Phase current measurements in a three phase inverter using a single common DC-link current sensor

A method for sampling phase currents in a three-phase inverter being controlled by three pulse width modulated (PWM) signals with corresponding duty cycles, using a common resistor sampling a direct current (DC)-link current, the method comprising:

• computing an index of modulation for each of the three PWM signals for a current demand voltage vector and comparing it with a threshold value;
• if the respective duty cycle of one of the three PWM signals differs from the duty cycle of the other two PWM signals by less than a minimum value, then determining which of the following three conditions is true
• a) the duty cycles of at least two of the PWM signals differ by less than the minimum value and the duty cycle of the third PWM signal is greater than the duty cycles of the other two,
• b) the duty cycles of at least two of the PWM signals differ by less than the minimum value and the duty cycle of the third PWM signal is smaller than the duty cycles of the other two, or
• c) the duty cycles of the three PWM signals differ among each other by less than the minimum value;
• and
• when at least one of the conditions a), b), and c) is true, then creating sampling windows for sampling at least two phase currents by performing at least one of
• inverting and complementing at least one of the PWM signals, and
• inserting a fixed time active vector in null sub-intervals of at least one of the PWM signals and maintaining a compound duty cycle over every PWM period of the affected signal,
• the inserting being based upon the comparison of the index of modulation with the threshold value and true condition.

A system for sampling current data for each phase of a three-phase inverter driving a motor, the three-phase inverter controlled based upon three PWM drive signals having corresponding duty cycles as set by a motor control system for respective phase windings of the motor, the system comprising:

• a resistor periodically sampling direct current (DC)-link current;
• a sense amplifier coupled to said resistor and for outputting a signal representing a voltage drop on said resistor;
• at least one analog-to-digital converter (ADC) for receiving the signal output by said sense amplifier, for operating based upon a trigger signal to convert the signal output by said sense amplifier to numerical values supplied to the motor control system;
• a pulse width modulated (PWM) drive signal generator circuit for generating the three PWM drive signals and at least a fourth PWM drive signal as the trigger signal;
• and
• a controller for
• computing an index of modulation for each of the three PWM drive signals for a current demand voltage vector and comparing it with a threshold value,
• when the duty cycle of at least one of the three PWM drive signals differs from the duty cycle of any of the other two by less than a minimum value, thereby impeding sampling of the current on said resistor of at least two of the three PWM drive signals and if so, then determining which of the following three conditions is true
• a) the duty cycles of at least two of the PWM drive signals differ by less than the minimum value and the duty cycle of the third PWM drive signal is greater than the duty cycles of the other two,
• b) the duty cycles of at least two of the PWM drive signals differ by less than the minimum value and the duty cycle of the third PWM drive signal is less than the duty cycles of the other two, or
• c) the duty cycles of all three of the PWM drive signals differ among each other by less than the minimum value;
• when at least one of the conditions a), b), and c) is true, for creating sensing windows for sampling at least two phase currents and modulating the fourth PWM drive signal to act as a trigger signal of said at least one ADC by performing at least one of
• inverting and complementing at least one of the PWM drive signals, and
• inserting a fixed time active vector in null sub-intervals of at least one of the PWM drive signals and maintaining a compound duty cycle over every PWM period of the affected signal,
• the inserting being based upon the comparison of the index of modulation with the threshold value and the true condition.

A system for sampling phase current data for each phase of a three-phase inverter operating based upon three PWM signals having corresponding duty cycles, the system comprising:

• a resistor periodically sampling direct current (DC)-link current for each phase of the three-phase inverter;
• a sense amplifier coupled to said resistor and for outputting a signal representing a voltage drop on said resistor;
• and
• a controller cooperating with said resistor and said sense amplifier for
• computing an index of modulation for each of the three PWM signals for a current demand voltage vector and comparing it with a threshold value,
• if the duty cycle of at least one of the three PWM signals differs from the duty cycle of any of the other two by less than a minimum value, then determining which of the following three conditions is true
• a) the duty cycles of at least two of the PWM signals differ by less than the minimum value and the duty cycle of the third PWM signal is greater than the duty cycles of the other two,
• b) the duty cycles of at least two of the PWM signals differ by less than the minimum value and the duty cycle of the third PWM signal is less than the duty cycles of the other two, or
• c) the duty cycles of all three of the PWM signals differ among each other by less than the minimum value, and
• when at least one of the conditions a), b), and c) is true, then performing at least one of
• inverting and complementing at least one of the PWM signals, and
• inserting a fixed time active vector in null sub-intervals of at least one of the PWM signals and maintaining a compound duty cycle over every PWM period of the affected signal,
• the inserting being based upon the comparison of the index of modulation with the threshold value and the true condition.