Lightweight Electric/Hybrid Vehicle Design

Battery/fuel-cell EV design packages 125
The EV1 can be charged safely in all weather conditions with inductive charging. Using a
220 volt charger, charging from 0 to 100% for the new lead–acid pack takes up to 5.5–6 hours.
Charging for the nickel–metal hydride pack, which stores more energy, is 6–8 hours. Braking is
accomplished by using a blended combination of front hydraulic disk, and rear electrically applied
drum brakes and the electric propulsion motor. During braking, the electric motor generates
electricity (regenerative) which is then used to partially recharge the battery pack. The aluminium
alloy structure weighs 290 pounds and is less than 10% of the total vehicle weight. The exterior
composite body panels are dent and corrosion resistant and are made from SMC and RIM polymers.
The EV1 is claimed to be the most aerodynamic production vehicle on the road today, with a 0.19
drag coefficient and ‘tear drop’ shape in plan view, the rear wheels being 9 inches closer together
than the front wheels. The EV1 has an electronically regulated top speed of 80 mph. It comes with
traction control, cruise control, anti-lock brakes, airbags, power windows, power door locks and
power outside mirrors, AM/FM CD/cassette and also a tyre inflation monitor system.
5.5.3 AC DRIVES
An interesting variant on the AC-motored theme, Fig. 5.16, is the use of a two-speed transaxle
gearbox which reduces the otherwise required weight of the high speed motor and its associated
inverter. A system developed by Eaton Corporation is shown at (a) and has a 4 kW battery charger
incorporated into the inverter. A 3 phase induction motor operates at 12 500 rpm – the speed being
unconstrained by slip-ring commutator systems. A block diagram of the arrangement is at (b) and
is based on an induction motor with 18.6 kW 1 hour rating – and base speed of 5640 rpm on a 192
V battery pack. The pulse width modulated inverter employs 100 A transistors. The view at (c)
shows the controller drive system functions in association with the inverter. In an AC induction
motor, current is applied to the stator windings and then induced into the windings of the rotor.
Motor torque is developed by the interaction of rotor currents with the magnetic field in the air
gap between rotor and stator. When the rotor is overdriven by coasting of the vehicle, say, it acts
as a generator. Three phase winding of the stator armature suits motors of EV size; the rotor
windings comprise conducting ‘bars’ short-circuited at either end to form a ‘cage’. Rotation speed
of the magnetic field in the air gap is known as the synchronous speed which is a function of the
supply-current frequency and the number of stator poles. The running speed is related to
synchronous speed by the ‘slip’.
If two alternators were connected in parallel, and one was driven externally, the second would
take current from the first and run as a ‘synchronous motor’ at a speed depending on the ratio of
each machine’s number of poles. While it is a high efficiency machine which runs at constant
speed for all normal loads, it requires constant current for the rotor poles; it is not self-starting and
will stop if overloaded enough for the rotor to slip too far behind the rotating stator-field. Normally,
the synchronous motor is similar in construction to an induction motor but has no short-circuited
rotor – which may be of the DC-excited, permanent-magnet or reluctance type.
The view at (d) shows a stator winding for a 2 pole 3 phase induction motor in diagrammatic
form. If supply current frequency is f , then stator field speed is f /p for number of poles p. Rotor
s s
current frequency f = sf where s is the slip. Power supplied to a 3 phase motor can be expressed
r p
as EI(3) 1/2f n where n is efficiency. When a synchronous motor has no exciting voltage on the rotor
s
it is termed a ‘reluctance’ motor which has very simple construction and, when used with power
transistors, can be applied as a variable speed drive. Axial air-gap versions are possible as at (e);
such an electronically commutated motor can operate with a DC source by periodic reversal of the
rotor polarities.
The general form of control, with DC link inverter, for induction motors is shown
diagrammatically at (f). The thyristors of the inverter are generally switched so as to route current