comprehensive overload protection is fitted. The inverter demand is a torque signal and a speed feedback is provided for the vehicle builder to close the speed loop.
Both signals are PWM format (10-90%) on a 400 Hz carrier. The drive can be adapted for induction motor control but this is not so efficient, as explained in the motor section below.
1.2.13 TURBO ALTERNATOR SYSTEM FOR GAS TURBINES
Figure 1.13 illustrates a turbo alternator scheme for gas turbines. This scheme has two purposes: it starts the turbine, and provides a stabilized DC link voltage for a 2:1 change in turbine speed and changes in DC link current from no-load to full-load. The alternator itself is the result of many years' development in high speed gas compressors. It is a 4 pole unit which allows iron losses to be kept low and in particular the tooth tip temperature reasonable whilst still using silicon steel laminations (2 pole permanent magnet alternators are potential fireballs!). The magnet material is samarium cobalt with a carbon fiber or Kevlar sleeve. At these speeds, one needs every bit of strength possible. The magnets are capable of operation at 150° C. The use of metallic magnets is not a problem here because the weight is small. Hall sensors are fitted for machine timing during starting and voltage control purposes. A small L/C filter limits the amplitude of the carrier ripple on the alternator windings.
Fig. 1.12 Electric vehicle 45 kW inverter.
1.2.14 MODULAR SYSTEMS
From the foregoing considerations, it will be apparent that the motor car of the future needs power electronics to be viable. Fortunately, we now have the technology to satisfy the most demanding applications. There may be some rivalry between different types of power switches but cost will be the final judge. A manufacturer who constructs the power electronics as an all-insulated system in a single module permits module exchange as the first means of maintenance. Liquid cooling also makes sense. It can cool the motor, warm/cool the sealed batteries and provide power steering at the same time. This concept will make it possible to convert existing chassis as well as develop new ones, thus enabling product to be brought to market quickly. Standard electronics packages are the only way to achieve the unit costs necessary for product acceptance in the market.
Interchangeable batteries will make it possible for maximum vehicle utilization in intensive duty applications, such as taxis and delivery vehicles. This method of construction also opens the door to new methods of financing EVs; for example, the user buys vehicle then rents battery/power electronics.
1.3 Selecting EV motor type for particular vehicle application
1.3.1 INTRODUCTION
Fig. 1.13 Turbo alternator.
Motor and drive characteristics are selected here for three different applications: an electric scooter; a two-seater electric car and a heavy goods vehicle, from four motor technologies: brushed DC motor, induction motor, permanent-magnet brushless DC and switched reluctance motor 2 . Any of the four machines could satisfy any application. This is not a battle of 'being able to do it', it is a battle to do it in the most cost-effective manner. There are two schools of thought regarding EVs - group A believe they should create protected subsidized markets for environmental reasons and are not too concerned with cost. Group B realize that until this technology can compete with piston engines in terms of performance and cost there will be no significant competition, hence no major market share. Polaron are putting their money on group B. What is clear is that the economics will come right at lower powers first, then work upwards. Another fact is that a market needs to be established before custom designs can be justified and the most immediate need is for conversion technology for existing vehicle platforms.
Fig. 1.14 Efficiency map and Gemini motor.
1.3.2 BRUSHED DC MOTOR
This consists of a stationary field system and rotating armature/brush-gear commutation system.
The field can be series or shunt wound depending on the required characteristics. The technology is well established with more than a century and half of development. The main problem is one of weight compared with alternative technologies, consequently Polaron believe DC is best at lower powers overall, due to the built-in commutation scheme. As the power level rises many problems become significant: commutation limited to 200 Hz for high speed operation; problems with commutator contamination; significant levels of RF interference; brush life limitations and cooling/ insulation life limitations. Polaron's Nelco division has made these machines for many years and has introduced a new design to help overcome some of the problems. The so-called Gemini series consists of an armature with a face commutator at both ends of the armature. This permits two independent windings which may be connected in series or parallel. Improvements in the torque speed curve are seen in Fig. 1.14, while Fig. 1.15 shows a recently developed controller. While existing controllers have single quadrant choppers with contactors for reversing and braking, and field control is effected by a separate chopper unit, Polaron feel such a design gives limited overall performance and is better replaced by the arrangement shown. Brushed DC motors have a role in applications below 45 kW but, if power rises above this figure, mechanical considerations such as the removal of heat from the rotor become more important. There are also factors to take into account in terms of efficiency when partially loaded. In many of these respects, the use of brushless DC motors could provide a better alternative. These have a number of features acting in their favor, including high efficiency in the cruise mode and a readily adjustable field, plus the practical benefits of a more easily made rotor.
