Could induction motors accelerate the electrification of CVs? - Just Auto

2022-05-28 07:03:52 By : Ms. Cherry Zhang

Changes in legislation to electrify vehicles have arguably hit the commercial vehicle (CV) industry the hardest. We spoke to Murray Edington, Head of Electrified Powertrain, Drive System Design (DSD) to learn more about the opportunities and challenges.

As an electrified powertrain engineering consultancy, DSD is well placed to provide an independent view on the key challenges in designing new electrified commercial vehicles as well as the potential electrification of existing fleets. Its research has found that, contrary to popular opinion, commercial vehicles can be better suited to induction machines (IM) that don’t require the use of rare earth materials.

What are the key challenges in electrifying commercial vehicles?

Perhaps the most obvious challenges arise from the size of the vehicle involved. For N1 category vehicles, such as a typical courier van, that might not present so much of a problem, but for larger vehicles such as trucks and buses, the sheer power and torque required demand a more specialised solution. That has knock-on effects elsewhere in the powertrain – the currents involved, even with high-voltage systems, place particular demands on items such as cabling and connectors, for example. With a 350-400kW set-up, thermal management becomes critical, even more so when you consider that the commercial sector demands not just reliability but also longevity. A typical tractor unit might be expected to function perfectly for a million miles; designing a system that can meet that requirement often requires a different mindset to passenger automotive development, which might focus on maximising performance for a given size and weight, factors which are less critical in a commercial application.

Of course, the key challenge today surrounds the issue of energy storage. A heavier vehicle requires more energy to propel it and that means carrying a heavier battery. Packaging that volume given today’s energy densities is a challenge. There are solutions, however – splitting it into smaller units to distribute around the vehicle, for example. Some of the newer manufacturers perhaps have an advantage here in that they have the freedom to design a vehicle structure from the ground up with this in mind, rather than attempting to adapt an existing architecture.

This is a problem vehicle converters often run into, with some traditional structures unsuited to electrification simply because there isn’t the space to accommodate the systems required. In some bus conversions, for example, it’s even proved necessary to engineer a replacement spaceframe or ‘spine’ chassis.

For N3 category vehicles, such as long haul HGVs, it may be necessary to sacrifice some cargo capacity – both in terms of weight and volume –to carry sufficient batteries, although other solutions are also being trialled. In some countries and states, short stretches of roads have been equipped with overhead powerlines, similar to those used by trains and trams, with a pantograph that extends from the roof of the truck. That leaves the onboard batteries to power the vehicle through the ‘last-mile’ portion of their journey, but also means smaller – and therefore lighter – batteries can be used.

Power constraints presumably highlight the need to maximise efficiency. What effect do induction motors have on this?

IMs are inherently less efficient and less torque dense than equivalent permanent magnet (PM) machines, and therefore require a larger machine to achieve the same level of performance. However, the efficiency map is a different shape and generally features a larger high-efficiency island. A PM machine may have a headline efficiency figure of, say, 98%, while an equivalent IM may achieve 95% efficiency. However, that efficiency will be spread over a larger area instead of a single peak, and once you start overlaying typical drive cycles over this map, it soon becomes clear that commercial vehicles tend to spend more time operating in this ‘sweet spot’.

For many IMs, the efficiency window is to be found towards the high-speed, high-load range. That correlates with heavy commercial vehicles’ tendency to spend a large proportion of their time at highway speeds where, by comparison, the losses in a PM machine are beginning to take effect.

This highlights the need to carefully consider the application any technology will be used in. That’s part of the reason why we developed our ePOP (Electrified Propulsion Optimisation Process) simulation tool, as it allows us to quickly compare tens of thousands of different motor topologies, gear ratios, operating voltages, and so on, over a series of different drive cycles.

Do induction motors have any other advantages that may earn them consideration even if they are not the optimal solution for a given use case?

