This invention relates to switching-over to a higher speed clock from a lower speed clock. The present invention includes starting the operation of a motor including windings and a rotor; placing the motor in a first operational state; driving the motor from the first operational state to a second operational state; and sensing a variation in an electrical waveform induced by driving of the motor to determine a cycle period from which the speed of the motor can be determined. Preferably two laterally spaced integral sealing ribs are provided in each recess, which sealing ribs extend transversely completely across the recess and terminate at each end in a sealing projection that extends upwardly from the associated planar surface.