1.3.3 BRUSHLESS DC MOTOR
The term 'brushless DC motor', however, is a misnomer. More accurately it should be described as an AC synchronous motor with rotor position feedback providing the characteristics of a DC shunt motor when looking at the DC bus. It is mechanically different from the brushed DC motor in that there is no commutator and the rotor is made up of laminations with a series of discrete permanent magnets inserted into the periphery. In this type of machine, the field system is provided by the combined effects of the permanent magnets and armature reaction from vector control.
Similar in principle to the synchronous motor, the rotor of this machine is fitted with permanent magnets which lock on to a rotating magnetic field produced by the stator. The rotating field has to be generated by an alternating current and in order to vary the speed, the frequency of the supply must be changed. This means that more complex controllers based on inverter technology have to be used.
Induction motors are used by many US battery-electric cars. The rotors are cooled with internal oil sprays which also lubricate the speed reducer. Operation at 12 000 rpm is common to minimize the torque and some designs operate under vacuum to reduce the noise. The one good point is that these motors are reasonably efficient under average cruise conditions (8000 rpm, 1/3 FLT). Polaron's view is their use will be short lived. Induction motors always have lagging power factors which cause significant switching losses in the inverter, and vector control is complex.
Fig. 1.15 Integral 4-quadrant chopper.
1.3.4 SWITCHED RELUCTANCE MOTORS
SRMs, Fig. 1.16, use controlled magnetic attraction in the 6/4 arrangement to produce torque.
Existing SR drives are unipolar, in that the voltages applied to windings are of only one polarity.
This was done to avoid shoot through problems in the power devices of the inverter. The 6/4 machine has a torque/speed curve similar to a DC series motor with a 4:1 constant power operating region. Torque ripple can be serious at low speed (20%).
In an attempt to improve the SR drive, two groups have made significant contributions: SR drives have worked with ERA Drives Club in developing the 8/12 SR motor, with much smoother operation; a University of Newcastle upon Tyne company, Mecrow, have postulated a bipolar switched reluctance machine using wave windings. This doubles copper utilization and increases output torque. It also uses a standard 3 phase bridge converter. Existing SR motors are both heavier and less efficient than PM BDC machines, for example a 45 kW unit (3.5:1 constant power/5000 rpm) would weigh 65 kg and have an efficiency of 94%. The new bipolar design should give a motor which is close to PM BDC in terms of weight (45 kg). However, in terms of efficiency, the BDC has the edge, both in the machine and the inverter, because it operates with a leading power factor under constant power conditions. However, SR motors are excellent for use in hostile environments and it is Polaron's expectation that they will be successful in heavy traction, where magnet cost may preclude brushless DC.
1.3.5 ELECTRIC MOTORCYCLE
An electric motorcycle is an interesting problem for electric drives. The ubiquitous 'Honda 50', an industry standard, is typical of personal transport in countries with large populations.
The petrol machine weighs 70 kg and has an engine capable of about 5.5 bhp. Honda have developed an electric version where the engine is exchanged for an electric motor and lead- acid batteries. Honda's solution weighs 110 kg and has a range of 60 km; it is offered in prototype quantities at £2500 ($3500), 1996 prices. Some elementary modeling shows that the key problem is battery weight - especially using lead-acid. To minimize this requires good efficiency for both motor and driveline. The standard driveline from engine to wheel is about 65% efficient. A better solution is to use a low speed motor with direct chain drive onto the rear wheel. This solution offers a driveline efficiency of 90%. However, we need a machine to give constant power from 700 to 1500 rpm. Cruising power equates to 1.5 bhp at 40 km/h and 5 bhp at 60 km/h. Vital in achieving good rolling resistance figures is to use large diameter tyres of, say, 24 inches.
Fig. 1.16 Switched reluctance motor.
