at the same time also the material in the apparatus has to be comminuted in the start up phase in this case also the apparatus is started up from the rest condition until it reaches the nominal rotary speed at the maximum current that is to say maximum torque if however that nominal torque is not sufficient to drive the comminuting shaft when starting up the motor the control system detects that on the basis of the signal from the rotary sensor in response to such detection of a blockage the motor is actuated to perform a reverse rotary movement by the control device by way of the frequency converter 8 when in that phase the control system detects a rotational movement of the motor the reverse movement is maintained over a predetermined period of time or number of revolutions and the apparatus is then switched into the forward direction again if no reverse movement is detected by way of the rotary sensor the apparatus is switched into the forward direction again without any time delay and that recurring procedure is effected at very short time intervals so frequently as to produce a rotational oscillating effect or rocking motion whereby the.