DEVICE FOR DETERMINING THE COMMON FREQUENCY OF TWO INDEPENDENTLY VARIABLE ALTERNATING VALUES, IN PARTICULAR IN A ROTATING FIELD MACHINE

摘要

An apparatus for determining the common frequency w of two independently variable a-c quantities is described. In a rotating-field machine, the flux, for instance, is described by two electrical a-c quantities which together determine the magnitude and direction of the field. The two a-c quantities are variable independently of each other and the instantaneous direction of the field is given by an angle alpha according to the relation a1/a2=cotan alpha , where a1 and a2 are the two a-c quantities. In order to determine the frequency of rotation w=d alpha /dt of the field from these quantities, the quantities a1 and a2 are each fed to smoothing stages and thence to a computing circuit, wherein the quotient (d1xa2-d2xa1)/(d1xa1+d2xa2) proportional to the frequency is formed from a1 and a2 and the smoothed quantities d1 and d2. The use of differentiators is thereby avoided, which would cause a phase error as well as difficulties when harmonics of the quantities a1 and a2 are present.