It is assumed that sealed batteries are to be used and consequently a battery voltage of 96 V was chosen to optimize the efficiency of motor and controller and particularly with an eye to controller cost. 200 V MOSFETS are near optimal at 100 V DC. A battery of 15 Ah 96 V weighs 40 kg (for comparison 24 V 60 Ah weighs 35 kg). In lead-acid 36 Wh/kg is achieved, while for comparison nickel hydride cells could offer 80 cells x 1.2 V x 25 Ah in a weight of 30 kg. The motor has to deliver a torque of about 40 Nm maximum and consequently a pancake-type design was chosen. Induction motors were rejected due to low efficiency and large mass for this duty. The four practical contenders are: permanent magnet brushless DC; permanent magnet DC brush pancake motor; DC series motor or switched reluctance motor. A tabulated comparison at Fig. 1.17(a) compares results. As can be seen, the permanent magnet brushless DC motor is the optimum performer at the two key cruise conditions. It has been estimated that with regenerative braking and flat terrain, a range of 70 km could be achieved with a 96 V 15 Ah lead-acid battery. The 25 Ah nickel hydride pack could give 120 km. However, 70 km is quite adequate for average daily use.
1.3.6 SMALL CAR
The small electric car is in the Mini or Fiat 500 class. Such a vehicle would weigh 750 kg and accelerate from 0 to 50 mph (80 km/h) in 12 seconds and have a range of 80 km with lead-acid batteries. The motor power would be 20 kW peak. As originally there were only aqueous batteries available, battery voltage was limited to 120 V DC by the tracking that took place across the terminals of the batteries due to electrolyte leakage. Two battery technologies were available: lead-acid and nickel-cadmium and vehicles were designed with efficiency = 25%, that is 188 kg of batteries if efficiency is expressed as battery mass/gross vehicle mass (for lead-acid 60 Ah 120 V 7.2 kWh and for nickel-cadmium 85 Ah 120 V 9.9 kWh).
Single quadrant MOSFET choppers were developed by Curtis and others to supply DC brushed series motors. The main advantage of this system was low cost (for example, lead-acid battery
£900 in 1996; quadrant chopper £500; motor DC series £750). However, the apparent cheapness of this system is deceptive because: (a) fitting regeneration can raise the battery voltage to 150 V - an unsustainable level for some choppers - consequently friction braking was often used; (b) a separate battery charger was required. More recently sealed battery systems have become available and batteries of around 200 V are possible in two technologies, lead-acid foil and nickel hydride.
These batteries are used with 600 V IGBT transistors which can operate at voltages up to 350 V DC. Battery capacity becomes limited if other services such as cabin temperature control/lighting/ battery thermal management are taken into consideration. A small engine driven generator transforms this problem and it is perhaps worth noting Honda have achieved full CARB approval for their small lean burn carburetor engines with the discovery that needle jet alignment is critical to emissions control and negates the need for catalytic converters.
All motor technologies are viable at 196 V; however, the practical consideration is that inverters are more costly than choppers which accounts for the popularity of DC brushed motors/choppers.
To counteract the inverter cost premium, the electronically commutated machines have been designed for 12 000 rpm, to reduce the motor torque (DC brush machine 20 kW at 5000 rpm; other types 20 kW at 12 000 rpm). Another benefit of the higher transistor voltage capability is that the inverters/choppers can function as battery chargers direct off 220/240 V without additional equipment. High rate charging is possible where the supply permits. All electronically commutated machines provide regeneration. The motor comparison is tabulated at Fig. 1.17(b). All the machines deliver constant power (20 kW) over a 4:1 speed range, making gear changing unnecessary. The induction/brushless motors are assumed to use vector control.
Fig. 1.17 Motor comparisons for three vehicle categories (the four motor types are also discussed in Section 4).
1.3.7 HGV
The heavy goods vehicle is an articulated truck which weighs 40 tons. Often omitted from clean air schemes on the grounds of low numbers they travel intercontinental distances every year and are major emitters of NO x and solid particles. Their presence is felt where there are congested urban motorways, and each one typically deposits a dustbin-full of carbon alone into the atmosphere every day, the industry declining to collect and dispose of this material! What is the solution? Use hybrid drivelines based on gas turbine technology; these vehicles would be series hybrids.
A gas turbine/alternator/transistor active rectifier, Fig. 1.18, provides a fixed DC link of 500 V.
