Asynchronous motors in electric traction

In many applications the electric motors must possess a high overload capacity. That is particularly necessary in electric traction, in lifting gears and in various industrial applications. Initially either d.c. motors were employed with series excitation, able to assure high working flux value and to produce high torque, or d.c. motors with shunt excitation in which the armature reaction is compensated by auxiliary poles. Some time ago flux value requirements in the motors were obtained by resorting to excitation with permanent magnets having high coercitive force, both in d.c. motor and in asynchronous motors (brushless motors), giving the possibility of obtaining a torque of the order of 4 times the nominal value of the motors. The aynchronous motor has always represented the ideal motor for its robustness, reliability besides its low cost, but for a long time it was considered unsuitable for traction applications because of its difficulty to support overloads. The vector control type inverter has increased its performance but, because of its high dependence on the motor parameters value and stability, it is not suited to applications requiring a torque 1,5 to 2 times higher than the nominal value. Using the vector control it is still necessary to resort to gear changes in the electric passengers car or to over dimension motors in industrial trucks. The new DFC system presented in this article is based on a choice of variables and of control systems so to avoid any dependence on motor parameters and it is suited to any overload. In DFC the constancy of rotoric flux Φ2 is particularly observed in every condition of speed and load. The response illustrated on Fig. 1 is obtained. We note the perfectly linear increase of torque with slip. We see the advantage of this condition in respect to the cases of flux constancy in airgap φm and in the input Φ1. The performance of DFC is superior to that of Brushless motor not having physical limit to the maximum torque value nor magnetisazion problems. The asynchronous motor required is much more reliable and cheaper than Brushless.