The role of inverter technology in the shift to electric vehicles

This article originally appeared in the June'24 magazine issue of Electronic Specifier Design – see ES's Magazine Archives for more featured publications.

Ben DeLand, Director of Electrical Hardware Engineering, GKN Automotive 

Demands on and from OEMs are key factors driving innovations, with a push for more powerful yet efficient, more sustainable, smaller, and lighter technologies, to bring EVs closer to their ICE counterpart and enable their mass adoption.  

Perhaps the unsung hero of democratising EVs, and already a crucial component in an electric drive system, is the inverter, which plays a central role in increasing range, reducing weight, and lowering cost. 

The inverter works by switching the direct current from the battery into the alternating current, driving a vehicle’s traction motor and producing torque. It delivers this power by controlling the switching of voltage to a copper stator inside the electric motor, creating the magnetic field that spins the rotor.  

In many of the first-generation EVs, the inverter sits separately from the traction motor, relying on a heavy cable harness to deliver the AC output to the motor. By integrating the inverter into the electric drive unit (EDU), though, manufacturers can replace the cable harness with busbars which make an internal connection between the inverter output and traction motor input.  

This increased integration not only reduces the weight and packaging of the system – alleviating the cost barrier – but also leads to improvements in power density by dramatically reducing the resistance between the inverter and traction motor. The overall unit can also be made more compact as the inverter can be designed to fit within the natural contours of the traction motor and gearbox, further reducing costs through package down-sizing.  

Also central to the successful uptake of EVs is efficiency, with consumers often still citing ‘range anxiety’ or, perhaps more aptly referred to as ‘charging anxiety’. This relates to how far a vehicle can travel on a single charge, how efficiently it uses its power, and how quickly it can be charged during a journey. Efficient power conversion from the battery to the driven wheels is central to this, and the inverter acts as a primary enabler for higher efficiency – often through better thermal management and superior power electronics.  

Advancements in power semiconductor materials, for example, have the potential to offer considerable efficiency gains. This is an area we continue to explore, looking at chemistries including silicon carbide (SiC) and perhaps even gallium nitride (GaN).  

The higher switching frequency capabilities of SiC, compared to classical Si IGBTs, enable the use of higher speed – potentially smaller – traction motors, leading to an overall reduction in the size of the electric drive system. The current needed for operation of this downsized system is also reduced, resulting in a more cost-effective power stage within the inverter. The faster switching slew rate of SiC also reduces the negative effect of switching losses, meaning they are more efficient. 

The largest trade-off for the efficiency gains offered by SiC, though, is its high cost, given the intensive production process involved for this technology. This is where GaN could come in. Capable of much higher switching frequencies than even SiC, we expect the production of GaN to be more cost-effective than SiC. The key challenge to enabling this technology for traction systems will be overcoming the inherent deficiencies regarding power delivery. 

When we consider the relationship between power and voltage, we can understand that increasing the battery voltage levels from 400 to 800V enables a reduction in the current to deliver the same amount of power with reduced losses. This, in turn, creates weight savings via lighter cabling and lower inverter mass, as well as the opportunity to use a smaller capacity battery without losing range.  

As one of the heaviest and most expensive components in an EV, we believe the biggest range improvements in the short to medium term will come from focusing on the optimisation of components, rather than on larger batteries.  

By being able to reduce battery size, we can reduce cost and weight, therefore improving efficiency. The ability to increase the amount of power a charger can deliver to the battery also significantly reduces charging times, which remains firmly on the minds of many prospective EV owners.  

As such, the optimisation of components is a key focus of ours to allow for efficiency gains, and an area we continue to invest in heavily. Take the 800V inverter, for example. It delivers 20% more power – a power-density increase of 50% and a power-to-weight improvement of 60% - compared to the previous model, all while reducing the amount of copper used by over 60%.   

We consider this efficiency gain to be a key architecture in unlocking many of the most important benefits – namely, reduced vehicle weight, power losses, charging times and costs – all of which are essential to the successful uptake of EVs.