Reversible variable frequency oscillator for smooth reversing of AC motor drives

In a static variable speed reversible ac motor drive including an inverter which supplies adjustable amplitude and adjustable frequency ac power to the motor, said reversible ac motor drive system including an improved inverter frequency control circuit responsive to an inverter frequency command signal and generating gating signals for inverter main thyristors so as to control inverter thyristor conduction, said improved inverter frequency control circuit comprising:

• a reversible variable frequency oscillator responsive to an inverter frequency command signal with a substantial ripple component and a dc component whose magnitude and polarity are indicative of the speed and direction of motor rotation, said oscillator including:
• an integrator operative to generate a ramp output signal which rises at a rate dependent on the dc component of said frequency command signal;
• a positive comparator and a negative comparator, each of said comparators coupled at a respective input to the integrator output;
• a logic gate having a first and second input coupled at the output of said positive and said negative comparator, respectively, said logic gate producing an output signal in accordance with a predetermined relationship between signals supplied to said first and second logic gate inputs;
• a switch coupled across the input and the output of said integrator for resetting said integrator responsive to logic gate output signals;
• and
• a direction flip-flop connected to each of said positive and said negative comparators for producing forward and reverse direction signals;
• a delay element;
• a reversible ring counter coupled between said delay element and said logic gate for generating gating pulses for inverter main thyristors when actuated by output signals produced by said logic gate;
• and
• minimum inverter frequency limiter means including a delay circuit and coupled to said ring counter for supplying clock pulses to said ring counter after a preset interval in accordance with output signals from said logic gate.

In a static variable speed ac induction motor drive system including a controlled current inverter which supplies adjustable amplitude and adjustable frequency ac current to the induction motor and has an inverter frequency control circuit that is responsive to an inverter frequency command signal and generates gating pulses for the inverter main thyristors in the forward and reverse phase sequence, said drive system further having a load angle feedback control for generating an angle error signal representative of the difference between the sensed instantaneous angle between motor current and motor flux and a reference value and which angle error signal is a component of the inverter frequency command signal, the improvement wherein said inverter frequency control has inherent multipulse operation at low motor speeds and comprises

• a reversible variable frequency oscillator having as an input an inverter frequency command signal with a substantial ripple component and a dc component whose magnitude and polarity are indicative of the speed and direction of motor rotation,
• said oscillator including an integrator which generates a ramp output dependent only upon the dc component of the input signal, and further including means for generating timing pulses and direction signals as the integrator output repetitively rises to preset levels, and
• means responsive to said timing pulses and direction signals for generating gating pulses for the inverter main thyristors in the forward and reverse phase sequence.

In a static variable speed ac induction motor drive system including a controlled current inverter which supplies adjustable amplitude and adjustable frequency ac current to the induction motor and has an inverter frequency control circuit that is responsive to an inverter frequency command signal and generates gating pulses for the inverter main thyristors in the forward and reverse phase sequence, said drive system further having a torque feedback control for generating an error signal representative of the difference between the sensed instantaneous torque and a reference value and which error signal is a component of said inverter frequency command signal, the improvement wherein said inverter frequency control has inherent multipulse operation at low motor speeds and comprises

• a reversible variable frequency oscillator having as an input an inverter frequency command signal with a substantial ripple component and a dc component whose magnitude and polarity are indicative of the speed and direction of motor rotation,
• said oscillator including an integrator which generates a ramp output dependent only upon the dc component of the input signal, and further including means for generating timing pulses and direction signals as the integrator output repetitively rises to preset levels, and
• means responsive to said timing pulses and direction signals for generating gating pulses for the inverter main thyristors in the forward and reverse phase sequence.