This is backed up by a battery plus DC/DC converter. A battery of 220 V (totally insulated) is used for safety. High quality thermal management would be vital to ensure long battery life; 2 tons of lead-acid units would be needed (144 ´ 6 V ´ 110 Ah) to be able to draw 400 bhp of peak power.
It is likely that capital cost would be offset by fuel cost savings. Another benefit is that the gas turbine can be multifuel and operation from LNG could be especially beneficial. The drive wheels are typically 1 meter in diameter giving 683 rpm at 80 mph. Usually there are 3:1 hub reductions in the wheels and a 2:1 ratio in the rear axle, giving a motor top speed of 4000 rpm. Translated into torque speed this means 2866 Nm at 1000 rpm, falling to 716 Nm at 4000 rpm. All motors are viable at this power; however, two factors dominate: (a) low cost and (b) low maintenance. DC brushed motors with 3000 hour brush life are unlikely contenders! PM brushless DC is unlikely on cost grounds, requiring 36 kg of magnets for 2900 Nm of torque. Both induction motors and switched reluctance are viable contenders but switched reluctance wins on efficiency and weight.
The contenders are tabulated at Fig. 1.17(c).
In the above review of four motor technologies for three vehicle categories, there is no clear winner under all situations but a range of technologies is evident which are optimal under specific conditions. Continuing development should improve the electronically commutated machines especially brushless DC and switched reluctance types. The relative success of these machines will be determined by improvements in magnet technology, especially plastic magnets, and cost reduction with volume of usage. On the device front, development is approaching a near ideal with 1/2 micron line width insulated gate bipolar transistors (40 kHz switching/l.5 V VCE saturated) but reduction in packaging cost must be the next major goal.
Fig. 1.18 Gas turbine technology.
1.4 Inverter technology
Inverters are one area where progress is being made in just about every area 3 : silicon, packaging, control, processors and transducers. The task is to find a way down the learning curve as quickly as possible. Polaron believe the lowest cost will come from packaging motor and inverter as a single unit. The major development this year is that of reliable wire bond packaging for high power silicon. New wire bond materials can offer a fatigue life of up to 10 million full current cycles with a Delta T of 25°C across the wire bond. The shorter pins on the package coupled with liquid cooling give best results. In Fig. 1.19, note that the temperature differential is the temperature difference between the connection pins and the baseplate in °C.
Fig. 1.19 Econopack 3 wire bonded package (left) and typical lead frame packaging (right). Traditional insulated packaging uses lead frame construction with wire bonding to the chip to give fuse protection. This technique has a guaranteed life of 25 000 full current cycles but package cost is high. Connections are by bolted joints. Wire bonded packaging uses a plastic pin frame which is wire bonded to the die. This construction technique is standard for low power six packs (complete 3 phase bridge on a chip as used in air conditioners). What is new is the capability to offer this packaging in a high power device. In USA designers seem to prefer MOS gated thyristor MCTs. In Europe and the Far East insulated gate bipolar transistors (IGBTs) are popular. In fact both devices are converging on a common specification of: (a) maximum volt amp product per unit area of silicon; (b) saturation voltage of 1.5 V at Ic max; (c) high frequency forced commutation capability.
Fig. 1.20 Silicon cost for 70 kW drive. Currently MCTs have the better saturation but IGBTs have better commutation. In the coming years makers will see better saturation figures for IGBTs and even lower switching energies. This is the result of smaller line widths and thinner silicon device structures. Currently a 1200 V, 100 A six pack can switch 600 V DC at up to 16 kHz (V ce Sat) 2.2 V/100 A, E on 18 mJ E off 14 mJ, cycle 32 mJ. The 600 V, 200 A six packs are now available as samples. Since the chips use non-punch through (NPT) technology, they may be connected in parallel without matching due to the inherent equalization characteristics of the die. Many vendors offer IGBTs in lead frame packaging, but this construction is not cost effective for electric vehicles. Devices of 1000 A, 1200 V are available.
Intelligent power modules are also available, for example Semikron, SKIpacks Fuji, Toshiba and Mitsubishi. These integrate gate control with the power devices and have protection integral with the device. The cost of this approach is high at present; it is wire bonded packaging that offers the lowest device costs. A 1200 V, 100 A six pack is around 100 dollars (1997 price), Fig. 1.20.
