Method for detecting the field position of an electric motor

Abstract

To provide a method for detecting a magnetic field position of an electric motor that makes it possible to drive at low speed by reliably detecting the magnetic field position of the electric motor even when a drive voltage fluctuates when a three-phase brushless motor is driven at low speed by sensorless drive by PWM control with 120° energization.SOLUTION: When the drive voltage fluctuates, an energization time t2 is calculated from a motor current Im and a drive voltage V1, and a drive voltage V2. The motor current Im and the drive voltage V1 are calculated on the basis of an inductance Lm of an electric motor, a resistance value Rm, a drive voltage V, an energization time t, and a motor current Im. The energization time t2 is divided by a PWM cycle to calculate and update a duty ratio. An MPU 51 operates by applying the drive voltage V2 with the duty ratio updated by an inverter circuit 52.SELECTED DRAWING: Figure 8

Description

[Technical Field]

[0001] The present invention relates to a method for detecting the field position of an electric motor when the electric motor is operated at a low speed. [Background technology]

[0002] Traditionally, brushed DC motors have been used for small direct current motors, but problems with brush noise, electrical noise, durability, etc. have led to the emergence of brushless DC motors.More recently, sensorless motors, which do not have position sensors, have been attracting attention from the perspective of being smaller, lighter, more robust, and less expensive, and were first adopted in hard disk drives and other devices in the information equipment field, but with the development of vector control technology, they have also begun to be used in home appliances and automotive applications.

[0003] In sensorless drive, the rotor position is detected from the induced voltage, but since no induced voltage occurs when the motor is stationary, the rotor position cannot be determined and starting is not possible.To detect the rotor position when the motor is stationary, one method is to provide a coil current sensor and current detection circuit, and use an inverter to pass a sine wave coil current through the coil using PWM drive, thereby estimating the position from the current response. The rotor position when stationary can be detected from the inductance deviation using the methods described above. Alternatively, the rotor can be rotated by forced commutation without position sensing, and the position can be determined.

[0004] However, once starting begins, the motor begins to rotate, making it difficult to detect rotor position from inductance deviation using sensing pulses. For example, detecting inductance deviation by superimposing a high-frequency current on the excitation current is considered, but this requires extensive hardware and software. Furthermore, the effects of magnetic saturation and induced voltage must be considered, as well as factors that are difficult to estimate, such as inherent errors in the motor and drive circuit. For this reason, the ramp-start method, which forcibly positions the rotor using fixed excitation without position detection and then gradually increases the rotation speed while achieving synchronization, is widely used. However, this method requires a long time to position the rotor and can even cause reverse rotation. Furthermore, synchronization is achieved using open-loop control, which requires time for acceleration and is prone to loss of synchronization due to load fluctuations. To avoid this, a large current is used for starting, reducing efficiency and requiring a large DC power supply. Load fluctuations cause loss of synchronization, limiting its applications. It cannot be used for reciprocating mechanisms, externally driven rotation, or applications with viscous or fluctuating loads.

[0005] Therefore, as a field position detection method that aims to reduce costs using simple hardware and software and can detect the rotor position in excitation interval units at 120° current conduction without generating sensing noise at startup, a method has been proposed in which, when a voltage is applied to a three-phase brushless motor driven at 120° current conduction, the induced voltage generated in the non-current conducting phase changes depending on the position of the permanent magnet field (rotor) to detect the rotor position (Patent Document 1; JP 2019-17235 A). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2019-17235 Summary of the Invention [Problem to be solved by the invention]

[0007] However, when the motor is energized with 120° energization, the induced voltage generated in the non-energized phase changes depending on the magnitude of the drive voltage to the motor. In particular, in the case of PWM drive, it changes depending on the magnitude of the duty ratio of the drive voltage (energization time). Figure 10 is a graph showing the change in induced voltage generated in the non-energized phase according to the rotor rotation position when the drive voltage fluctuates at a duty ratio of 20% (fixed) of a certain motor. This graph shows that if voltage fluctuations exceeding a certain level occur, such as fluctuations in the battery system or power supply voltage, the waveform of the induced voltage will change significantly, and there is a risk that it will become impossible to detect the field position.

[0008] In order to solve this problem, Patent Document 1 proposes a method of multiplying a correction coefficient according to the duty ratio, but depending on the motor, even if the induced voltage is multiplied by a correction coefficient, position detection becomes difficult at low duty ratios below a certain level or high duty ratios above a certain level. Therefore, only a limited range of duty ratios can be used. [Means for solving the problem]

[0009] The present invention has been made to solve these problems, and its object is to To provide a method for detecting the field position of an electric motor, which can reliably detect the field position of the electric motor and operate the electric motor at low speed even if the drive voltage fluctuates when operating a two-phase brushless motor at low speed by sensorless drive using PWM control with 120° energization.

