Control of an EV drive with reduced unsprung mass

Traditionally, the motors used in electric vehicles as direct wheel drives were built entirely into the vehicle wheel; this, however, increases the weight of the wheel. The drive arrangement described here utilises a permanent magnet, brushless, axial field double-sided disc motor. This type of machine, particularly the double-sided stator configuration, is attractive because rotor misalignments perpendicular to the motion of the drive shaft can be accommodated. In such an arrangement, the stators of the machine can be mounted onto the chassis of the car and the rotor is allowed two degrees of freedom (rotational and transverse), and thereby directly drives the road wheel. Hence, a reduction in the unsprung mass of the wheel drive is achieved. However, there is a drawback: since the wheel is perturbed by the road surface roughness effects, the rotor moves between the outer stator assemblies (transversely), and consequently induces torque pulsations in the machine. Two separate control strategies to eliminate the torque pulsations caused by the rotor perturbation are investigated: modulation of the input motor current, and instantaneous torque control using a variable structure controller. The experimental results in each case show that both control strategies are effective in eliminating the torque pulsations