Electric vehicle
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| Figure 7.24 (a–c) Diagram showing the basis of operation of the brushless DC motor B A B C C A N S N S Figure 7.25 Diagram showing an arrangement of three coils on the stator of a BLDC motor Electric Machines and their Controllers 173 Figure 7.26 A 100 kW, oil-cooled BLDC motor for automotive application. This unit weighs just kg. (Photograph reproduced by kind permission of Zytek Ltd.) These BLDC motors need a strong permanent magnet for the rotor. The advantage of this is that currents do not need to be induced in the rotor (as with, for example, the induction motor, making them somewhat more efficient and giving a slightly greater specific power. Permanent magnet synchronous motors which area type of BLDC motors are increasingly used in electric vehicles. Modern electronics allow the supply frequency to be continuously varied so that it can be used to control the motor speed and hence the vehicle speed. Permanent magnet synchronous motors are highly efficient and tend to replace induction motors in many applications. This is due to the fact that permanent magnet motors have a higher torque-to-volume ratio as compared with the induction motors. Also, the decrease in the manufacturing cost of permanent magnets makes the permanent magnet motors appealing. 7.3.3 Switched Reluctance Motors Although only recently coming into widespread use, the switched reluctance motor (SRM) is, in principle, quite simple. The basic operation is shown in Figure 7.27. In Figure a the iron stator and rotor are magnetised by a current through the coil on the stator. Because the rotor is out of line with the magnetic field a torque will be produced to minimise the air gap and make the magnetic field symmetrical. We could lapse into rather ‘medieval’ science and say that the magnetic field is reluctant to cross the air gap, and seeks to minimise it. Medieval or not, this is why this type of motor is called a reluctance motor. At the point shown in Figure b the rotor is aligned with the stator, and the current is switched off. Its momentum then carries the rotor on round over a quarter of a turn, to the position of Figure c. Here the magnetic field is reapplied, in the same direction 174 Electric Vehicle Technology Explained, Second Edition on on off Magnetic field turns rotor to minimise air gap Momentum keeps rotor moving Magnetic field turns rotor to minimise air gap (a) (b) (c) Download 3.49 Mb. Share with your friends:
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