[0010] a control means for storing current-flow angle information and current-flow direction information for each 60° current-flow section that allows continuous rotation, and for controlling the switching of the output means to switch the current-flow state based on the stored information; and a measuring means for A / D-converting the three-phase coil voltage and sending it to the control means. The control means measures the current-flow angle information and current-flow direction information for each 60° current-flow section that allows continuous rotation, and for controlling the switching of the output means based on the stored information. In this method for detecting the field position of an electric motor, 120° energization including off cycles is performed periodically in a direction of energization where the position at which self-excitation stops due to energization coincides with the start position of the 60° energization section, and the field position of the electric motor is detected by measuring the energized phase voltage and the non-energized phase voltage in the on cycle of PWM energization by the measuring means, and the method is operated in a sensorless manner, and when the drive voltage fluctuates when the electric motor is operated by applying a drive voltage to the three-phase coils at a predetermined duty ratio, the method calculates the field position of the electric motor by:

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[0011] In this way, when the drive voltage V1 is applied to the three-phase coil at a predetermined duty ratio and the drive voltage changes to V2 during low-speed operation,

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[0012] When operating a three-phase brushless motor at low speed using PWM control with 120° energization, a method for detecting the field position of the motor can be provided that can reliably detect the field position of the motor and operate it at low speed by updating the duty ratio even if the drive voltage fluctuates. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 10 is a waveform diagram of inductance and voltage of a non-energized phase coil during UV excitation. [Figure 2] FIG. 10 is a waveform diagram of inductance and voltage of a non-energized phase coil during UW excitation. [Figure 3] FIG. 10 is a waveform diagram of inductance and de-energized phase coil voltage during VW excitation. [Figure 4] FIG. 10 is a waveform diagram of inductance and voltage of a non-energized phase coil during VU excitation. [Figure 5] FIG. 10 is a waveform diagram of inductance and voltage of a non-energized phase coil during WU excitation. [Figure 6] FIG. 10 is a waveform diagram of inductance and non-energized phase coil voltage during WV excitation. [Figure 7] FIG. 10 is a diagram showing the measured waveform of a non-energized phase coil voltage. [Figure 8] FIG. 1 is a block diagram of a drive circuit for a three-phase brushless DC motor. [Figure 9] This is a diagram showing the configuration of a star-connected three-phase brushless DC motor. [Figure 10] FIG. 10 is a waveform diagram of an induced voltage induced in a non-energized phase coil when the drive voltage is varied with a fixed duty ratio of 20% in PWM drive. [Figure 11] FIG. 10 is a waveform diagram of an induced voltage induced in a non-energized phase coil when the duty ratio is corrected in accordance with voltage fluctuations from a drive voltage with a duty ratio of 20% in PWM drive. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, an embodiment of a method for detecting the field position of an electric motor according to the present invention will be described with reference to the accompanying drawings. The present invention will be described using, as an example of an electric motor, a sensorless motor having a permanent magnet field in the rotor, windings in the stator arranged with a phase difference of 120° in a star connection, and phase ends connected to a motor output means.

[0015] As an example, a method for detecting the permanent magnet field position of a sensorless motor that drives a three-phase DC brushless motor without using a sensor will be described below, along with the configuration of a sensorless motor drive device. An embodiment of a three-phase brushless DC motor according to the present invention will be shown with reference to FIG. 9. As an example, a three-phase brushless DC motor equipped with a two-pole permanent magnet rotor and a stator 4 with three slots will be shown. The motor may be either an inner rotor type or an outer rotor type. Furthermore, the permanent magnet field may be either an interior permanent magnet (IPM) motor or a surface permanent magnet (SPM) motor.

[0016] In Figure 9, a rotor 2 is integrally provided on a rotor shaft 1, and a two-pole permanent magnet 3 is provided as a field magnet. Pole teeth U, V, and W are arranged on a stator 4 with a phase difference of 120°, facing the permanent magnet 3. Windings u, v, and w are provided on each of the pole teeth U, V, and W of the stator 4, and the phases are star-connected with a common C, forming a three-phase brushless DC motor that is wired to a motor drive device (described later). Note that the common wire is omitted because it is not necessary.

[0017] Next, an example of a drive circuit for a three-phase DC brushless motor is shown in FIG. The drive method assumed at startup is 120° energization bipolar square wave excitation. The MOTOR is a three-phase sensorless motor. The MPU 51 is a microcontroller (control means). The MPU 51 stores field position information that specifies six energization directions for the three-phase coils (U, V, W) and the 120° energization excitation switching intervals (intervals 1 to 6) corresponding to each energization direction, and controls the switching of the output means in response to rotation commands from the upper controller 50 to arbitrarily switch the excitation state.

[0018] The inverter circuit 52 (INV: output means) energizes the three-phase coils and performs switching operations such as excitation phase switching or PWM control to control the motor torque. The inverter circuit 52 includes diodes connected in antiparallel to the switching elements, and is provided with half-bridge switching circuits for three phases that can be arbitrarily connected to the positive power supply line and the ground power supply line. The current sensor 53 measures the coil current when PWM current is applied to the three-phase coils. Specifically, a shunt resistor r is provided between the common ground terminal of the inverter circuit 52 and the ground. Because only a low voltage of several volts, which is the voltage drop, is applied to the shunt resistor r, it can be used even when the coil applied voltage is as high as several hundred volts. The operational amplifier 54 amplifies the coil voltage corresponding to the coil current and sends it to an A / D conversion circuit 55 (ADC: measurement means).

