Method of eliminating self oscillations of an induction motor powered from a frequency changer and means for effecting same

Claim 3

A control system comprising a power supply and a power circuit which is energized by said power supply to produce an a.c. output voltage, the amplitude and frequency whereof are regulated proportionally, and a control circuit which is energized by said power supply to control said power circuit, which latter includes: (a) a controlled d.c. supply circuit to provide a controlled d.c. voltage; (b) an inverter incorporated in said power circuit and energized through an LC-filter, including a capacitor, by said controlled d.c. voltage to produce an output voltage to feed an a.c. motor; said output voltage having a controlled amplitude which is proportional to said d.c. voltage and the regulated frequency; (c) a bank of valve-action elements of said inverter, which conduct the current of said motor in the opposite direction in order to maintain a voltage across said capacitor that is proportional to the counter-electromotive force of said motor; (d) means incorporated in said control circuit, including a controlled reference voltage circuit to produce a reference voltage, a first feedback circuit to produce a first feedback voltage proportional to said output voltage of the inverter, and a comparison circuit to obtain a deviation voltage proportional to the difference between said reference voltage and said first feedback voltage; (e) means incorporated in said control circuit, sensitive to said deviation voltage, for controlling said d.c. power circuit; (f) frequency controlling means incorporated in said control circuit to control said inverter; said controlled reference voltage circuit being associated with said controlling means to produce a reference voltage which determines the output frequency of said inverter under steady-state conditions; (g) the improvement in the control system residing in a second feedback circuit to produce a second feedback voltage proportional to the low-frequency component of the reactive current oscillations of said motor; (h) a frequency regulator energized by said reference voltage and said second feedback voltage, to provide periodic voltage pulses at a frequency proportional to the sum of said voltages; and (i) means sensitive to said voltage pulses, for controlling said inverter.

Claim 4

A control system comprising a power supply and a power circuit which is energized by said power supply to produce an a.c. output voltage, the amplitude and frequency whereof are regulated proportionally, and a control circuit which is energized by said power supply to control said power circuit, which latter includes: (a) a controlled d.c. supply circuit to provide a controlled d.c. voltage; (b) an inverter incorporated in said power circuit and energized through an LC-filter, including a capacitor, by said controlled d.c. voltage to produce an output voltage to feed an a.c. motor; said output voltage having a controlled amplitude which is proportional to said d.c. voltage and the regulated frequency; (c) a bank of valve-action elements of said inverter, which conduct the current of said motor in the opposite direction in order to maintain a voltage across said capacitor that is proportional to the counter-electromotive force of said motor; (d) means incorporated in said control circuit, including a controlled reference voltage circuit to produce a reference voltage, a first feedback circuit to produce a first feedback voltage proportional to said output voltage of the inverter, and a comparison circuit to obtain a deviation voltage proportional to the difference between said reference voltage and said first feedback voltage; (e) means incorporated in said control circuit, sensitive to said deviation voltage, for controlling said d.c. power circuit; (f) frequency controlling means incorporated in said control circuit to control said inverter; said controlled reference voltage circuit being associated with said controlling means to produce a reference voltage which determines the output frequency of said inverter under steady-state conditions; (g) the improvement in the control system residing in a second feedback circuit to produce a second feedback voltage proportional to the low-frequency component of the reactive current oscillations of said motor; (h) a frequency regulator energized by said reference voltage and said second feedback voltage, to provide periodic voltage pulses at a frequency proportional to the sum of said voltages; and (i) means sensitive to said voltage pulses, for controlling said inverter.

Claim 5

The control system as defined in claim 2, wherein said second feedback circuit includes: (j) first means for obtaining a voltage proportional to the reactive component of said motor current, said first means being energized by said bank of valve-action elements; (k) second means for obtaining a voltage proportional to said low-frequency component of said reactive current oscillations; said first means feeding said second means; and (1) means for differentiating said last-named proportional voltage.

Claim 6

The control system as defined in claim 3, wherein (m) said first means includes a resistor connected between common anodes of said bank of valve-action elements and a negative bus of said d.c. supply circuit, a capacitor, one of the plates whereof is connected to said common anodes, while the other plate is connected in series with a primary winding of a differentiating transformer, the free end whereof is connected to said negative bus; and (n) said second means includes a single-phase full-wave rectifier circuit made up of non-controlled valve-action elements, the input whereof is connected to a secondary winding of said differentiating transformer, a capacitance-resistance differentiating circuit, whose input is connected through a low-frequency LC-filter to the output of said rectifier circuit, and the output of said differentiating circuit is connected in series with the output of said reference voltage circuit, which determines the output frequency of said inverter under steady-state conditions.