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A motor driving apparatus includes a neutral point difference voltage detecting unit operable to detect a difference voltage between a virtual neutral point of a resistor circuit connected in parallel to motor coils and an actual neutral point which is a common connection point of the motor coils, a

A motor driving apparatus includes a neutral point difference voltage detecting unit operable to detect a difference voltage between a virtual neutral point of a resistor circuit connected in parallel to motor coils and an actual neutral point which is a common connection point of the motor coils, a rotor position detecting unit operable to detect a position of the rotor based on the difference voltage, and a controller operable to control commutation of an inverter. A rotation start torque applying process applies a rotation start pulse for providing rotation torque to the rotor based on a determined rotor position.

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What is claimed is: 1. A motor driving apparatus which includes a rotor and a stator having motor coils of a plurality of phases, the motor driving apparatus comprising: a virtual neutral point unit that has a virtual neutral point, and provides a voltage obtained by averaging motor terminal voltag

What is claimed is: 1. A motor driving apparatus which includes a rotor and a stator having motor coils of a plurality of phases, the motor driving apparatus comprising: a virtual neutral point unit that has a virtual neutral point, and provides a voltage obtained by averaging motor terminal voltages at the virtual neutral point; an inverter comprising a plurality of switching elements; a back electromotive voltage detecting unit operable to detect a back electromotive voltage of the motor; a neutral point difference voltage detecting unit operable to detect a neutral point difference voltage which is a difference voltage between the virtual neutral point and an actual neutral point which is a common connection point of the motor coils; a rotor position detecting unit operable to detect a position of the rotor on the basis of the neutral point difference voltage; and a controller operable to control commutation of the inverter, wherein the controller has, as operational modes, a searching start mode for determining conduction phases at the start of the motor on the basis of the detection result by the rotor position detecting unit and energizes the coils of the determined conduction phases; and a back electromotive voltage feedback mode for controlling commutation on the basis of the back electromotive voltage detected by the back electromotive voltage detecting unit, the controller alternately performs a rotor position searching process and a rotation start torque applying process in the searching start mode, the rotor position searching process applies a search pulse for detecting the rotor position in forward and backward directions of at least a pair of conduction phases to detect neutral point difference voltages, and adds up the neutral point difference voltage obtained in the forward direction and the neutral point difference voltage obtained in the backward direction, to determine the rotor position on the basis of a polarity of the added neutral point difference voltage; and the rotation start torque applying process applies a predetermined rotation torque to the rotor on the basis of the rotor position determined by the rotor position searching process. 2. The motor driving apparatus according to claim 1, wherein, in a first rotor position searching process, the controller detects neutral point difference voltages with respect to all conduction patterns, judges polarities of the detected neutral point difference voltages, and determines the rotor position on the basis of the polarity determination result, and in the second and subsequent rotor position searching processes, the controller applies the search pulse only to a pair of conduction phases determined on the basis of the result of polarity determination by the previous rotor position searching process, detects a neutral point difference voltage, and determines the rotor position on the basis of the detected neutral point difference voltage. 3. The motor driving apparatus according to claim 1, wherein the controller further has a retry mode for changing a setting parameter in the case of failure of the rotor position determination in the rotor position searching process, and subsequently performing again the rotor position determination by the rotor position searching process. 4. The motor driving apparatus according to claim 1, wherein the controller further has a retry mode for applying a kick pulse to the motor coil in the case of failure of the rotor position determination in the rotor position searching process, and subsequently performing again the rotor position detection by the rotor position searching process. 5. The motor driving apparatus according to claim 1, wherein the rotor position detecting unit converts the polarity of the added neutral point difference voltage into a logic value, and determines the rotor position using the logic value. 6. The motor driving apparatus according to claim 5, wherein the rotor position detecting unit weights the logic value for each combination of conduction phases, sums the weighted logic values of the conduction phases, and determines the rotor position using the summed value. 7. The motor driving apparatus according to claim 1, further comprising a command unit operable to control a current peak value of the search pulse. 8. The motor driving apparatus according to claim 1, further comprising a command unit operable to control a current peak value of the rotation start pulse. 9. The motor driving apparatus according to claim 1, wherein, when the motor rotating speed exceeds a predetermined value in the searching start mode, the controller shifts the mode to the back electromotive voltage feedback mode. 10. The motor driving apparatus according to claim 1, wherein the neutral point difference voltage detecting unit comprises a differential amplification circuit. 