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POWER ELECTRONICS


and braking and proactively contribute to maintaining vehicle efficiency. Instead of internal combustion engines, EVs use a braking chopper to convert the energy generated by high-speed braking, typically when on the motorway. Urban drive cycles have a considerable amount of acceleration and decelerating periods due to traffic control in place around towns and cities. While braking, a car’s motor continues to spin even though the vehicle is trying to slow down, creating excess energy. Integrating a braking resistor allows this otherwise wasted energy to be dissipated as heat and recovered to warm the vehicle’s cabin in cold weather or to regenerate the kinetic energy to improve efficiency. The concept is widely implemented on electric trains, where wasted energy is dissipated back into the power line and consumed by other trains on the track. On the road, regenerative brakes can not only significantly improve energy efficiency, but can also contribute to maintaining the overall upkeep of the vehicle. Regenerative braking means that mechanical friction brakes are only used in emergency situations, such as sudden stops, so there is far less need for maintenance. In addition, accessory components, including resistors, don’t require any maintenance, unlike the high cost of replacing brake pads and discs. To increase a braking resistor’s dissipation capability, it’s important that the component is prevented from overheating.


WATER AND AIR The effect of temperature on a product’s performance is an important design consideration. Overheating can reduce the efficiency and longevity of components, and regulating this is critical. From our experience, the challenge of managing temperature is critical when ensuring the efficiency of electric vehicles, and standard electrical items may not always fulfil this requirement. Electric brake resistors are well known in the domain of power electronics, railway or elevator technology to dissipate heat and slow down a mechanical system to avoid overheating. Most brake resistors are typically encased in a frame to create a safe


36 www.engineerlive.com


distance between surrounding components. These frames feature a choice of either cooling fans or liquid coolants. Air cooling has traditionally been the


most common cooling method, as its fan technology is robust and able to withstand a significant amount of wear and tear without malfunctioning. If, for example, a single fin broke on the fan, it would still function safely. This method of dissipating heat does,


however, come with its limitations. Air fans create an additional source of noise, which interferes with the quiet running that is typically one of an EV’s benefits. Using a fan also means that its enclosures are generally bulkier and take up more space, which limits the range of vehicles that can use it. Air-cooling fans are further limited by their reliance on ambient temperatures to cool down heating components, which impacts the consistency of their efficiency. On the other hand, water-cooled


resistors, such as Cressall’s EV2, can be easily integrated into existing cooling systems and are able to deliver cooling to areas where fans often fall short. Pipes containing a liquid coolant circulate around an enclosure and out of the device to help keep components cool using the principal of heat exchange. Excess heat is thermally conducted


by the water in the pipes, which is transferred out of the device and regenerated for other purposes. As the water can be stored in pipes at temperatures below the application’s


ambient level, cooling can happen faster and with greater reliability. The efficiency of air-cooled fans also depends on the size of the application. Manufacturers may need to integrate multiple fans, instead of just one, if they are being used to cool components in large electric vehicles such as trucks and buses. As water has a high heat conductivity, a water-cooled resistor only requires a single pump. This considerably lowers energy consumption and overall cost, while increasing the amount of space left inside the vehicle.


THE NEW NORM With the electric vehicle market continuing to flourish, EVs are no longer a new concept. Although there is still a way to go before our entire transport system is electrified, manufacturers need to work to make sure that vehicles meet efficiency targets. We’re no longer only electrifying commercial vehicles. If the planet is to truly feel the impact of the move away from petrol and diesel, all vehicles on our roads need to undergo an upgrade. For this to happen, vehicle technology needs to keep pace. It isn’t all about batteries. Other components, such as braking resistors, must also be perfected to maximise the drive for greener roads. n


The author is managing director at Cressall Resistors


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