The invention relates to a method for power control of an electric motor,
in particular a motor for driving the suction fan of a vacuum cleaner
connected to an AC mains power supply. The power control is effected by
varying the phase angle of the electric alternating quantity supplied to
the motor by the use of semiconductor switches. AC power supplied to the
motor is switched by using different delay times for the switching in
each half-period of a full AC period to suppress odd harmonics,
particularly the third harmonic of the mains power line frequency.
Half-periods of a full period are mirrored in the next consecutive full
period in order to suppress even harmonics, particularly the second
harmonic of the mains power line frequency. A set of delay times t1 and
t2 are defined and selected to be used in further avoiding critical delay
time areas for harmonics and minimize the generation of vibrations.