Feature: System Design
application requirements. 316 grade contains two per cent
molybdenum, which makes it more resistant to deformation and degradation at high temperatures. For this reason, 316 grade is preferred for DBRs that need to operate under continuous, extreme heat exposure. However, 304 grade is a more cost effective alternative if the application does not involve continuous exposure to high temperatures.
Extra protection Resistance to corrosion is also an essential factor when selecting resistor materials. For example, DBRs within marine propulsion systems need to be strategically designed to prevent wear and degradation from exposure to moisture, salt and abrasion. High resistance to corrosion makes 316 grade an ideal choice for these harsh conditions, which is why it is oſten referred to as marine grade. To further enhance durability in marine environments, Cressall also uses titanium-sheathed elements in super duplex stainless-steel vessels to ensure high, continuous performance even in hot sea water. In some industries, high-level sealing is
required. For instance, mining machinery is exposed to dust and debris that could damage the internal components. Enclosing the resistor elements from the surrounding environment is therefore essential for mining applications. Te International Electrotechnical
Commission (IEC) uses Ingress Protection (IP) ratings, made up of two numbers, to show how well equipment resists intrusion from solids like dust and liquids like water. Each number indicates the level of protection for that element. Te first grades the level of protection against solids from zero to six, and the second grades the level of protection against liquids from zero to nine. Resistors used for mining benefit from sealing to IP56 level, preventing harmful dirt, dust and pollution from damaging the elements. Across different applications, DBRs
are exposed to variable damage factors. For this reason, it is crucial to consider the primary risks associated with the end use environment when selecting resistor materials.
EV2 resistor allows for individual units to be combined into a scalable high-power solution that can handle up to 1.0MW power. Tis modular design means manufacturers can save space by only combining the exact number of units required for the application.
Heavy-duty without heaviness Another common challenge with DBRs is their size and weight. Since DBRs are required to dissipate large amounts of energy, resist chemical and thermal wear, and withstand vibrations and mechanical shock, weight can become an issue if not factored into the component’s design. Te cooling method used also impacts the
size of the resistor. Natural convection or air cooling, in which warm air naturally rises and cools in a steady air flow, is used for low power drive systems. However, for heavy- duty applications with high heat transfer, a stronger approach is required, such as forced convection cooling. Tis method of cooling relies on fans, which add to the resistor’s overall bulkiness. Liquid cooling is the more efficient choice
for high power applications with limited space. A water-cooled resistor takes up 10 per cent of the size and 15 per cent of the weight of an equivalent air-cooled resistor. Additionally, water has a higher thermal conductivity than air and can transfer heat much quicker, allowing for faster cooling. Liquid cooling is also suited to continuous, high temperature applications as it can sustain stable temperatures for long durations, reducing the risk of overheating. For high power automotive, traction and marine systems, Cressall’s water-cooled
Balancing safety and efficiency In industrial processes, efficient performance yields high productivity, consistent quality and cost reductions. However, by their nature, DBRs waste energy by dissipating it as heat rather than reusing it. For this reason, some manufacturers dismiss dynamic braking in favour of regenerative systems that restore energy generated during braking to be reused. For example, in electric vehicles (EVs), regenerative braking allows for the vehicle battery to be recharged by excess energy. However, DBRs are still needed to make regenerative braking safe and effective. If the battery is already full or there is a
failure, the energy cannot be transferred to the battery and needs to be safely dissipated instead. Integrating a DBR into the system of an EV with regenerative braking offers a solution which still optimises energy efficiency but safeguards the braking system. Tese two technologies can work together to make the most of excess energy generated during braking and safely dissipate that which cannot be used to charge the battery. In addition, some DBRs are able to recycle
this excess energy within the vehicle. For instance, Cressall’s advanced EV resistors convert energy into heat that can be used to pre-heat the battery or warm the cabin of the vehicle. DBRs do not have to be equated with inefficiency, as they perform a crucial role in systems with regenerative braking and can be optimised to reuse kinetic energy where possible. Across different applications, potential
DBR challenges will vary. With so many factors to consider, working with a trusted partner can streamline the process of selecting the appropriate DBR. With decades of experience designing and manufacturing resistors for a range of applications and demanding environments, Cressall can provide expert guidance and support, helping manufacturers to achieve the best outcome for their electric drives.
www.electronicsworld.co.uk November 2024 27
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