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Selecting EV motor type for particular vehicle application

Current EV design approaches

1.3 Selecting EV motor type for particular vehicle application

1.3.1 INTRODUCTION

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 motor2. 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

Specification

Speed 60 000 rpm

Power 90 kW

Voltage 200 V

Frequency 2000 Hz

Weight 20 kg (housed)

Dimensions 150 mm OD x 175 mm long

Current 262 amps

Efficiency 99%

Resistance 14 milliohms

Inductance 15 microhenries Cooling Liquid (oil or water)

Face commutator 1.

Face commutator 2.

NEXUS GEMINI

1000 2000 3000 4000 5000

A

A 82.5 Nm

65% 75%

85%

87%

89%

22 kW 90

80 70 60 50 40 30 20 10

TORQUE Nm

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.

1.3.2 BRUSHED DC MOTOR

This consists of a stationary field system and rotating armature/brushgear 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

Fig. 1.14 Efficiency map and Gemini motor.

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 favour, 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

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 modelling 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.

CONSTANT T CONSTANT P

POWER (kW)

10 20 30 40

1000 2000 3000

SPEED (RPM)

82%

85%

89% 87%

90%

50

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 carburettor 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 Chapter 4).

(a)

PM BDC Brushed PM DC series Switched

pancake motor reluctance

Size (mm) 200 × 100 200 × 100 200 × 175 200 × 150

Weight (kg) 10 10 18 14

Rating 3 @ 750 3 @750 3 @750 3 @1500

(kW@rpm@V) 40 40 60 70

3 @1500 3 @1500 3 @1500

70 80 80

Efficiency 0.3/750 80% 3/750 75% 3/750 70% 3/750 80%

(motor 0.75/750 94% 750/750 80% 750/750 70% 750/750 85%

only) 3/1500 93% 3/1500 85% 3/1500 80% 3/1500 85%

(b)

Brushless Induction Switched Brushed

DC PM motor motor reluctance DC motor

Speed (rpm) 3000 3000 3000 1250

Torque (Nm) 64 64 154

rising to:

Speed (rpm) 12 000 12 000 5000

Torque (Nm) 16 16 38.5

Voltage (V) 150 AC 150 75–150 192

Current (A) 126–81 AC 164–106 AC 180–90 DC 122 DC

Power (kW) 20 20 20 20

Frequency (Hz) 800 400 800 (equiv. 125 Hz)

Weight (kg) 12 25 20 50

Efficiency

% @ 3000 95 90 92 (1250) 80

% @ 12 000 97 92 94 (5000) 85

Cooling oil oil oil air

(c)

PM (DC) Induction Switched DC

brushless motor reluctance brush

Speed (rpm) 1000 1000 1000 1000

Torque (Nm) 2866 2866 2866 2866

at speed (rpm) 4000 4000 4000 4000

Torque (Nm) 716 716 716 716

Voltage (V) 380 380 190/380 500

Current (A) 753–486 980–630 1000/500 520

Power (kW) 300 300 300 300

Frequency (Hz) 1056 133 266 (133 equiv.)

Weight (kg) 300 600 500 1000

Efficiency

% @ 1000 95 93 94 85

% @ 4000 97 95 96 89

Cooling oil oil oil air

1.3.7 HGV

The heavy goods vehicle is an articulated truck which weighs 40 tonnes. Often omitted from clean air schemes on the grounds of low numbers they travel intercontinental distances every year and are major emitters of NOx 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 tonnes 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 metre 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.