Perhaps one of the biggest benefits is that they contain no rare-earth materials, now a well-known highly cost-volatile commodity. In fact, in 2011, the price increased ten-fold in little more than a year. That makes it difficult to settle on an architecture that may become economically unviable in fairly short order.

There are other benefits, too. IMs have higher temperature limits, whereas PM machines can suffer permanent damage, even irreparable demagnetisation should the temperature rise too high.

IMs are relatively simple from a technical perspective, and their operation is well understood throughout the industry, having been used for various applications for more than a hundred years. As commercial vehicle manufacturers face the prospect of transitioning away from decades of experience with combustion engines, there’s a certain amount of comfort to be found in the array of off-the-shelf systems available, even if they have to be adapted to suit a particular application.

In fact, in some ways, the technologies are quite compatible – IMs have been used in steel mills, machine tools, and other heavy-duty applications for years, where they share the requirement to operate with very long lifetimes. They’re reliable and well-proven, and that may make them ideal candidates for use in electric conversions.

No solution is ever perfect; what might some of the potential drawbacks of IMs be?

Getting the required power and torque from an IM can be more of a challenge than a PM machine when space constraints are a factor, such as in front-wheel-drive applications where the motor must mount between the wheels. The general rule of thumb is that an IM needs 20% more volume – whether that’s diameter, length, etc. – to achieve the same level of performance. In part that stems from the way IMs work, creating a magnetic field on the rotor to interact with the magnetic field on the stator, thereby creating motion. Inevitably, there’s a slip between the two, and while the induced currents in the rotor have a loss, a PM doesn’t, demanding a certain amount of compensation.

Although induction motors are quite a mature technology, are designs changing as a result of being used in new automotive applications?

Industrial IM applications have typically used a fixed frequency, perhaps 50Hz to match the mains supply, whereas traction applications use variable frequency drive (VFD) to permit operation across a wide speed range. In addition, optimisation may be needed to achieve efficiency at higher road speeds – while you don’t want to be carrying around more motor than you need, small optimisations and fine geometry changes within the machine design may serve to ensure the efficiency window is as broad as possible.

It’s not a linear relationship, but if you can run the machine faster, then potentially the power density is greater. That does, however, create issues around mechanical integrity to ensure the rotating parts are designed and rated to suit.

Cooling is becoming more important. Fundamentally, motors are thermally limited, so if they’re not approaching the limits of their temperature rating then we’ve over-designed them. That of course means that the better you can cool them, the greater the power density that can be extracted from them. At DSD, for example, we do a lot of work looking at not just water jacket cooling, but also oil spray cooling of the windings.

IMs work by inducing an electrical current within the rotor, which is located at the heart of the machine and thermally isolated by bearings and other elements. This significantly increases the complexity of cooling strategies and makes the losses created here notoriously difficult to manage. In the commercial arena, the focus on long-term reliability tends to favour opting for a larger machine rather than a smaller, more power-dense unit that requires better cooling.

Although we’ve talked at length about induction motors, presumably they are not the only show in town?

Indeed – wound-field motors, for example, are also free of magnets, and they have a long history in industry for certain applications. Some automotive suppliers have been using wound-field designs in electric vehicles for some time. Axial flux machines, meanwhile, may find a home in low-speed, high-torque applications.

Each design has its issues to contend with – some elements within wound-field motors typically have a finite life and might be considered a serviceable item – making it important to consider the system as a whole when designing for a particular application.

We’re already seeing the market accepting that different motor designs favour certain applications, and that enables a broad spread of options when designing any powertrain architecture. We’re also seeing ‘novel’ motors that use unusual technologies that can help in applications where the last few percentages of efficiency are important.

With motor technologies developing so rapidly, manufacturers must evaluate all the options at the design stage of a programme to make sure the product is optimised for any given application. To do this they will need a broad range of motor expertise and the right simulation tools to provide an independent assessment.

The perfect motor may never exist, but that doesn’t mean there isn’t a right tool for the job.