Control system for AC motor

A control system for an AC motor, comprising:

• an inverter that converts DC voltage into AC voltage for driving and rotating the AC motor;
• and
• a rectangular wave voltage controller that controls a voltage phase of a rectangular wave voltage applied to the AC motor so as to perform torque control, wherein
• the rectangular wave voltage controller includes:
• a linear approximation unit that calculates a first slope as a ratio of a change in torque to a change in the voltage phase at a first operating point, based on the voltage phase and an operating state of the AC motor, said first operating point corresponding to the current operating state and voltage phase;
• a phase change amount calculation unit that calculates a torque compensation amount based on a command value of the torque control, and calculates a first phase change amount by dividing the torque compensation amount by the first slope;
• a positive/negative checking unit that determines whether a first voltage phase to which the voltage phase is changed by the first phase change amount from the current value thereof and the command value are of the same sign, and corrects the first voltage phase to a predetermined value when the first voltage phase and the command value are of opposite sign, while maintaining the first voltage phase when the first voltage phase and the command value are of the same sign;
• a first torque calculation unit that calculates a torque value at a second operating point corresponding to the corrected or maintained first voltage phase and the current operating state, according to a torque equation using at least one motor variable associated with the operating state of the AC motor and the voltage phase as variables;
• a torque difference determination unit that determines whether a difference between the torque value at the second operating point and the command value is smaller than a predetermined value;
• an updating unit that determines a second phase change amount by dividing the torque compensation amount by a second slope as a ratio of a torque difference to a voltage phase difference between the currently obtained second operating point and the first operating point when the torque difference determination unit determines that the difference is equal to or larger than the predetermined value, and updates the second operating point to an operating point corresponding to the current operating state and a second voltage phase to which the voltage phase is changed by the second phase change amount from the current value;
• and
• a second torque calculation unit that calculates a torque value at the updated second operating point according to the torque equation each time the second operating point is updated by the updating unit, wherein
• the torque difference determination unit determines whether a difference between the torque value and the command value is smaller than the predetermined value each time the torque value is calculated by the first or second torque calculation unit;
• and
• the phase change amount calculation unit calculates an amount of change of the voltage phase according to a voltage phase difference between the currently obtained second operating point and the first operating point if the torque difference determination unit determines that the difference is smaller than the predetermined value.