Origin offset calculation method of rotational position detecting device of electric motor and motor control device using the calculation method

An origin offset calculation method of a rotational position detecting device of an electric motor used in an electric motor control device that includes:

• a current detector that detects a three-phase current of a three-phase synchronous motor;
• a rotational position detecting device that detects a rotational position of said three-phase synchronous motor;
• a three-phase to two-phase converter that makes coordinate transformation of said three-phase current into a magnetic flux component and an orthogonal component orthogonal to said magnetic flux component based on said rotational position;
• a current command generator that divide a current command value, which is provided from outside, into a current command value of said magnetic flux component and a current command value of said orthogonal component;
• a current control operator that operates voltage command values of mentioned each component so that a current value of said magnetic flux component and a current value of said orthogonal component are equal to command values of said each component of said current command generator respectively;
• a two-phase to three-phase converter that makes the coordinate transformation of a voltage command value of said each component into a three-phase voltage command value;
• and
• an inverter that outputs a voltage to be applied to said three-phase synchronous motor based on an output from said two-phase to three-phase converter;
• the origin offset calculation method of a rotational position detecting device of an electric motor comprising:
• a first process of forcibly making a current command value of said each component zero;
• a second process of controlling offset to make the addition to an output from said rotational position detecting device so that said magnetic flux component voltage command value is zero by multiplying a deviation between said magnetic flux component voltage command value and zero by a predetermined proportional integral gain when said current command value becomes zero;
• a third process of letting said offset when said magnetic flux component voltage command value becomes zero a phase difference between said rotor reference position and an origin of said rotational position detecting device, and of storing this phase difference as an origin offset;
• and
• a fourth process of adding said origin offset to an output from said rotational position detecting device.