Adjustable speed AC drive system control for operation in pulse width modulation and quasi-square wave modes

A method for controlling an adjustable speed polyphase AC motor drive including an inverter, said inverter comprising a plurality of parallel phase legs connected between first and second conductors of a source of DC electric power, each of said phase legs including an AC output terminal for connection to a different phase of said motor, upper and lower switching devices, and upper and lower flyback diodes, said upper switching device and said upper flyback diode being connected in parallel relationship between said first conductor and said AC output terminal, and said lower switching device and said lower flyback diode being connected in parallel relationship between said AC output terminal and said second conductor, the steps of said method comprising:

• sensing the instantaneous magnitude of current through each respective phase leg of said inverter and producing a current feedback signal representative thereof;
• providing a plurality of reference AC waveforms normally representative of the desired currents in the respective motor phases, said waveforms having controllable fundamental frequency and amplitude;
• turning on and off said upper and lower switching devices in each leg of said inverter so as to normally maintain each current feedback signal within a hysteresis band of the corresponding reference waveform by alternately decreasing and increasing the phase current magnitude as the feedback signal attains upper and lower limits of said hysteresis band during the conduction interval of the corresponding motor phase;
• counting the number of current chops caused by the turning on and off of a preselected one of said switching devices during a time interval that varies with the period of one of said reference waveforms;
• and
• selectively controlling the operation of said inverter in either a PWM mode or a quasi-square wave mode including changing from PWM mode to quasi-square wave mode in response to said number of current chops decreasing below a first predetermined number and changing from quasi-square wave mode to PWM mode in response to said number of current chops increasing above a second predetermined number.

A hysteresis band current chopping control for a full-wave inverter in a polyphase AC motor drive system including a plurality of parallel phase legs connected between first and second conductors of a source of DC electric power, each of said phase legs including an AC output terminal for connection to a different phase of said motor, upper and lower switching devices, and upper and lower flyback diodes, said upper switching device and said upper flyback diode being connected in parallel relationship between said first conductor and said AC output terminal, and said lower switching device and said lower flyback diode being connected in parallel relationship between said AC output terminal and said second conductor, said hysteresis band current control being operable in a PWM mode and a quasi-square wave mode and comprising:

• a plurality of current sensing means for deriving current feedback signals representative of the instantaneous magnitudes of currents in the respective phases of said motor;
• reference waveform generating means for providing a plurality of reference AC waveforms normally representative of the desired currents in the respective motor phases, said waveforms having controllable fundamental frequency and amplitude;
• and
• a plurality of summing means for deriving a current error signal for each respective motor phase that varies with the difference between the corresponding values of the current feedback signal and the reference waveform;
• controllable hysteresis circuit means associated with said generating means for providing a hysteresis band having upper and lower limits being varied as functions of a variable hysteresis circuit means including chopping means for turning on and off said upper and lower switching devices in each leg of said inverter so that each current feedback signal is normally maintained within the hysteresis band of the corresponding reference waveform by alternately decreasing and increasing the corresponding phase current magnitude as the feedback signal attains said upper and lower limits of said hysteresis band during the conduction interval of the corresponding motor phase;
• said chopping means comprising a plurality of comparator means for comparing the respective current error signals with the hysteresis band signal, said hysteresis band signal having a positive value corresponding to said upper limit and a negative value corresponding to said lower limit, each said comparator means generating a first logic level signal when the respective current error signal increases to said positive value and generating a second logic level signal when the respective current error signal decreases to said negative value;
• and
• said hysteresis circuit means further comprising switch means associated with each said comparator means for providing said negative value of said hysteresis band signal to the respective comparator means in response to the output signal thereof changing from said second logic level to said first logic level and for providing said positive value of said hysteresis band signal to the respective comparator means in response to the output signal thereof changing from said first logic level to said second logic level.