Liquid-cooled brake resistors in electric vehicles

Manufacturers can take advantage of REO liquid cooling technology for brake resistors without worrying about power-to-weight ratio.

A fundamental principle of good product design is power-to-weight ratio. This concept is particularly important in the design of components for electric vehicles, such as brake resistors. This is because electric vehicles already have a high base weight in the design due to the lithium-ion batteries and electric motors. Weight optimization of vehicle components is therefore an important aspect of development. Steve Hughes, Managing Director at REO UK, explains the advantages of liquid cooled brake resistors and gives an insight into why REO manufactures them in a weight and space optimized way.

Power-to-weight ratio is a calculation applied to motors and mobile power sources. It allows a comparison between different units or designs. To calculate power-to-weight ratio, the power of a drive or motor is divided by the total weight of the vehicle. The result is a quantity that is independent of the vehicle size and is often used as a measure of the overall performance of a vehicle. The greater the weight of the individual components, the higher and therefore “worse” the power-to-weight ratio. Consequently, saving weight in the construction of components is an important aspect in reducing the power-to-weight ratio.

Brake resistors from REO

A central part of the design of all electric vehicles is the braking resistor, which dissipates heat and slows down mechanical systems in a process known as dynamic braking. There are two forms of dynamic braking: rheostatic and regenerative. In rheostatic braking, the energy is dissipated as heat in a resistor. Regenerative braking, on the other hand, feeds electrical energy back into the system. Braking resistors with regenerative brakes are more expensive in comparison due to their construction.

Common to both systems is the development of heat that must be dissipated from the system. To improve power dissipation, resistors are therefore often equipped with active fans. However, thanks to advances in liquid cooling technology, electric vehicles are increasingly incorporating solutions that use water or a coolant to dissipate heat. REO can use this technology to design and build highly efficient cooling systems that are about 80 per cent smaller than conventional braking resistors. Electric vehicle manufacturers appreciate the more compact dimensions, lighter weight and improved cooling capability when used in their vehicles.

The greatest advantage of liquid cooling is due to the physical properties of the coolant used: liquids have a higher density than air, which leads to a higher heat-carrying capacity. The temperatures and loads on electrical components are greatly reduced and the service life is significantly extended. The elimination of a large surface area needed for ambient or forced air cooling also means that liquid cooling systems take up less space and can therefore be designed to be much more compact. In addition, the water-cooled resistor is virtually inaudible due to its encapsulated design. This contrasts with conventional resistors, which can produce audible noise at harmonic frequencies of the DC wave current. This can be disturbing in areas where they are near people.

But where electricity flows and liquids are used, there is a particularly high demand for safety. REO therefore separates the cooling system or coolant from the electrical circuit. This ensures electrical insulation, and the component can continue to run for a short period without coolant. This is important to allow safe shutdown of the system in the event of a coolant problem.

REO uses liquid cooling in many modern series because of these advantages. A good example of this is REO’s BW D 158 series of aluminum-coated water-cooled resistors. The components are designed for continuous power dissipation of up to 60 kW and are sealed to IP65 in an anodized aluminum housing. All connections and fixings are either stainless steel or anodized aluminum to protect against corrosion. REO offers the resistors between 2 and 850 ohms and, depending on the size, with a nominal power of 1 kW to 60 kW. The rated voltage of the component is 1 kV in all cases.