Lightweight Electric/Hybrid Vehicle Design

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Notation H = Field strength B = Magnetic intensity μ = Permeability φ = Magnetic flux N = Number of field turns Z = Number of armature turns I = Current V = Voltage L = Length of windings n = Rotational speed E b = Back-EMF (a) (c) (d) D a b S S Brushes against rotation Brushes normal Brushes forward Volts (b) N M RPM (e) Full dynamo field 5000 100 4600 90 4200 80 3800 70 3400 60 3000 50 2600 40 2200 30 1800 20 1400 10 Effy % BHP 132.8W 1000 RPM 102.8W 50 100 150 Amps 47.0% Divert RPM Full field RPM 65.5% Divert RPM 42.8W 72.8W Full field effy % 47% Divert effy % BHP Full field BHP 47.0% Divert BHP 65.5 % Divert 47.0% Divert torque Introduction xxvii 7.2W in resistance box 19.8W in resistance box 1000 0 0 40 80 120 160 Amps at 250 volts 200 240 280 0 320 50 K.W. 200 250 300 65.5% Divert effy % Full field torque 65.5% Divert torque Constant watt curve Fig. 0.5 Electric transmission basics: (a) ‘clutching’ of electric transmission; (b) high EMF at low loads; (c) horned interpoles; (d) brush movement effect; (e) motor characteristics. 300 250 200 150 100 50 Torque lb ft
xxviii Lightweight Electric/Hybrid Vehicle Design help in rapid build-up of generator EMF. The resulting problem is heat build-up of these series windings under heavy vehicle-operating loads. Efforts to counteract this by reducing the length of the shunt coil creates the further difficulty of slow excitation after vehicle coasting. Since the brushes of the generator or motor short-circuit one or more sections of the armature winding, it is important that these sections are in the neutral zone between field magnets of opposite polarity at the moment they are shorted. To otherwise avoid destructive arcing under heavy load, the machine characteristic may be altered by moving the brushes either with or against the direction of armature rotation. This will provide more or less droop of the characteristic as shown at (b), but on interpole machines there is the added problem of the interpoles being prevented, under brush movement, of fulfilling their role of suppressing arcing. Horned interpoles, (c), may be used to offset this effect. The shape of the horn is made such that the magnetic flux under the foot of the interpole is not altered but the additional shoe section is magnified sufficiently to act on a few turns of the armature, these turns providing sufficient induced EMF to give the required compounding effect for rapid excitation from standstill and under heavy loads. The view at (d) shows the performance characteristics by a machine of this type. While the curve for the full field (no series resistance) approximates to the constant power characteristic, its EMF rises at light loads. The effect of inserting resistance is also shown. However, for a given motor torque, speed is proportional to EMF applied so that if the engine speed is reduced, motor and thus vehicle speed will fall. To avoid this, the motor field windings have a diverter resistance connected in parallel to them, to weaken the motor field; the counter-EMF is reduced, and more current is taken from the generator, which increases motor speed again. Thus a wide speed ratio is provided. In earlier times resistance was altered by handles on the steering column; with modern electronics, auto-control would, of course, be the norm. Regenerative braking can be obtained by reversing the field coil connections of the motor which becomes a ‘gravity-driven’ series-wound generator, running on short-circuit through the generator armature. However, the currents involved would be too heavy and an alternative approach is required. The theme is taken up by H.K. Whitehorne in a slightly later paper 5 , who pays especial tribute to Burton’s skewed horn interpole invention. He goes on to consider motor characteristics and favours the series-wound machine because its speed is approximately inversely proportional to the torque delivered, adjusting its current demand to the speed at which it runs and to the work it has to do. Characteristic curves of a motor running on a fixed voltage are shown at (e). Conditions are shown for full field, and for two stages of field diversion. Examination of the 50 kW line makes it apparent that the torque/amp curve is independent of voltage; speed is practically proportional to voltage and generally characteristics vary on the size of the motor, its windings and length of its core. However, on low voltage and heavy current, the efficiency falls rapidly which makes electric transmission a difficult option for steep gradients. There is considerable flexibility, though, as engine and generator running at 1500 rpm deliver 50 kW at 250 V, 200 A, the electric motor for this output being designed to run at 3800 rpm giving torque of 70 lb ft, for overdrive cruising, yet at 800 rpm giving 315 lb ft for gradients. 0.5 EV classification EVs in common current use include handling trucks, golf carts, delivery vans/floats and airport people movers/baggage handlers. The more challenging on-road application is the subject of most of what follows in this book, where the categories include motor scooter, passenger car, passenger service vehicle, taxi and goods vehicle. The smallest road-going EVs are probably the electric bicycles such as the Sinclair Zike and the Citibike product. Both these companies also produce bolt-on pedal assist systems for
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100 Lightweight Electric/Hybrid Veh
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PART TWO Battery/fuel-cell EV desig
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5 Battery/fuel-cell EV design packa
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(a) cell voltage 2,5 V 2,0 1,5 1,0
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Varta Electric Power: Nickel-metal-
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700 600 500 400 CurrentmA 300 (a) 2
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Battery/fuel-cell EV design package
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Intake filter Intercooler High spee
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Fig. 5.22 Bradshaw Envirovan. Batte
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6.1 Introduction 6 Hybrid vehicle d
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6.2 Hybrid-drive prospects Hybrid v
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TRAVEL ELECTRIFIED, % 80 70 60 STRA
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Hybrid vehicle design 147 750 kg to
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M [Nm] 500 450 400 350 300 250 200
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6.3.6 TAXI HYBRID DRIVE Hybrid vehi
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Hybrid vehicle design 153 The Rover
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Hybrid vehicle design 155 as a star
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Hybrid vehicle design 157 injection
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Hybrid vehicle design 159 In ‘sta
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Hybrid vehicle design 161 power and
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Hybrid vehicle design 163 The batte
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Hybrid vehicle design 165 into mech
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Hybrid vehicle design 167 drive ele
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Hybrid vehicle design 169 The view
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Hybrid vehicle design 171 6.5.4 ADV
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Lightweight construction materials
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(a) (d) (e) Lightweight constructio
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4.2 4.2 5.6 4.2 7.0 5.6 4.2 5.6 5.6
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(a) Creepstrain dE/dt (b) 3.0 2.0 1
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(a) M b Mb Mt 3 2 1 T (b) (c) t s T
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Design for optimum body-structural
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ψ e 1.O σ av (a) (b) Design for o
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Rolling resistance coefficient 0.03
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252 Lightweight Electric/Hybrid Veh
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Lightweight Electric/Hybrid Vehicle Design

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