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 transistors⁴. 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.

IM ^Brushless_{DC Motor}

(a)

(b)

50 kw 200 V

POWER

FUEL CELL VOLTS

250 V

100 V

45 kw 150 kw

POWER

MINIMUM VOLTAGE

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 ternperature limited to 60^oC 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.

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^oC. 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

Fig. 1.23 Voltage vs power relationship for (a) lead–acid battery and (b) fuel cell.

(a)

(b)

MOTOR REVERSIBLE

CHOPPER UNIT

0 V +300 V

100–250 V

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^oC. 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 refuelling either by recharging the battery or by exchanging the battery module.

1.5.5 MOTORPAK CONSTRUCTIONAL CONSIDERATIONS

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

Fig. 1.24 Reversible chopper.

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 litres/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

Temp. range −20°C to + 40°C

45 kW traction motor

Type Brushless DC permanent magnet

Size 375 long × 250 diameter, weight 50 kg

Fig. 1.25 Motorpak concept.

Vacuum pump for brake servo Optional air conditioner / heater

ACCELERATOR

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 litres/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.

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 tonne 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 refuelling 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.

References

1. Hodkinson, R., 45 kW integrated vehicle drive, EVS 11, Florence, 1992

2. Hodkinson, R., Machine and drive characteristics for hybrid and electric vehicles, ISATA 29, Stuttgart, 1996

3. Hodkinson, R., Towards 4 dollars per kW, p. 4 et seq., EVS 14, Orlando, December 1997 4. Hodkinson and Scarlett, Electric Vehicle Drives, Coercive Ltd report, December 1991 Further reading

Electric vehicle technology, bound volume of SAE papers, 1990

Electric and hybrid vehicle technology, bound volume of SAE papers, 1992 Electric and hybrid vehicle design studies, bound volume of SAE papers, 1997 Technology for electric and hybrid vehicles, bound volume of SAE papers, 1998 Strategies in electric and hybrid vehicle design, bound volume of SAE papers, 1996 Electric vehicle design and development, bound volume of SAE papers, 1991

Breaking paradigms, the seamless electro-mechanical vehicle, Convergence 96, SAE, 1996

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