Lightweight Electric/Hybrid Vehicle Design: Current EV design approaches (part 2)

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3. Selecting EV motor type for particular vehicle application

3.1 INTRODUCTION


FIG. 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. Any of the four machines could satisfy any application. This isn't a battle of 'being able to do it', it's 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. 14 Efficiency map and Gemini motor.

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. 14, while FIG. 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.

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. 15 Integral 4-quadrant chopper.

3.4 SWITCHED RELUCTANCE MOTORS

SRMs, FIG. 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's Polaron's expectation that they will be successful in heavy traction, where magnet cost may preclude brushless DC.

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's offered in prototype quantities at ($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. 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. 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.

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's 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 $1200 in 1996; quadrant chopper $900; motor DC series$1250). 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's 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 12000 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. 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. 17 Motor comparisons for three vehicle categories (the four motor types are also discussed in Section 4).

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. 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's 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. 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. 18 Gas turbine technology.

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. 19, note that the temperature differential is the temperature difference between the connection pins and the baseplate in °C.


FIG. 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. 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 isn't 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's wire bonded packaging that offers the lowest device costs. A 1200 V, 100 A six pack is around 100 dollars (1997 price), FIG. 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. 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 don't 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 isn't 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. 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's 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 isn't 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. 22 (a) Single bridge inverter, (b) double chopper inverter.

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.

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. 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 isn't 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 deg. 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.

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. 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. 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 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.

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. 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.

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.

5.5 MOTORPAK CONSTRUCTIONAL CONSIDERATIONS


FIG. 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. 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 $3900 at 1000 off ex batteries ($7000 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's 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. 25 Motorpak concept.

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.

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