Variable-frequency AC induction motor controller

A system for driving a mechanical energy converter from a source of multiphase alternating current, said system comprising:

• a mechanical energy converter including mechanical drive means, which produces a mechanical drive for application to a load which may vary under some load condition;
• drive characteristic sensing means coupled to said mechanical energy converter, for producing characteristic signals representative of said mechanical drive;
• a multiphase alternating-current induction motor mechanically coupled to said mechanical drive means of said mechanical energy converter, and including at least a plurality of windings associated with a predetermined number of poles, each winding being driven by a different phase of said alternating current, for driving said mechanical energy converter in response to electrical power applied to said plurality of windings;
• rectifying means adapted to be coupled to said source of multiphase alternating current, for generating direct voltage;
• controllable multiphase inverter means coupled to said rectifying means and to said plurality of windings of said motor, for converting said direct voltage into multiphase alternating voltages of controllable amplitude and frequency applied to said windings of said motor;
• error signal generating means coupled to said characteristic sensing means and to a source of signals representing a reference characteristic, for generating a difference signal indicative of a characteristic error;
• motor speed signal generating means coupled to said error signal generating means, for converting said characteristic error representative difference signal into motor speed signals representative of a desired motor speed which corresponds to the current load condition;
• motor drive means coupled to said motor speed signal generating means and to said inverter means, for converting said motor speed signal into alternating drive signals for said windings of said motor, said drive signals having a fundamental frequency and amplitude, said fundamental frequency being equal to said desired motor speed multiplied by a quantity, which quantity is equal to one-half the number of said poles of said motor, said amplitude being normalized by the ratio of said fundamental frequency to a reference frequency, said alternating drive signals at said fundamental frequency including a component at the third harmonic of said fundamental frequency.