Patent ID: 6940250

Claim:
A control device for a synchronous motor, comprising: a rotational position detector for detecting a rotational position of a synchronous motor; an angular rate computing unit for determining a rotational angular rate from the rotational position; a current instruction generator for determining a d-axis current instruction and a q-axis current instruction from the rotational angular rate and a torque instruction; a three-phase to two-phase converter for converting three-phase currents flowing into the synchronous motor into a d-axis current and a q-axis current; a current controller for determining a d-axis voltage instruction and a q-axis voltage instruction from a difference between the d-axis current instruction and the d-axis current and a difference between the q-axis current instruction and the q-axis current; a phase computing unit for determining a rotor positional angle from an output from the rotational position detector; a voltage converter for determining three-phase voltage instructions based on the rotor positional angle, the d-axis voltage instruction, and the q-axis voltage instruction; a phase correction quantity detector for determining the amount of offset in which the d-axis voltage instruction becomes zero when the phase correction instruction is inputted and the d-axis voltage instruction is not zero, and turning off phase correction instruction when the phase correction instruction is inputted and the d-axis voltage instruction is zero; and an adder for adding the amount of offset to the rotor positional angle to determine a fresh rotor positional angle, and outputting the fresh rotor positional angle to the voltage converter, wherein, when the phase correction instruction is inputted, the current instruction generator sets the d-axis current instruction and the q-axis current instruction as zero; the current controller determines the d-axis voltage instruction and the q-axis voltage instruction from the difference between the d-axis current instruction which is set as zero and the d-axis current, and the difference between the q-axis current instruction which is set as zero and the q-axis current; the phase correction quantity detector determines the amount of offset in which the d-axis voltage instruction becomes zero when the d-axis voltage instruction is not zero; and the adder adds the rotor positional angle and the amount of offset to determine the fresh rotor positional angle in which the d-axis voltage instruction becomes zero, thereby correcting a deviation produced between the rotational position of the synchronous motor which is determined from the output from the rotational position detector and an actual rotational position of the synchronous motor.