Due to the further development of power electronics, today three-phase alternating-current drives are seeing wider application. The inverter-controlled asynchronous motor is suitable as a variable speed drive not only in industry but also in automotive applications. Only the bypass rotor is a possibility because it is maintenance free and robust. By employing an expensive field-oriented control, an operating behavior with basic setting and field weakness areas can be achieved which corresponds to the behavior of dc motors. Particularly for a good efficiency, relatively large areas of field weakness can be presented. The power output is achieved at high speeds but not at high torques. This results in smaller machines. The efficiency is higher compared to direct-current motors but lower than synchronous motors.
The externally-excited synchronous motor consists of an excitation winding on the rotor which can be supplied with the necessary direct current either through commutator rings or using a brushless design. This motors achieves a field weakening which leads to a favorable torque curve. Here, on the one hand it is a disadvantage that the commutator rings are not maintenance-free while on the other hand a contact-free supply is very expensive. Permanently-excited synchronous machines are often used as drives. In this case rare earth magnets are employed that are available at acceptable prices today. In case of constant excitation using permanent magnets, a field weakening can only be realized at additional expense. The advantages of these machines besides the compact design and low power weights are the high efficiency.
A diagram concerning the control of a direct-current motor (GM) and asynchronous (ASM) or synchronous motor (SM) is shown in the figure.
As a result of its characteristics, the electric motor fulfills most requirements associated with an automobile drive. In addition, it can be overloaded up to 100% for a short time, operates with low vibrations and noise and is emission-free. Operation and maintenance are relatively simple and along with the constructional possibility of regenerative braking. Its competitiveness has essentially failed so far because of insufficient energy storage. This was already presented earlier. The electric drive becomes more interesting within the framework of hybrid propulsion concepts.