Motor controller and image forming apparatus

An image forming apparatus comprising:

• a feed roller on which recording media are fed one by one;
• an image forming unit that forms an image on each of the recording media that are fed;
• a brushless DC motor of a sensorless-type for driving the feed roller;
• and
• a motor controller that controls the brushless DC motor, wherein
• the motor controller includes:
• a drive circuit that applies a voltage to each phase of a stator winding of the brushless DC motor;
• a current detection circuit that detects a current flowing through the stator winding;
• and
• a control circuit that controls the drive circuit and estimates an initial position of a magnetic pole of a rotor of the brushless DC motor by causing the drive circuit to continuously or intermittently apply a constant voltage to the stator winding during a prescribed energization time period at each of a plurality of energization angles while sequentially changing the plurality of energization angles,
• estimating the initial position includes:
• causing the drive circuit to apply the constant voltage at a first energization angle arbitrarily selected from among the plurality of energization angles and to subsequently apply the constant voltage at a second energization angle, wherein the drive circuit starts voltage application at the second energization angle after the drive circuit ends voltage application at the first energization angle and before a residual current flowing through the stator winding returns to zero;
• obtaining, at each of the plurality of energization angles, from the current detection circuit,
• a value of the residual current remaining immediately before voltage application is started or at a time when voltage application is started, and
• a peak value of the current flowing through the stator winding during the prescribed energization time period;
• and
• correcting the peak value of the current by the detected value of the residual current to estimate the initial position of the magnetic pole of the rotor based on the corrected peak value of the current that is obtained at each of the plurality of energization angles,
• wherein correcting the peak value includes:
• correcting a peak value of a γ-axis current by multiplying a detected peak value of the γ-axis current by a correction coefficient corresponding to a calculated γ-axis residual current based on a correction table showing a correspondence relation between a value of the γ-axis residual current and the correction coefficient.