Mitigation of Harmonic Currents and Conservation of Power in Non-Linear Load Systems

Abstract

An AC power controller system applies three-phase AC operating power to an induction motor that drives a non-linear mechanical load. A primary low pass filter is connected in series between branch phase conductors and a power controller of the type that uses gate-controlled switching thyristors for controlling power to the motor. KVAR capacitors connected between the power controller and the induction motor phase windings form a secondary low pass filter across the controller output terminals. The primary and secondary low pass filters isolate the power controller and induction motor with respect to spurious noise and harmonics generated by local as well as remote sources, and also improve real power transfer efficiency from the power generating source to the induction motor by transforming the effective impedance of the power source and induction motor load.

What is claimed is: 
     
         1 . A power controller system including one or more supply input terminals for receiving AC voltage from one or more supply phases of an AC power source, and one or more supply output terminals for conducting AC current to one or more stator phase windings of an AC induction motor, comprising in combination:
 an electronic power controller including one or more power input terminals for receiving AC voltage from each of the power controller system supply input terminals, one or more power output terminals electrically connected for conducting AC current to each of the power controller system supply output terminals, and switching means coupled between the power input terminals and power output terminals for controlling the conduction of current to each of the power controller system supply output terminals;   a primary low pass filter circuit including one or more input terminals coupled to each of the supply input terminals and one or more output terminals coupled to each power input terminals of the electronic power controller; and   one or more shunt capacitors connected in shunt phase to neutral relation across each of the electronic power controller power output terminals,   wherein the electronic power controller interrupts the AC current flow conducted to stator phase windings in each phase during an interval that is proportional to the measured phase difference between the voltage waveform and the current waveform zero crossings in the preceding half cycle,   wherein the capacitance value of each shunt capacitor is selected and coordinated with the inductance value of one or more stator phase windings of the AC induction motor that is to be connected to the power controller system, connecting each of the shunt capacitors with each of the stator phase windings and thereby forming a secondary low pass filter circuit,   wherein the primary low pass filter circuits transform the impedance of the AC power source to an effective source impedance, the secondary low pass filter circuits transform the impedance of the AC induction motor into an effective load impedance, and the effective load impedance is approximately equal to the power source impedance,   the primary low pass filter circuit and the secondary low pass filter circuit have low attenuation from DC up through the fundamental power distribution of the AC power source to a cutoff frequency, and, attenuates current flow at frequencies above the cutoff frequency.   
     
     
         2 . A power controller system as recited in  claim 1 , wherein the capacitance value of each shunt capacitor is selected and coordinated with the inductance value of each of stator phase windings of an AC induction motor that is to be connected to the power controller system. 
     
     
         3 . A power controller system as recited in  claim 2 , wherein the primary low pass filter circuit comprises at least one low pass LC filter connected between each of the supply input terminals and each of the power input terminals. 
     
     
         4 . A power controller system as recited in  claim 3 , the electronic power controller comprising first and second gate-controlled switches, each switch having a respective control gate and the switches being connected in parallel, opposing polarity relation with each other between a first node and a second node, for each phase of the AC voltage, wherein the first node is electrically coupled to one of the power controller system supply input terminals and the second node is electrically coupled to one of the power controller system supply output terminals. 
     
     
         5 . A power controller system as recited in  claim 1 , wherein the primary low pass filter circuit comprises one low pass LC filters connected between each of the supply input terminals and each of the power input terminals. 
     
     
         6 . A power controller system as recited in  claim 1 , the electronic power controller comprising first and second gate-controlled switches, each switch having a respective control gate and the switches being connected in parallel, opposing polarity relation with each other between a first node and a second node, for each phase of the AC voltage, wherein the first node is electrically coupled to one of the power controller system supply input terminals and the second node is electrically coupled to one of the power controller system supply output terminals. 
     
     
         7 . A method of controlling the application of AC voltage from a AC power source in one or more phases of AC voltage to each of one or more stator phase windings of an AC induction motor to match the power requirements of a mechanical load being driven by the AC induction motor, the method comprising the following steps:
 coupling a gate-controlled switch in an electronic power controller in series between a selected phase of the AC voltage and a selected meter stator winding wherein the gate-controlled switch includes first and second control gates, one for each polarity of the AC voltage applied to the switch and the AC induction motor;   alternately triggering in each phase the gate-controlled switch into a conductive state during each alternation of the AC voltage;   inhibiting in each phase the conduction of the gate-controlled switch during each alternation of the AC voltage for a time interval proportional to an interval beginning when the AC voltage of an alternation in the stator winding passes through a first zero-crossing and ending when the corresponding AC current of an alternation in the stator winding passes through a second zero-crossing;   filtering in each phase the AC voltage that is conducted through the gate controlled switch; and   connecting a shunt capacitor in shunt phase to neutral relation across one of the electronic power controller power output terminals, comprising a primary low pass filter circuit between the selected phase of the AC voltage and the gate-controlled switch,   wherein the capacitance value of the shunt capacitor is selected and coordinated with the inductance value of one of the stator phase windings connecting each of the shunt capacitors with each of the stator phase windings and thereby forming a secondary low pass filter circuit,   wherein the primary low pass filter circuits transform the impedance of the AC power source to an effective source impedance, the secondary low pass filter circuits transform the impedance of the AC induction motor into an effective load impedance, and the effective load impedance is approximately equal to the power source impedance,   the primary low pass filter circuit and the secondary low pass filter circuit have low attenuation from DC up through the fundamental power distribution of the AC power source to a cutoff frequency, and, attenuates current flow at frequencies above the cutoff frequency.   
     
     
         8 . A method of controlling the application of AC operating power to the AC induction motor as recited in  claim 7 , including the step of controlling the operation of the gate controlled switch by the electronic power controller, and further including the step of applying the filtered voltage output from the primary low pass filter as operating power to the electronic power c