Cool customer

A liquid-cooled electric motor offers a neat opportunity for automotive designers to increase the power and cut the size of motors while avoiding the problems of overheating. Jon Excell reports.

To say that water and electricity don’t mix well is something of an understatement, yet more and more engineers are exploiting the cooling properties of water in an ever-increasing variety of electrical applications.

In one of the latest developments in this field, engineers at Hitachi’s Tokyo research laboratory have developed a prototype automotive electric motor that is cooled by water.

As cars include more and more driver-friendly features, the number of electric motors built in to them continues to grow. To accommodate these extras, designers are under increasing pressure to make motors both smaller and more powerful.

Clearly, however, as the power of electric motors is increased and their size minimised, managing the higher amounts of heat that they generate becomes more of a problem. High and unpredictable loads frequently cause the bearings and rotors of automotive electric motors to overheat. This leads to damage and can dramatically reduce the useful life of the motor.

The water-cooling concept developed by Hitachi means that because the problem of overheating is controlled, the size of a motor can be smaller for a given output – or greater output can be produced from motors of a given size.

Hitachi’s engineers believe the design of its water cooling system will efficiently dissipate heat generated by a motor’s stator, rotor, and bearings, and allow it to operate at a relatively uniform temperature.

The company explained that by keeping the bearing temperature down the problem of sticking is avoided, and by keeping the rotor temperature low, rapid deterioration of the wiring insulation is also avoided.

Lquid coolant, such as water, flows through the stator and around the bearing parts. Stationary cooling discs are attached to the stator while annular rotor discs rotate close to them.

Most of the heat generated by the rotor is transmitted to the stator across the 0.1-0.6mm gap between the cooling discs, helping to maintain a constant rotor temperature. The cooling discs themselves are made of materials with high thermal conductivity and can take on a variety of shapes, to increase the exposed surface area between the discs.

The coolant in Hitachi’s system enters at one bearing end and makes a half-turn around the bearing. It is then ducted to the stator, where it passes along passages that zigzag around the stator.

The liquid then cools the bearing at the opposite end from the place where it entered, returns through the stator, and cools the other half of the entry-end bearing before leaving the motor.

The company added that the amount of heat transferred from the rotor to the stator can be increased even further by applying thermally conductive grease or resin between the discs.

The performance of the water-cooled motor was verified through the testing of a 30kW prototype. The results of these trials indicated that the temperature increase inside the rotor and stator is reduced by 60 per cent and 40 per cent.

A Hitachi spokesperson said that while there are no immediate plans to commercialise the product, the company is keen to license the technology to an interested manufacturer.

In a separate development, drives expert Danfoss recently announced the launch of Shower Power, a drive unit for high-powered automotive applications in which coolant is sprayed on to the hot baseplate of the drive unit.

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