Adjustable speed A-C motor drive with smooth transition between operational modes and with reduced harmonic distortion

Improved static electric power apparatus having relatively positive and negative d-c input terminals adapted to be connected to a source of voltage, a plurality of a-c output terminals adapted to be connected to a variable frequency alternating current load, and means for connecting each of said output terminals to both of said input terminals in a manner to convert unipolarity voltage applied to said input terminals into alternating voltage at said output terminals, said connecting means comprising at least one pair of alternately conducting controllable electric valves interconnecting one of said output terminals and the respective input terminals of said apparatus, firing and commutating means for periodically switching the conducting states of said valves, and means responsive to frequency and amplitude command signals for controlling the operation of said firing and commutating means so as to vary, as functions of said command signals, the frequency and the amplitude of the fundamental component of said alternating voltage, wherein the control means includes:

• a. first means responsive to said frequency command signal for generating a periodic reference signal having a frequency which depends on said frequency command signal;
• b. second means keyed by said reference signal for determining the consecutive moments during each cycle of the fundamental component of said alternating voltage at which the control means causes said firing and commutating means to switch the conducting states of said valves, the timing of a first switching moment in each cycle being established by said reference signal, a subsequent switching moment occurring one-half cycle later, and additional switching moments occurring after said first moment at variable angles measured in electrical degrees of a cycle of fundamental voltage, the per-cycle schedule of said additional switching moments comprising:
• i. a moment following said first switching moment by a variable angle beta (.beta.),
• ii. an immediately succeeding moment occuring 180.degree. -.beta. after said first moment,
• iii. a moment following said subsequent switching moment by the same angle .beta., and
• iv. another moment occurring 180.degree. -.beta. after said subsequent moment;
• c. third means for varying .beta. between predetermined maximum and minimum angles as a function of said amplitude command signal to thereby vary the amplitude of the fundamental voltage component;
• and
• d. fourth means operative when .beta. is equal to or smaller than a predetermined value closely approaching 0.degree. for eliminating the above-listed additional switching moments from said per-cycle schedule;
• e. said fourth means including means responsive to said reference signal for preventing an initial operation thereof except during predetermined intervals which are spaced from said first and said subsequent switching moments by at least said predetermined value.