There’s no such thing as a DC motor. If you find that statement surprising, think about it for a moment or two. All motors depend for operation on a rotating magnetic field so, even if a motor is powered from a DC supply, some form of switching must be provided to produce this, which means that the motor cannot be strictly considered as a DC machine. Nevertheless, well into the 20th century motors fed from DC supplies were the workhorses of industry, and the required switching was achieved with brushes and commutators.
As AC supplies became more widespread, wound-rotor synchronous motors took over as the dominant technology, followed by the induction motors that remain ubiquitous in industry today. The main benefit of these machines compared with their predecessors is simplicity. In effect, the ‘switching’ needed to produce a rotating field is an inherent feature of the AC supply. The commutator and brushes, which are costly to produce and require regular maintenance, are no longer necessary.
All that’s needed to make an induction motor rotate is to connect it to a three-phase AC supply. Superficially, this is a very simple arrangement, but it is not without its limitations. There is no control over speed and torque, and, when a stationary motor is connected to the supply, the initial inrush current is very large. While these limitations are acceptable in some applications, in others, such as electric vehicle drive systems, the ability to vary the motor speed and torque are essential.
This led to further development of machines operated from DC supplies, first with wound fields and then with permanent magnet fields. The characteristics of these machines when fed from a DC variable voltage source allow continuous speed control to be achieved. The DC supply for these motors was typically derived from the AC supply network using, in the early days, ignitron and thyratron ‘vacuum’ tube technology.
Developments in semiconductors later allowed the ignitrons and thyratrons to be replaced by solid-state thyristors, making it possible to produce drive systems that were more compact, efficient and reliable. Thyristors were in turn displaced when further semiconductor development led to the introduction of the newer power devices that are widely used today, such as the IGBT, MOSFET and more recently the up-and-coming SiC. With these, further size reductions can be achieved, along with improved control and even greater efficiency.
Parallel development of motor materials saw permanent magnets evolve from ferrite, to alnico to samarium cobalt to neodymium iron boron, with each step providing higher magnetic field strength for a given size of magnet and improved resistance to demagnetisation. The steels used in the magnetic circuits of the motors also evolved from basic mild steel to silicon steel of various grades, then to cobalt steel and ultimately to amorphous and nanocrystalline niobium-enhanced materials. This progress in magnets and steels has made it possible to reduce motor sizes while increasing efficiency.
The progress described so far in this article brings us to a point that corresponds to the motors typically in use today. An important observation, however, is that these are almost always three-phase machines comprising a number of individual coils distributed around the motor periphery and interconnected to form a three-point star set. The motors are powered by three-phase inverters fed from batteries in mobile applications and the AC mains, rectified and smoothed, in fixed applications.
This is not the optimum system architecture, however, particularly for electric vehicles, which are set to become the dominant mode of transport – land, sea and air – in the near future. What’s needed here is safe, environmentally-sustainable drive systems with high efficiency. Kilometres per watt-hour will be a critical metric.
From the point of view of safety, three-phase systems in electric vehicle applications have an inherent limitation. Any single failure in the inverter or the motor will stop the vehicle. And, from the point of view of efficiency, three-phase systems involve large system/interconnection inductances, which along with large semiconductor switches, means relatively slow switching and limited efficiency.
The drive systems of tomorrow will address these critical issues by segmenting the motor into multiple segments, each of which will be isolated from the others and powered by its own mini inverter. These small mini inverters can be designed to switch very fast and very efficiently. They can also be manufactured cost effectively in volume using conventional pick-and-place PCB assembly machines – as there are no physically large and awkward-to-handle power devices that need to be mounted manually.
In operation, the output of each mini inverter is fractionally phase shifted from that of its neighbours, which minimises electromagnetic interference (EMI) and reduces the size of the crucial and normally voluminous DC link capacitor needed. Another crucial benefit is that, because the mini inverters are isolated from each other, a failure in one of them – or the motor segment it is feeding – merely results in a small overall loss of power from the motor which, in every other way, continues to operate normally.
Since they are physically small, the mini inverters can be readily integrated into the motor structure to produce motorised hubs that eliminate the need for high current cables between the motor and the inverter. This not only reduces costs but also saves space and greatly reduces the potential of the drive system for generating EMI.
The motor technology favoured for use in the new segmented drive systems may well be based on enhanced reluctance machines as these require little or no permanent magnets. However, research is also continuing into magnet materials, and there are now promising alternatives to neodymium, 85% of the world’s known reserves of which are in China. In particular tetrataenite, although at an early stage of its development, is a very exciting prospect as a potential replacement for and improvement on neodymium.
Alongside the developments in the magnetic materials for the motors, niobium-enhanced steels offer huge increases in permeability creating further opportunities for efficiency improvements. This is of particular interest in high performance applications, such as those associated with aircraft.
Motor technology has come a long way since Ferdinand Porsche demonstrated his Electromobile electric vehicle with wheel hub motors at the 1900 Paris Expo. That proved to be a false dawn for electric vehicles, but with today’s new drive technologies based on greatly improved materials, segmented motors and mini inverters, along with the environmental imperative to ditch the polluting internal combustion engine, it can be confidently predicted that this time around, electric vehicles are here to stay.