Fundamental to the cost equation is that inverter cost is proportional to motor current. Electric and hybrid vehicles are tending to use drives of 70 kW because the vehicles weigh 1500 kg. What Fig. 1.21 illustrates is that the induction machine requires almost 1.8 times the current capacity of the brushless DC inverter for 3.5:1 constant power speed range. Typical circuit diagrams are illustrated in Figs 1.22a and b. The view in (a) is a typical induction motor drive with just six switches. This drive will need 3-off 600 V, 200 A six packs in parallel. Under US conditions, cars seldom require 70 kW for more than 10 seconds during overtaking. With current designs of battery peak power falls to 55 kW at minimum battery voltage limited by internal resistance (typically 1.75 V/cell for lead-acid). The view in (b) is a brushless DC drive using a double chopper circuit.
Essentially a 300 V battery is increased to 600 V link with a 460 V motor. This inverter can be built with just two 1200 V, 100 A six packs. With oil at 40°C the package can operate at 140 A continuous. It will operate at 96 A RMS, 136 A peak on a 50% duty cycle for short periods. The brake resistor in the circuit prevents battery overcharging during regeneration. If the battery is overcharged its life may be reduced. In flat terrain the friction brakes may fulfill this role; however, in steep terrain the energy per 1000 meters height is 14.7 million joules or about 2400°C on average family car disc brakes. Electric cars do not have engine braking.
There are many benefits of using the high voltage circuit. First the motor current is 100 A or less. This makes the motor easier to wind and permits the use of printed circuit technology in the inverter. Second there is a major control benefit. An optimum control strategy is to use current-source PWM at low speed and voltage-source square wave at high speed. If a 300 V battery is used the DC link voltage is kept low until the motor voltage exceeds the DC link and then increased as the speed and voltage rise. This strategy reduces PWM carrier losses and permits better efficiency along the no-load-line of the vehicle. Use of printed circuit technology not only assists automatic assembly but also reduces EMC. EMC compliance is not too difficult in steel body cars but is much more of a challenge in composite structure vehicles. Having considered the inverter core some thoughts concerning the peripheral components are needed. Clearly this configuration requires an L/C filter for the chopper and an output filter for the motor to limit dv/dt on the motor windings.
The dimensions of the L/C filter are determined by two factors: permitted inductor current ripple and permitted capacitor current ripple.
Fig. 1.21 Base speed/max speed operating points for induction and brushless DC motors. Polaron prefer to split the inductor to give good common mode rejection with respect to the battery. A value of 100 micro-henries is suitable with a capacitance of 1250 microfarads. The inductors are made as air-core units with 10 mm micro-bore copper pipe. The turns may be close spaced by insulating the outside of the copper with epoxy powder coat paint. The spacing can be reduced further by using X extrusion copper which permits bending in two planes. The capacitors are ripple current dominated. With 100 A of motor current a capacitor that can handle 100 A peaks (30 A RMS) at temperatures of up to 50°C so oil immersion is the requirement. Polaron Group have chosen electrolytic units of 470 microfarads, 385 V, arranged five in parallel in series with five more in parallel. The cans are of the solder mount type choosing five more pins for mechanical strength in 40 mm ´ 50 mm cans.
The inductors for the dv/dt units are more challenging. An inductance of 10/20 microhenries is needed but it is advantageous if the inductance is more at low current. Consequently this application favors a cored inductor with low permeability iron powder and oil immersed litz wire winding.
The core needs to have a molded bobbin to provide inter-turn insulation for the litz wire and as a casting mould for the core material. A final point is that if one were prepared to hand wind the motor Polaron believe it would be possible to eliminate the dv/dt inductors by the use of an insulation extrusion to control the ground capacitance of the winding - the capacitance/inductance characteristic as a uniform transmission line.
In summary, for motors, Polaron believe brushless DC will prove to be the dominant technology especially for hybrid vehicles where efficiency at peak power matters. Machines for 12 000 rpm are well established. Successful operation of 70 kW machines at 20 000 rpm has been demonstrated and 150 kW machines are in development. Currently, higher speeds present a number of technical/ cost obstacles (there are successful company designs operating to 150 000 rpm but not using low cost methods). Improvements in materials could radically change this in the next few years. In the inverter area, cost is proportional to current and the brushless DC motor requires 60% of the current of the induction motor to achieve a 3.5:1 constant power operating envelope. The double chopper circuit offers many benefits over the single bridge solution and is cheaper to construct.