11. The motor driving apparatus according to claim 1, wherein the virtual neutral point unit is connected in parallel to the motor coils, includes a plurality of resistances in common connection, and the common connection point of the resistances is the virtual neutral point. 12. A disk driving system comprising: a motor for rotating a disk; and a motor driving apparatus for driving the motor according to claim 1. 13. A motor driving method for driving a motor which includes a rotor and a stator having motor coils of a plurality of phases, the method comprising: performing a searching start mode which applies a search pulse to the motor coils, determines conduction phases on the basis of a response signal to the applied pulse, and energizes the determined conduction phases; and performing a back electromotive voltage feedback mode which detects a back electromotive voltage generated in the motor, and controls commutation on the basis of the detected back electromotive voltage, wherein the searching start mode alternately performs a rotor position searching process and a rotation start torque applying process, the rotor position searching process applies a search pulse for detecting a rotor position in forward and backward directions of at least a pair of conduction phases, detects each in the forward and backward directions a neutral point difference voltage as a voltage difference between a voltage at a virtual neutral point which is a voltage obtained by averaging motor terminal voltages and a voltage at an actual neutral point which is a common connection point of the motor coils, adds up the neutral point difference voltage obtained in the forward direction and the neutral point difference voltage obtained in the backward direction, and determines the rotor position on the basis of a polarity of the added neutral point difference voltage, and the rotation start torque applying process applies a rotation start pulse for providing a predetermined rotation torque to the rotor on the basis of the rotor position determined by the rotor position searching process. 14. The motor driving method according to claim 13, wherein the added neutral point difference voltage is compared with a predetermined threshold to determine the polarity of the neutral point difference voltage in the searching start mode. 15. The motor driving method according to claim 13, wherein the search pulse is applied to at most three pairs of conduction phases to detect the neutral point difference voltage, only in the first rotor position searching process. 16. The motor driving method according to claim 13, wherein, in detection of the polarity of the rotor position by the second or subsequent rotor position searching processes, the search pulse is applied to a pair of conduction phases determined on the basis of the polarity of the neutral point difference voltage obtained by the previous rotor position searching process, to obtain a neutral point difference voltage, and a rotor state transition is determined on the basis of the obtained neutral point difference voltage, and in a case where the rotor state transition is not recognized, in a next rotor position searching process, the search pulse is applied to the pair of conduction phases used for successful detection of the rotor state transition in the previous rotor position searching process. 17. The motor driving method according to claim 13, wherein, in the second or subsequent rotor position searching processes, the search pulse is applied to a pair of conduction phases determined on the basis of the polarity of the neutral point difference voltage obtained by the previous rotor position searching process, to obtain a neutral point difference voltage, and the rotor state transition is determined on the basis of the obtained neutral point difference voltage, and in a case where the rotor state transition is recognized, in a next rotor position searching process, the search pulse is applied to a pair of conduction phases which enables recognition of the rotor state transition to a next position. 18. The motor driving method according to claim 13, wherein, during the rotor position searching process, in the case of failure of the rotor position detection, a kick pulse is applied for changing the rotor position, and subsequently, the rotor position searching process is performed again. 19. The motor driving method according to claim 18, wherein the kick pulse includes two or three pulses with phases different from one another by a predetermined angle. 20. The motor driving method according to claim 19, wherein the predetermined angle is substantially either one of 90, 60, and 120 degrees. 21. The motor driving method according to claim 13, wherein the rotation start pulse includes a plurality of pulses with current peak values limited to predetermined values. 22. The motor driving method according to claim 13, wherein the search pulse includes a plurality of pulses with current peak values limited to predetermined values. 23. The motor driving method according to claim 13, wherein, when a predetermined times of rotor state transitions are detected in the searching start mode, the mode is shifted to the back electromotive voltage feedback mode. 24. The motor driving method according to claim 13, wherein, when the rotational speed of the motor reaches a predetermined value in the searching start mode, the mode is shifted to the back electromotive voltage feedback mode. 25. The motor driving method according to claim 13, wherein first detection of a back electromotive voltage after the mode shift to the back electromotive voltage feedback mode is performed on the basis of zero-cross timing expected from a commutation period in the searching start mode immediately before the back electromotive voltage feedback mode. 26. The motor driving method according to claim 13, wherein the rotor position is determined on the basis of a neutral point difference voltage at a time when the search pulse current reaches a predetermined value.