Lightweight Electric/Hybrid Vehicle Design

Current EV design approaches 17
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
2
If
Ef
3
4
Ea
Ia
A 1
FORWARD BRAKING FORWARD MOTORING
2
Ia
If
Ef
If
Ef
Ea
4
3
1
2
Ia
If
Ef
If
Ef
Ea
REVERSE BRAKING REVERSE MOTORING
Ia Ea Ia Ea
4
3