For 70 kW a 600 V DC link is best. The use of a controlled DC link becomes even more important in hybrid vehicles where smaller batteries lead to greater voltage variations between peak motoring and peak regeneration. The use of high voltage is not a safety hazard so long as the motor and inverter are contained in a single enclosure where the active components are not accessible. Oil immersed construction offers the lowest temperature rises and the best component reliability, especially for the silicon and filter capacitors. This method of construction permits complete subsystem testing before mounting in a vehicle.
Fig. 1.22 (a) Single bridge inverter, (b) double chopper inverter.
1.5 Electric vehicle drives: optimum solutions for motors, drives and batteries
Optimum supply of voltage for the power electronics of EVs is around 300 V DC using the latest IGBT power transistors 4 . This also provides a sensible solution for the motor because in the power range of 30-150 kW the line currents are quite reasonable. A consequence of using a 300 V battery is that the rail voltage will vary from 250 to 400 V under different service conditions.
1.5.1 BATTERY CONSIDERATIONS
A good commercial battery for deep discharge work is the Trojan 220 Ah 6 V golf cart unit. This gives 75 A for 75 mins and weighs 65 lb, consequently a 108 V stack weighs 1170 lb and cost $1080 in 1991. It also requires considerable maintenance and occupies a projected area of 1342 square inches and is 10 5/8 inches high.
In comparison, sister company Nelco have available a sealed lead-acid battery of 12 V, 60 Ah and arranged into 18 cells to give 108 V. It occupies 720 square inches of plan area and weighs 697 lb. This arrangement can also provide 75 A for 75 minutes. The problem area is cost. This battery cost $2700 in 1991. If the vo1tage was increased to 312 V, with the same stored energy, the cost rises by 20% at 45 kW. Such 300 V battery systems require great attention to safety; 100 V batteries may be feasible at 45 kW but this ceases to be true at 150 kW. In fact, one can draw the graph in Fig. 1.23(a) to define minimum voltage for a given output power. Other areas worthy of comment are maintenance and battery life. High voltage strings of aqueous batteries are dangerous and should be banned by legislation. This is not so of sealed lead-acid batteries as there is no need for maintenance access. However, no voltage greater that 110 V should be present in a single string or an individual connector. Long series strings present a potential maintenance problem with respect to cell equalization. The problem may only be resolved by keeping all cells at the same temperature. A final problem is fast charging; this is temperature limited to 60 o C max cell temperature. The newer cells may be fast charged so long as the temperature is contained and the individual cell voltage is below 2.1 V.
1.5.2 FUEL-CELL CONSIDERATIONS
There is no doubt that the long-term power supply for electric vehicles will be some form of hydrogen fuel cell, the leading current technology being the PEM membrane system as manufactured by Vickers/Ballard. This is a complete system measuring 30 ´ 18 ´ 12 inches which produces about 5 kW at 45% efficiency.
Fig. 1.23 Voltage vs power relationship for (a) lead-acid battery and (b) fuel cell. The unit consists of 36 plates of 250 A rating and the fuel gases operate at 3 bar and the exhaust temperature is around 80 o C. This arrangement leads to the relation in Fig. 1.23(b). Hence for a vehicle with a storage battery approximately one-third maximum power +10 kW is the peak fuel-cell load. Hence for 45 kW this amounts to five modules producing 100 V at 250 A. For a 150 kW system, vehicle builders will need ten modules giving 50 kW at 200 V. The voltage may not rise above 200 V due to problems relating to the hydrogen. Warm up takes about 5 minutes from cold with units producing 50% output at 20 o C. Once hot, response is 1-2 seconds for load steps and endurance has been confirmed as greater than 20 000 hours. One of the more intriguing possibilities offered by fuel cells is to use the power converter to produce 50 Hz for powering lights and portable tools on site vehicles.
1.5.3 OVERALL SOLUTION
There is a basic incompatibility between the power source voltage and the motor voltage; so how can this problem be addressed? The solution is to put a reversible chopper between the battery/fuel cell and the inverter (Fig. 1.24). This means that the supply to the inverter is stabilized under all conditions resulting in full performance during receding battery conditions and no overvoltage during battery charging mode.
By using the inverter as the battery charger express charging can be performed, where mains supply permits, in approximately 3 hours.
