Motor feedback systems rotary incremental with commutation
For a reversal of the direction of current, the polarity of synchronous motors used to be reversed via so-called brushes Current reversal is required for continual rotary motion.
With the introduction of brushless motors, incremental motor feedback systems with commutation tracks have taken on this task. The mechanical commutation, which is very much prone to wear, is no longer required. This makes brushless synchronous motors practically maintenance-free.