Water cooling multiplies brake resistor power density

Jon Lawson

Electric vehicle braking resistor is also suitable for industrial applications, says Martin Nicholls

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 firm Cressall Resistors traditionally operates. However, the common denominator is a lightweight 25kW water-cooled resistor.

Cressall Resistors’ involvement in the electric vehicle market allowed it to develop the first of a series of Electric Vehicle (EV) resistors, culminating in a product the company christened the EV2.

Advanced ceramic

Cressall later 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.

On the one hand, the EV2 has the same insulation properties as any other ceramic, with insulation withstand to 20kV. However, the thermal properties are such that it will conduct heat almost as effectively as aluminium - normally 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.

Launch customers for the EV2 include Rolls Royce in Canada and Samsung for a Korean vehicle prototype. This is driven by the need for minimising onboard space previously occupied by traditional water-cooled resistor designs. Because the resistor is completely encapsulated, it has a lower explosion risk for offshore rigs and can conduct heat away more quickly from areas where this would be a problem, like the bowels of a ship for example.

Cressall’s French distributor has a customer that makes winches for offshore oil platforms, which use Caterpillar drives to lower pipes down to the seabed, and this requires braking. The customer now uses Cressall water-cooled resistors at the same price as the traditional air-cooled resistors but occupying one tenth of the space and the company has plans to get certifications from both Lloyds and Atex for the EV2.

Independent braking systems

In 15-20 years, wherever possible, braking will be regenerative rather than mechanical: this creates the possibility of storing and re-using the regenerated braking energy, rather than just dissipating it as waste heat. Storage of the recovered energy can be in the vehicle’s batteries or in 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?

Martin Nicholls is with Cressall Resistors