48 Protean’s motors are of the outrunner type, meaning that the rotor housing is on the outside and revolves around the central stator. The seal, therefore, has to keep contaminants from passing through the gap between them around a relatively large circumference compared with seals needed for in-runner motors. According to one of Protean’s patents, the seal is a ring with a V-shaped crosssection. Made of a rubber material such as neoprene, it makes use of centrifugal force to minimise friction and ensure an effective seal. When the rotor is stationary or rotating slowly, the seal fully closes the gap, but at higher speeds the seal is forced away from the stator housing enough to minimise drag torque and wear due to friction, while the centrifugal force prevents the ingress of contaminants. Integrated bearings The motors integrate the wheel bearings, making the whole assembly more compact, and this contributes to its stiffness and strength, with wheel bearings being inherently strong components. Lambert notes that while the bearings are fairly simple, getting them right presents some complications, with different versions of the motor distinguished by bearings with varying load ratings, and the higher-rated bearings naturally used in commercial vehicles. A further consideration involves remaining compatible with standard vehicle systems that connect with the wheel bearings, such as wheel speed sensors. The motor is designed to be compatible, with standard wheels and tyres. Looking inwards from the wheel, the first component encountered is the rotor. This is attached to the rotating portion of the wheel bearing, independent of the studs that hold the wheel on. The stator is attached to the non-rotating portion of the wheel bearing and supports the field coils. On the other side are the capacitor ring (more of which later) and its two DC cables, the inverter and the cover. The brake caliper mount bolts through to the non-rotating portion of the wheel bearing, while the brake disk is secured to the rotor with a set of bolts around its circumference and is gripped by the caliper from the inside. The entire assembly bolts to the suspension arm. The rotor and the stator are made from an aluminium alloy that gives the motor the ruggedness it needs at a price point that fits the market. The remainder of the bill of materials is accounted for by the magnets attached to the inner surface of the rotor, the copper windings, the ‘back iron’ core that supports the windings and completes the magnetic circuit, and the power electronics of the integral inverter. The inverter inside the motor takes the form of two curved modules attached to the back of the stator. The inverter is electrically connected to a large capacitor that smooths out the fluctuations on the DC bus when the motor is operating in regen mode and supplying current to the battery. So far, so conventional, but because the inverter installation has to fit into a small, cylindrical space, the capacitor cannot be a large, square block, Lambert points out. Therefore, Protean designed the capacitor and its connections with the inverter as a large-diameter ring, which is referred to as an annular capacitor. Three-phase inverter Rated at 400 V, the inverter is a three-phase unit, based on silicon insulated-gate bipolar transistors (IGBTs) rated at around 750 V. “We buy those in from one of our technology partners who we’ve been working with for a while. They provide us with integrated power modules, which we then attach and weld into the inverter,” Lambert says. Using IGBTs capable of handling voltages higher than the inverter’s nominal rating provides some margin for it to cope with any voltage spikes and transients safely, and to allow for manufacturing tolerances that affect the components’ real-world voltage limits. The idea is to enhance the inverter’s reliability and longevity. Protean is looking at alternative power electronics technology in the form of silicon carbide MOSFETs for use in future inverter systems, Lambert notes. Even with the inverter integrated into the wheel motor, the positive and negative DC cables, CAN bus wires and coolant lines unavoidably enter the wheel hub, so their routing and protection are among the factors that must be considered in engineering the system to meet reliability and longevity goals. March/April 2024 | E-Mobility Engineering The major components of the hub motor revealed, with the integrated wheel bearing, capacitor ring and brake being notable design features (Image courtesy of Protean)
RkJQdWJsaXNoZXIy MjI2Mzk4