Water cooling multiplies brake resistor power density

5th September 2018
Lanna Deamer

There would seem to be a world of difference between the current generation of advanced electric sports cars and the heavy industries where Leicester based electrical resistor manufacturer, Cressall Resistors traditionally operates. However, the common denominator is a lightweight 25kW water-cooled resistor, as Martin Nicholls of Cressall explains.

There are now a number of very advanced electric sports cars on the market, from both brands known well for their combustion-powered cars and start-ups focused specifically on performance electric vehicles. These cars do, of course, come with the need for a similarly sophisticated wallet.  

Cressall Resistors’ involvement in the electric vehicle market allowed it to identify the need for the first of a series of Electric Vehicle (EV) resistors. Electric cars have always been at a disadvantage against their petrol-driven counterparts in terms of energy density. A can of petrol weighing 4kg equates to an electric vehicle battery weighing half-a-tonne, and costs six Euros compared to 40,000.

The issue of battery life exacerbates high capital cost and weight. As a result, many all-electric sports cars, remain status symbols more than economic ways of getting to work. Alternatively, hybrid vehicles run from much smaller engines, appropriate for the power consumed in an average car journey and which recharges the battery.

Advanced ceramic
In the meantime, Cressall also identified an industrial need for water-cooled resistors with minimal footprints in industrial applications, launching the EV2 water-cooled resistor suitable for medium voltage industrial applications. This features two plates made from an advanced ceramic with exceptional properties.

An important benefit of using an industrial electric drive is that reliable systems of regenerative and dynamic braking are available to complement or replace traditional mechanical braking systems. The advantages of electric braking include control, reliability, mechanical simplicity, weight saving and in certain cases, the opportunity to make use of the regenerated braking energy to top up batteries.

The EV2 has the same insulation properties as any other ceramic, with insulation withstand to 20kV, but it will also conduct heat almost as effectively as aluminium - usually a good insulator is a bad conductor.

The EV2 was developed using unique patented construction to manage temperature. This design was prepared using finite element analysis (FEA) software tools, allowing a peak temperature of about 350°C from an electrical load of over 25kW, although 40kW has been attained.

Because the resistor is completely encapsulated, it has a lower explosion risk, important for offshore rigs and can conduct heat away more quickly from areas where this would be a problem, like the bowels of a ship.

Independent braking systems
All vehicles have at least two or three independent braking systems for safety reasons. In a conventional car these are usually dual-circuit hydraulic brakes plus a handbrake.

In 15-20 years, we will probably all be driving hybrid or electric vehicles.

Wherever possible, electric vehicle braking will be regenerative rather than mechanical: to store and re-use the regenerated braking energy, rather than just dissipating it as waste heat. Recovered energy can be stored in the vehicle’s batteries or ancillary media such as flywheels or ultracapacitors.

Cressall’s EV2 water-cooled resistor is a 25kW unit available as a single unit or as a block of ten with a common cable box attached to 250kW braking power input, all using a common water supply. Cooling is achieved on the EV2 by pumping cold water, which comes into one end of the system and then absorbs the heat. It can be pumped through a radiator, which can be located some way from the heat generating equipment.

This hot water can then be used to provide heat to the cabin of the vehicle. This innovative method of heating reduces the amount of energy required from the battery and uses heat that otherwise would have been wasted.   

Perhaps the needs of the automation industry and the electric vehicle sector are not worlds apart after all?

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