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A magnetic-pole detecting system for synchronous AC motors and a magnetic-pole detecting method therefor are provided. The magnetic-pole detecting system for synchronous AC motors includes a position detector 10 for detecting the relative position between the moving element 3 and the stator of an AC

A magnetic-pole detecting system for synchronous AC motors and a magnetic-pole detecting method therefor are provided. The magnetic-pole detecting system for synchronous AC motors includes a position detector 10 for detecting the relative position between the moving element 3 and the stator of an AC motor 5; a phase-update command unit 52 for making a plurality of currents with different phases flow in coils 3a and 3b so that the moving element 3 moves to a plurality of stable points; a reversal command unit 56 for making currents flow in the coils 3a and 3b so that the moving element 3 reverses from the previous direction in which the moving element 3 has been moved by the phase-update command unit 52; a current cut-off command unit 54 for cutting off the currents flowing in the coils 3a and 3b when the position-detector 10 detects a movement of the moving element 3; a reversal-determining unit 64 for determining, based on a position-detecting signal from the position-detector 10, a direction in which the moving element has moved based on the phase-update command unit 52, and for determining that the determined direction has reversed between the previous and the present instances; and a stable-point-simulator for simulating a stable point by means of the phase of the currents flowing in the moving element 3 when the reversal-determining unit 64 has detected the reversal.

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What is claimed is: 1. A magnetic-pole detecting system for a synchronous AC motor, comprising: a synchronous AC motor having phase coils either in a moving element or in a stator; a position-detecting means for generating a position-detecting signal to detect positional relationship between the mo

