Frequency converter for controlling asynchronous motors, especially for use in automatic screwdrivers

摘要

In order to achieve precise results with automatic screwdrivers which are driven by an asynchronous motor, the motor must be stopped as quickly as possible on reaching the desired torque. The short desired braking time of approximately 50 milliseconds can be achieved only if the braking process is controlled optimally. To this end, on the one hand it is necessary to take into account the excitation of the armature before the start of the braking process and, on the other hand, it is necessary to match the supply frequency of the motor such that the maximum possible torque is applied throughout the braking process, because the conventional methods of direct-current or opposing-field braking are no longer adequate. The desired characteristics are achieved using a frequency converter which tracks the phase sequence in a counter and, in the event of a braking command, in each case calls up the braking command matching the present conducting phase, from a memory. The operating frequency during the braking process must be optimally matched to the mechanical time constants of the drive via a delay element and a voltage/frequency converter.