Challenges of running high-speed rotors
The production electric car is now a reality, with a number of the big automotive manufacturers selling them and many more having parallel electric hybrids in production. Parallel hybrids are used in Formula One and are widely used in endurance racing too, courtesy of some brave changes in the regulations. In 2014, we have also seen Formula E emerge as a global race series, pure electric racecars competing on circuits around the world, with some big names involved. Electric motorcycle racing led the way, and the average speed for the electric TT on the Isle of Man in 2014 rose by almost 8 mph to more than 117 mph.
For a given type and displacement of engine, it is a rule of thumb that maximum potential torque is a fixed amount – for example, all 600 cc four-cylinder, four-stroke engines with sufficient attention given to gas-exchange processes and friction will produce the same level of torque. Power depends on the torque and the speed at which it can be achieved. The same applies to electric machines, as the potential maximum torque is fundamentally a function of the electrical design choices and the construction of the motor, and the power depends at what speed this torque can be maintained. As with engines, there is a strong incentive to run an electric motor at high speed.
There are however a number of barriers to running at ever-increasing speeds, although they are very different from those in a reciprocating engine. There are no strong second-order forces and couples as we find in an engine, and fewer causes of excitation to provoke a strong resonant response.
The main enemy is centrifugal force. For a permanent magnet motor, it is not a simple task to keep the magnets in place at very high speed. The motors used on the electric machines in the exhaust heat recovery motors, driven by the exhaust turbine in Formula One, run at speeds up to 100,000 rpm. It is certainly not enough to simply rely on the magnetic forces to keep the magnets in place, and the mechanical strength of magnets leaves much to be desired – even if we were to be able to ‘stick’ them to the rotor, they are unlikely to remain intact.
It is necessary therefore to restrain the magnets radially to prevent both loss and breakage. In low- and medium-speed motors this is sometimes done with glass-fibre reinforced polymers, but for high-speed motors, higher strength composites such as carbon or PBO are used. The limitation is then the strength of the composite sleeve.
It is also important to have high-speed rotors properly balanced to a high degree of accuracy in order to prevent early bearing failures and to minimise the transmission of vibration to adjacent components. When dealing with very high-speed rotors, the machinery for balancing them can be complex.
Another problem can be maintaining the rotor magnets within their operating temperature range. At relatively modest temperatures the magnets can become demagnetised, and this is not a property that returns when temperatures fall to more desirable levels. Some companies resort to liquid or forced-air cooling of rotors in order to keep magnet materials sufficiently cool.
Written by Wayne Ward