A conventional DC electric motor uses a stationary permanent magnet, with electrical windings on the moving part of the motor, or rotor. Brushes are needed to provide the electrical connection to the rotor, and to switch current to the correct set of windings as the rotor turns.
The disadvantage is that the brushes wear. Speeds are limited by the problem of preventing the windings falling out under centrifugal force, and because the brushes start to ‘float’ at higher speeds, increasing losses. Stronger springs can hold them down, but this increases wear.
A brushless DC motor solves these problems by having the magnet on the rotor, and stationary windings so brushes are not needed for the electrical connection.
But there is still a need to switch the windings in the correct sequence. This is achieved using power transistors in Bluebird Electric, integrated gate bipolar transistors, which are switched by a control signal from optical sensors on the rotor.
The power of an electric motor is the product of the torque generated and the speed of rotation. ‘Torque dictates the size of the motor,’ says Sheffield University’s Tim Allen, ‘but if you double the speed you double the power.’ Because Bluebird’s electric motors can run at up to 20,000rpm, compared with 3,000 for a conventional industrial motor, it can generate more power for its size. Bluebird’s generate 100kW (135bhp) but weigh only 13kg.
Chassis: Steel tube space frameBody: high modulus twill (a widely used racing car material)Length: 7.0mWidth overall: 1.24mWeight: 1.1 tonneDrag coefficient: 0.17Motors: three-phase brushless DC, oil-cooledTorque: 70NmPower: 100kW at 20,000rpmBatteries: 48 Hawker Genesis lead-acid, 600VBrakes: disc at front, regenerative braking at rear}}