61 functionality that combines various techniques to increase inverter efficiency all over the operating map, and guarantee motor stability and performance, even when using IGBTs. While the few percentage points of efficiency yet to come in electrical efficiency may not seem like much, they can have a significant impact on cooling. For example, an inverter handling 200 kW at 99% efficiency still generates 2 kW of heat that must be removed from the system, according to the expert at our power-switching, motor control and software company, who reckons there is still considerable room for improvement in thermal management. “The efficiency of the cooling solution affects how much weight you need to add to the overall system. Hence, we’re seeing more use of liquid cooling and improvements to it, such as single oil cooling, direct cooling and nozzle-cooling systems,” the expert says. From another perspective, chasing further efficiency gains in the inverter when the percentage is already so high is of only marginal benefit, and there is more to be gained elsewhere. “I’m good with 99%, so I’m going to focus on other areas of the drive system where we can actually have an influence, particularly the electric machine,” says the expert from our developer of light, medium and heavy-duty traction inverters. Power density The variables determining power density in an inverter are volume/weight and current capability. For the same voltage and volume, the higher the current capability, the higher the power density, while the peak current capability defines the maximum output of the inverter. The difference between the peak and the continuous current is directly related to the heat management design of the inverter, says the expert at the traction inverter and BMS developer. “The better the thermal design, the smaller the ratio between peak and nominal, and thus the inverter will be more power-dense for continuous activity.” Specifically, the combination of thermal mass and thermal resistance drives the peak to continuous ratio, so a large heat sink with a large thermal mass compared to its thermal resistance will increase the ratio. The company’s development approach is to consider the inverter as a system formed of subsystems, and to focus on both efficiency and heat management. “This has allowed us to maximise all subsystem integration efficiency, and reduce volume and weight, while improving cost reduction and manufacturability,” he says. In low-voltage inverters (below 60 V) for light-mobility applications such as two-wheelers, he says the company has achieved 5x to 8x the current and therefore power density of typical solutions on the market. For high-voltage products, its benchmark is 30 kW/kg, while its large automotive inverters achieve between 15-30 kW/kg. Our light, medium and heavy-duty traction inverter specialist has developed a 48 V system capable of delivering up to 60 kW. Its expert says the limiting factor is the DC link current. Pulling 60 kW from a 48 V battery invariably draws over 1.5 kA. The DC connections to handle this current need large, heavy and expensive conductors. Thermal management problems are a frequent limiting factor when it comes to power density. As this parameter increases it becomes more difficult to remove the generated heat. Here, less lossy advanced semiconductors are complemented by the advanced cooling technologies mentioned above, along with others such as phase-change materials and the optimisation of cooling system designs, says the expert from our advanced semiconductor and control algorithm developer. Along with their advantages, advanced semiconductors also come with drawbacks, one being increased EMI, according to the power-switching, motor control and software company. “If you drive up the switching frequency to reduce component size, you also E-Mobility Engineering | March/April 2024 Efficiency maps for the Exro Coil Driver inverter, driving the same motor in series mode (left) at low speed and parallel mode (right) at high speed, compared with a conventional three-phase inverter (Image courtesy of Exro Technologies)
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