Page 164 - Automotive Engineering Powertrain Chassis System and Vehicle Body

Battery/fuel-cell EV design packages CHAPTER 6.1

6.1.6.4 Thyristor control and output from the device is a train of rectangular

voltage pulses. The steady current related to such a train
Thyristor control, Fig. 6.1-20, has been an important is obtained by connecting Dl across the motor load.
advance and units such as the controller by Sevcon are
Output voltage can thus be controlled by altering t off /t on
suitable for 24–72 V working with a current limit rating while holding t on þ t off constant – a constant frequency
of 500 A for 5 minutes. Continuous rating at any fre- variable mark/space chopper – or altering 1/(t on þ t off )
quency would be 160 A and a bank of eight 50 mF, 160 V while holding t on constant – a variable frequency constant
commutation capacitors are employed. Overall di- mark chopper.
mensions are 430 301 147 mm and weight is 14 kg.
Like other units of this type, it supplies the motor with
fixed width pulses at a variable repetition frequency to 6.1.6.5 Ford ecostar
provide stepless control over the average voltage supplied
to the motor. The view at (a) shows the typical pulse An electric vehicle with equivalent performance to an
width of 12 ms, giving a frequency range from 10 to 750 IC-engined one, and yet based on a conventional Escort
Hz and a good motor current form factor with minimal van platform, Fig. 6.1-23, has been achieved by Ford with
heating effect commensurate with some iron losses.
the use of a sodium–sulphur battery. The battery is
Ratio of on-time to the total period would be 30–50%
enclosed in a hermetically sealed metal casing, which
while the commutation circuit which switches the main
contains sodium and sulphur on each side of a porous
conducting thyristor operates every 2.5–3.5 millisec-
ceramic separator. In order to react on a molecular level,
onds. The graph shows motor consumption at current
the sodium and sulphur must be free to flow through the
limit; ripple is 170 while average motor current of 500 A
separator in liquid form. This requires maintaining an
compares with battery current of 165 A. At (b) is seen operating temperature of between 290 and 350 C. The

the energy being stored and reversed each cycle by the Ecostar’s battery pack is formed of a double-skinned,
main commutation capacitor. stainless steel case, with a glass fibre insulated cavity
A typical DC chopper circuit for thyristor drive is which has been evacuated to form a vacuum flask. Inside
shown at (c). Thyristor T1 is the ‘on’ one and T2 the ‘off’; the case, 480 individual sodium–sulphur cells are
inductive load of a series motor is shown as R1 and L1 embedded in a sand matrix to prevent movement and
while Dl is a so-called ‘freewheel’ diode. When capacitor contain the elements in the event of a severe impact.
C is charged to a voltage Vb in a direction opposing the Each cell is encased in a corrosion-resistant aluminium
battery voltage, T1 and T2 are off and load current Ilis cylinder and produces about 2 volts. They are linked to
flowing through R1, L1 and D. A pulse is applied to the provide a 330 volt power supply and a steady-load cur-
gate of T1 to turn it on; Dl becomes reverse-biased and rent of 80 amps. Power of up to 50 kW is available for
load current Il begins to flow through R1, L1 and T1. This short periods. The battery pack is mounted under the
causes a short-circuit, effectively, across CL and D2, vehicle floor, ahead of the rear axle. The vehicle is fitted
creating a tuned circuit. Its resonance drives a current with an intelligent on-board charging system that auto-
through T2, D2, L and C, sinusoidally rising to a maxi- matically adjusts to the supply voltage from the mains. It
mum then decaying to zero, at which point C is effec- can be used for overnight recharging and temperature
tively charged to V2 in the reverse direction. control of the sodium–sulphur battery. A full recharge
When the current attempts the second half of its takes between 5 and 7 hours. In the event of electrical
cycle, D2 prevents any reversal and the reverse charge on
malfunction, the battery pack is isolated by replacing
C is caught by D2, the ‘catching’ diode. At time t on
external 200 amp fuses at the battery connection box.
a pulse applied to the gate of T2 turns it on, applying
The batteries are also equipped with internal fuses that
reverse voltage on C across T1 – which rapidly turns off
isolate the electrical supply from the vehicle if there is
and diverts load current through R1, L1, C, L and T2. any kind of internal overload.
C is thus charged at a linear rate by Il passing through it. The Ecostar, Fig. 6.1-21, is powered by a high speed,
When capacitor voltage again approaches Vb, opposing 3 phase AC motor, rated at 56 kW (75 bhp). It is di-
battery voltage, load current begins to flow through R1, rectly coupled to the transaxle by a single-speed plane-
L1 and D1 again, current through T2 dropping below the tary reduction gear set. The motor operates at 330 volts,
holding value so turning off T2–completing the cycle provided by the sodium–sulphur battery pack. Com-
ready for reinitiation by pulsing the gate of T2 again. The pared to equivalent DC systems, the AC motor works at
circuit thus represents a rapid on/off switch which en- a higher rate of efficiency, is smaller, lighter, lower in
ables effective DC output voltage to be variable cost, easier to cool, more reliable and generally more
according to:
durable. Maximum speed is 13 500 rpm and maximum
½t on =ðt on þ t off ފVb torque 193 Nm at zero rpm. Regenerative braking comes

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