An electric motor (40) has a permanent-magnet rotor (46) and an apparatus for generating a three-phase sinusoidal current (i202, i204, i206) for supplying current to said motor (40), also a microprocessor (95) for executing the following steps: while the motor (40) is running at a substantially constant load, the motor is operated firstly at a predetermined operating voltage (U), and an amplitude of a current flowing to the motor is iteratively sampled, stored, and compared as applied voltage is decreased. If it is found, in this context, that the current flowing to the motor has not decreased as a result of reduction in the voltage amplitude, the motor (40) is operated at that current. If, however, it is found that the current flowing to the motor has decreased as a result of the reduction in the voltage delivered to the motor (40), the measurements and the comparison are repeated, optionally multiple times, in order to identify values for optimized efficiency.