1.5.4 MOTORPAK SAFETY CONSIDERATIONS
To charge and discharge the battery quickly whilst optimizing battery use requires perfect control of the battery temperature. Since the battery is sealed this is best achieved by immersing in silicon fluid. A circulating pump passes fluid to and from the motor. This keeps the batteries cool and at equal temperature during charging using the motor as a heatsink and, during discharge, the motor warms the batteries to give optimum performance. Hence the batteries are built into a tank and this prevents access by the operator.
The next concept is to make the battery module interchangeable. This permits refueling either by recharging the battery or by exchanging the battery module.
1.5.5 MOTORPAK CONSTRUCTIONAL CONSIDERATIONS
Fig. 1.24 Reversible chopper.
If costs are to be optimized, it makes sense to locate the power controller close to the battery. In the above case, Nelco have taken the concept one stage further. The power controller is located in the base of the battery tank. We call this concept Motorpak, Fig. 1.25, and as can be seen the mechanical execution could not be made much simpler. The motor and PCU pack are mounted under the vehicle either in place of or in addition to the conventional power train. No gearbox is needed and the motor provides nearly 300 Nm of torque directly. The following specification applies for a 45 kW Motorpak:
Input 50-240 V AC, 40-65 Hz single or 3 phase up to 30 A; recharge time 3 hours Output 0-220 V, 3 phase up to 750 Hz 60 kVA, 13.6 V DC 500 W Batteries 18 off, 12 V, 60 Ah sealed lead-acid units, may be configured as 108 or 216 V unit Weight 800 lb (362 kg) Dimensions 30 in long, 27 in wide, 14 in high Construction Weatherproof Controls Function switch, accelerator pedal, voltmeter/ammeter/amp hour meter, 13.6 V for auxiliaries, 2 oil pipes to motor (4 liters/min) Deep discharge 800 cycles to 80% performance Stored energy 10 kWh Cost in 1991 £3000 at 1000 off ex batteries (£5000 with batteries), price includes motor Torque 0-1500 rpm, 280 Nm falling to 70 Nm at 5000 rpm on 45 kW constant power curve Construction Flange mount with double ended shaft and integral encoder Cooling Silicon oil, 4 liters/min Electrical rating 220 V, 130 A, 750 Hz Power pack contains: batteries, power conversion unit, 12 V, 500 W supply for auxiliaries and hydraulic power steering supply/cooling for motor. This unit is interchangeable in seconds. A 45 kW unit weighs just 800 lb; the motor is oil cooled and weighs 130 lb.
Temp. range -20°C to + 40°C ; 45 kW traction motor
Type Brushless DC permanent magnet Size 375 long ´ 250 diameter, weight 50 kg
REMOVE Engine Gearbox Cooling system Fuel tank ADD Motorpak unit Motor Vacuum pump for brake servo Optional air conditioner / heater; AMMETER I VOLTMETER I AMPERE HOUR METER
Fig. 1.25 Motorpak concept.
1.5.6 ADVANTAGES OF THE SYSTEM DESCRIBED
If the conventional engine is replaced by a battery/motor the weight increases by approximately 300 lb for a 1 ton vehicle. This means the system can be fitted to existing chassis designs or retrofitted to cars. The system can be used standalone or as a hybrid. The complete electrics pack is interchangeable for instant refueling and the PCU works with any battery input supply for 100- 250 V. Batteries are rated for 800 deep discharge cycles to 80% depth of discharge. On a 45 kW unit, the battery can supply 75 A for 75 minutes at 108 V DC or 37.5 A for 75 minutes at 216 V.
Total safety is ensured by all electrical parts except the motor which is contained in a single totally insulated module with no parts distributed over the vehicle. Batteries are sealed to give best resistance to crash situations. Electrics are protected against short-circuits with both fuses and circuit breaker. The oil cooling system can supply the power steering if required. Minimized technical risk is ensured by a total package solution and if technology improves only one module has to be changed. The module approach makes many finance packages feasible, facilitating user acceptance; for example, the user buys the vehicle and motor but hires the battery and PCU. The battery pack can be recharged in 3 hours where mains supply permits. The PCU functions as a battery charger and the drive system can supply up to 45 kW of mains electricity for short periods - longer if used with a fuel-cell prime mover. The PCU makes use of portable power appliances viable which is particularly useful for the building industry. Finally, the concept makes conversion of existing vehicles possible.