In the industry, variable speed electrical drives are employed for various applications; their power bandwidth differs from few watts for a small servomotor up to megawatt for traction applications. For decades the dc-motor was the core of the mostly used drives, because of its simple speed controllability. Nevertheless, the disadvantage associated with these machines is the wear-associated commutator which leads to rise in maintenance expenses and also to a raised soiling of the machine by the brush abrasion. Unfortunately, a complete encapsulation of all electrical components is relatively complex due to the construction. On the other hand, rotating ac-machines are known since the initial times of the electrical machines. However, they were rarely used as the speed was not simply controllable, because it changes with the frequency of the supply grid. Consequently, machines of this kind were only used where features of constant speed were required, e. g. as generators in power stations or water pumps.The advances of the semiconductor technology increased the possibilities to feed ac-machines with variable frequency in the supply grid, permitting variable speed in a realizable and controllable way. Here, the supply voltage is rectified into a so-called dc-link. From here, three-phase voltages with variable frequency are generated with the help of a variable-frequency converter, better known as inverter. Herewith, the task to operate the machine with any speed was solved. However, a dynamic control similar to the dc-machines became a new problem. This was due to the fact that the separate control of field flux and torque is not inherent like in dcmachines. This problem was solved with the introduction of the so-called Field Oriented Control (FOC ) about 20 years ago. Today, strategies based on its principle fulfill nearly all demands of servo drives in the application field.