All electromagnetics energy conversion devices which incorporates permanent magnet technology are described by the term ‘permanent magnet machine’. A single or multiple permanent magnet are used for magnetic excitation. Variety of configurations could be found in the energy converter that employs permanent magnet technology, several examples are, generator, motor, stepper motor, alternator, actuator, linear motor, control motor, transducer, brush less DC motor, tachometer and many more. The stator of the motor is like the stator of the multiphase AC motor. The new component incorporated in the permanent magnet motor is rotor, the rotor is complete contrast to that of conventional rotors. In this application, rotor relies of magnetic excitation unlike excitation by electric current in the winding of multiphase AC motors. To achieve the higher efficiency for the desired load characteristics, high efficiency, high power factor as well as performance, it is necessary to optimise the configuration of the rotor, mechanical design and electromagnetic rotor and the design of the electromagnetic stator (Rizk et al, 2000).
The direct drive wind turbine application requires a machine with high power density, high power torque, high efficiency with low design operating speed. The primary reason to use direct drive permanent magnet machine is that the machine has an ability to reduce the cost of converting wind power to mechanical power simply by eliminating the necessity of step up gear box. The speed increasing gear box is usually incorporated in wind mill to amplify the small rpm of the wind rotor to higher rpm to produce electricity. The cost associated for the operation as well as maintenance of direct drive permanent magnet machine is very low as compared to gear coupled machines.
The role of low speed, high power direct drive electric machines are extensively limited to special applications such as large hydroelectric generators therefore they are not commonly used in the industry. Therefore, low speed and high torque motors need reliable evaluation and experimentation to analyse their suitability for the wind turbine application as a direct drive device. Such suitable application could be revolutionary due its high performance, low cost and simple structural requirements. (Baywaters et al, 2005)