What is claimed is: 1. A magnetic-pole detecting system for a synchronous AC motor, comprising: a synchronous AC motor having phase coils either in a moving element or in a stator; a position-detecting means for generating a position-detecting signal to detect positional relationship between the moving element and the stator; a movement-determining means for generating a movement signal upon determining that the moving element has moved, based on the position-detecting signal from the position-detecting means; a current command generating means for generating a first current command signal for making a plurality of different-phase currents flow so that the moving element moves to a plurality of stable points, and for generating a second current command signal that makes currents flow, the currents having a phase that makes the moving element move in reverse to the direction in which the moving element has moved based on the first current command signal; a current controlling means for making the currents flow in the phase coils, based on the first and the second current command signals; a current cut-off means for cutting off the currents flowing in the each-phase coils, based on the movement signal from the movement-determining means; a reversal-determining means for determining, based on a detection value from the position-detecting means, the direction in which the moving, element has moved based on the first current command signal, and for determining that the detected direction has reversed between previous and present instances; and a stable-point simulating means for simulating the position of the moving element by means of the stable point determined by the phase of the first current command signal when the reversal-determining means has detected a reversal, or by the phase prior to the phase at the reversal. 2. A magnetic-pole detecting system for a synchronous AC motor according to claim 1, wherein the current phase of the first or the second current command signal is a phase at which sinusoidal cogging torque generated in the synchronous AC motor is approximately zero. 3. A magnetic-pole detecting system for a synchronous AC motor according to claim 1, further comprising: a phase-updating means for updating the phase of the first current command signal in steps of Δθ; a standstill-determining means for generating a standstill signal upon determining that the moving element has come to a standstill, based on the position-detecting signal from the position-detecting means, after the currents are made to flow into the phase coils by the first current command signal; a first position-calculating means for obtaining a first position equal to the first stable point, based on the first phase of the first current command signal upon the occurrence of the standstill signal; a second position-calculating means for obtaining a second position equal to the second stable point, based on the second phase of the first current command signal when the reversal-determining means has made a determination; and a stable-point calculating means for obtaining, based on the first and the second positions, the position of the stable point; wherein the current controlling means maintains the phase of the movable element when at a standstill, even when the currents are made to flow in the phase coils, according to the first current command signal. 4. A magnetic-pole detecting system for a synchronous AC motor according to claim 1, wherein the currents generated by the current controlling means have a time constant longer than that determined from the resistance component and the inductance component of the AC motor. 5. A magnetic-pole detecting system for a synchronous AC motor according to claim 1, further comprising: a cut-off signal generating means for generating a current cut-off command signal when a predetermined time period elapses from the occurrence of the first or the second current command signal; wherein the current cut-off means cuts off the currents in phase coils, based on the current cut-off command signal from the cut-off signal generating means. 6. A magnetic-pole detecting system for a synchronous AC motor according to claim 5, wherein the current phase of the first or the second current command signal is a phase at which sinusoidal cogging torque generated in the synchronous AC motor is approximately zero. 7. A magnetic-pole detecting system for a synchronous AC motor according to claim 5, further comprising: a phase-updating means for updating the phase of the first current command signal in steps of Δθ; a standstill-determining means for generating a standstill signal upon determining that the moving element has come to a standstill, based on the position-detecting signal from the position-detecting means, after the currents are made to flow into the phase coils by the first current command signal; a first position-calculating means for obtaining a first position equal to the first stable point, based on the first phase of the first current command signal upon the occurrence of the standstill signal; a second position-calculating means for obtaining a second position equal to the second stable point, based on the second phase of the first current command signal when the reversal-determining means has made a determination; and a stable-point calculating means for obtaining, based on the first and the second positions, the position of the stable point; wherein the current controlling means maintains the phase of the movable element when at a standstill, even when the currents are made to flow in the phase coils, according to the first current command signal. 8. A magnetic-pole detecting system for a synchronous AC motor according to claim 5, wherein the currents generated by the current controlling means have a time constant longer than that determined from the resistance component and the inductance component of the AC motor. 9. A magnetic-pole detecting system for a synchronous AC motor, comprising: a synchronous AC motor having phase coils either in a moving element or in a stator; a position-detecting means for generating a position-detecting signal to detect positional relationship between the moving element 25 and the stator; a movement-determining means for generating a movement signal upon determining that the moving element has moved, based on the position-detecting signal from the position-detecting means; a current command generating means for generating a first current command signal for making a plurality of different-phase currents flow so that the moving element moves to a plurality of stable points, and for generating a second current command signal that makes currents flow, the currents having a phase that makes the moving element move in reverse to the direction in which the moving element has moved based on the first current command signal; a current controlling means for mating-the currents flow in the phase coils, based on the first and the second current command signals; a current cut-off means for cutting off the currents flowing in the each-phase coils, based on the movement signal from the movement-determining means; a reversal-determining means for determining, based on a detection value from the position-detecting means, the direction in which the moving element has moved based on the first current command signal, and for determining that the detected direction has reversed between previous and present instances; and a current-maintaining means for inactivating the current cut-off means based on the determination, by the reversal-determining means, that the reversal has occurred, and for continuing to make the currents with the phase at the reversal or currents with the phase prior to the phase at the reversal flow into the phase coils, until the moving element is determined, based on the detecting signal from the position-detecting means, to have come to a standstill. 10. A magnetic-pole detecting system for a synchronous AC motor according to claim 9, wherein the current phase of the first or the second current command signal is a phase at which sinusoidal cogging torque generated in the synchronous AC motor is approximately zero. 11. A magnetic-pole detecting system for a synchronous AC motor according to claim 9, further comprising: a phase-updating means for updating the phase of the first current command signal in steps of Δθ; a standstill-determining means for generating a standstill signal upon determining that the moving element has come to a standstill, based on the position-detecting signal from the position-detecting means, after the currents are made to flow into the phase coils by the first current command signal; a first position-calculating means for obtaining a first position equal to the first stable point, based on the first phase of the first current command signal upon the occurrence of the standstill signal; a second position-calculating means for obtaining a second position equal to the second stable point, based on the second phase of the first current command signal when the reversal-determining means has made a determination; and a stable-point calculating means for obtaining, based on the first and the second positions, the position of the stable point; wherein the current controlling means maintains the phase of the movable element when at a standstill, even when the currents are made to flow in the phase coils, according to the first current command signal. 12. A magnetic-pole detecting system for a synchronous AC motor according to claim 9, wherein the currents generated by the current controlling means have a time constant longer than that determined from the resistance component and the inductance component of the AC motor. 13. A magnetic-pole position detecting method for a synchronous AC motor having phase coils either in a moving element or in a stator, a position-detecting means for generating a position-detecting signal to detect positional relationship between the moving element and the stator, a current-command generating means for generating a first current command signal that makes a plurality of different-phase currents flow so that the moving element moves to a plurality of stable points, and for generating a second current command signal that makes currents flow, the currents having a phase that makes the moving element move in reverse to the direction in which the moving element has moved based on the first current command signal, a current controlling means for making the currents flow in the coils of each phase, based on the first and the second current command signals, the method comprising: a first step of making, by means of the current control means, currents with a first phase flow in the phase coils, so that the moving element moves, based on the first current command signal, to a first stable point; a second step of cutting off the currents by means of a current cut-off means, when a movement-determining means detects, based on the position-detecting signal, a movement of the moving element; a third step of making, based on the second current command signal, currents with a second phase flow in the phase coils, by means of the current controlling means, so that the moving element reverses from a direction in which the moving element has moved; a fourth step of cutting off the currents by means of the current cut-off means when the movement of the moving element is detected by the movement-determining means; a fifth step of changing the phase of the first current command signal to the phase for a second stable point that is different from the phase for the first stable point, and of making the currents flow in the phase coils by means of the current controlling means; a sixth step of determining, based on the position-detecting signal from the position-detecting means, a direction in which the moving element moves based on the first current command signal, and of determining whether or not the detected direction has reversed between the previous and the present instances; wherein the first step through the sixth step are sequentially carried out, and the stable point determined by the phase of the first current command signal when the reversal-determining means has detected a reversal, or by the phase prior to the phase at the reversal is simulated as the position of the moving element by a stable-point simulating means. 14. A magnetic-pole position detecting method for a synchronous AC motor according to claim 13, wherein the current phase of the first or the second current command signal is a phase at which sinusoidal cogging torque generated in the synchronous AC motor is approximately zero. 15. A magnetic-pole position detecting method for a synchronous AC motor according to claim 13, wherein the currents generated by the current controlling means have a time constant longer than that determined from the resistance component and the inductance component of the AC motor.