Electric motors: cooling concepts
The rise of hybrid drive technology, both in the production automotive sphere and latterly in racing, is something I hope will breathe new life into both realms of engineering. Years of racing powertrain regulation have led to some fantastic pieces of machinery powering cars and motorcycles around the circuits of the world, but they have become increasingly irrelevant to the direction of mainstream production vehicles.
If we take the energy storage aspect of hybrid powertrains (electrical energy only - flywheels will be dealt with in another article), we are still some way from matching gasoline, diesels or alcohols. However, electric motors are more than competitive with internal combustion engines. While not at anything like the same stage of development as a race engine, an electric motor can deliver huge power per unit mass - 5 kW per kg was not at all unreasonable even a decade ago, and we can assume that the current batch of electric motors used in Le Mans and Formula One competition are at or above the same level.
Just as an engine has losses that detract from its output, so does an electric motor. There are losses in the rotor and the stator which are apparent as heat; if the motor is not to overheat rapidly, its output must be limited or the heat must be rejected. The power is limited often by the ability of the conductors in the windings to carry the current without overheating and melting.
The limits of passive air-cooled motors - those using finned stator cases - in terms of current capacity per square millimetre is in single figures, with 5-8 A/sq mm being typical*. This may be improved by forced convection, for example using fans or air ducts to channel air from the outside of the vehicle to the motor, thus keeping the cooling fins at a lower temperature. Finned motor cases lead to a large increase in diameter, although for low-cost static machinery, this is not a concern.
Jacketed motor bodies offer an effective way of taking heat away from the stator. As with a water jacket around a single-cylinder engine, the same principles apply with an electric machine but, if the windings are to be kept dry, this method relies on the conduction of heat through the steel stator 'stack'. An accepted level of current in the windings of such machines is in the region of 10-15 A/sq mm. Complexity is added here because we now need to have a cooler in the system, and a pump.
A method of cooling the windings more directly without having them actually in contact with liquid coolant is to have enclosed coolant channels running along the stator slots and between the windings. This method most readily lends itself to conductors that are produced in rectangular section. By conducting heat through thin-walled coolant vessels rather than via the steel/iron stator, we can increase the allowable current capacity of the conductors to around 20 A/sq mm.
If we wish to go further than this limit, then we have to consider directly cooling the conductors with an electrically non-conductive coolant. There are many suitable fluids for this, ranging from pure water (deionised water is a poor electrical conductor) to special-purpose electrical oils, which are designed for cooling electric motors and transformers, and are hence often known as 'transformer oils'.
There are two common methods of cooling the winding conductors directly. The first is immersive cooling, where the winding coils are immersed in coolant which is then circulated via a cooler. The second method is to spray oil directly onto the end-turns of the coiled conductors. These are the areas which are most easily accessed, and in Fig. 1 here they are the parts of the coils at the end of this wound stator. Such cooling schemes where coolant removes heat directly from the coils allow a current capacity of about 30 A/sq mm.
* Gieras, J.F., "Advancements in Electric Machines", Springer, 2010, ISBN 9-0481-8051-1
Fig. 1 - Direct cooling of the stator windings allows high current densities to be used, resulting in a smaller motor for a given output
Written by Wayne Ward