[0019] The A / D conversion circuit 55 is connected to the coil output terminals U, V, and W, and simultaneously samples the coil voltages of each of the three phases in response to a conversion start signal from the MPU 51, performs analog-to-digital conversion in sequence, and sends the conversion results to the MPU 51. The ADC 55 is normally built into the MPU 51, and when using the built-in ADC 55, it is desirable to provide a voltage divider circuit using resistors because the maximum input voltage is low. In this way, the drive circuit can be configured very simply.

[0020] The inductance change (space harmonics) due to rotor angle θ is approximated as ΔL = -cos(2θ), and is known to have two periods per electrical angle.On the other hand, when two-phase current is applied to a three-phase coil using square wave PWM current, it is known that two-period voltage fluctuations centered on the neutral point potential according to θ are observed in the non-current-carrying phases.

[0021] Figure 1 shows the theoretical waveforms of the voltage change waveform ΔVw of the non-energized phase, the inductance changes (ΔLu, ΔLv) of the U and V phases, and the combined inductance change ΔLu-v of the two phases when rotated by one electrical angle while UV excitation is performed with PWM energization. Note that the voltage change waveform has the polarity of the combined inductance change waveform reversed, and swings positive and negative around a neutral point potential that is half the voltage applied to the coil.

[0022] Figure 7A shows the measured waveform of the coil voltage of the non-energized phase using an inner rotor motor. The theoretical voltage waveform of the non-energized phase was assumed to be reverse polarity, reflecting the inductance, and the waveform is nearly identical, demonstrating that this assumption is correct. Furthermore, when using square-wave energization, ringing occurs in the induced voltage, but measurements have shown that the ringing time is very short, converging within the measurement error range of several microseconds to several tens of microseconds for various motors. This means that the induced voltage can be accurately detected even with square-wave PWM energization pulses used to drive the motor.

[0023] When a large current flows through the three-phase coil, magnetic saturation occurs and the inductance stops changing, which is particularly noticeable in small outer rotor motors. When magnetic saturation occurs, the two-periodic inductance change waveform retains its peak and bottom adjacent to the setup position where self-excitation stops due to two-phase fixed current flow, but the other peak and bottom disappear, resulting in a single period. Figure 7B shows an example of an inductance waveform that has become periodic due to magnetic saturation. The motor used for the measurement was a small outer rotor motor, which is different from the motor used in Figure 7A. The setup position when UV is energized is 150°, and in the ΔVw waveform, only the peak and bottom adjacent to the setup position are clearly observed.

[0024] The setup position where self-excitation stops due to two-phase fixed current is the inductance zero cross point and also the induced voltage zero cross point, and the setup point and the adjacent peaks and troughs are stable against magnetic saturation. As can be seen from Figures 7A and 7B, the voltage fluctuations of the non-energized phase reflect the rotor angle θ, and moreover, monotonicity is guaranteed within the interval, so even when no induced voltage is generated at rest, it is possible to estimate the rotor position by passing an excitation current. The voltage fluctuation range is 10% or more of the voltage applied to the coil, and is on the order of several volts, which is overwhelmingly advantageous when considering that conventional methods detect induced voltages on the order of millivolts at start-up.

[0025] As explained above, by detecting inductance changes in the non-energized phase coil voltage using square wave PWM control for motor drive and using only the inductance changes near the setup position, stable position detection is possible from a stationary state to low speed rotation ranges. This simplifies the sensing procedure, improves efficiency as no power is required for sensing, and reduces noise as no sensing noise is generated.

[0026] The angle and current direction for each current section of 120° current flow are summarized in the table below. In the table, CW current flow is the current direction in which the angle increases, and CCW current flow is the current direction in which the angle decreases. Setup current flow is the current direction that stops self-excitation at the angle shown in parentheses in the table, and both the start and end points are listed for each section. For each current direction, the phase connected to the + power supply side is listed first, and the phase connected to the GND side is listed after a hyphen.

[0027] (The rest is blank) [Table 1]

[0028] The rotation direction in which the section number in Table 1 increases is defined as CW, and the rotation direction in which it decreases is defined as CCW. The section end position is the boundary point with the adjacent + side section when rotating in the CW direction, and the boundary point with the - side section when rotating in the CCW direction. For example, in the case of section 1, the boundary point with section 2 is 90° when rotating in the CW direction, and the boundary point with section 6 is 30° when rotating in the CCW direction.

[0029] In Figure 1, the start point of the energized section during clockwise rotation with UV excitation is shown as Point A, and the end point of the energized section is shown as Point B. The setup point is Point C, and the bottom section where Point B is located has a stable phase and can be used for position detection. Therefore, threshold values ​​Vth on the positive and negative sides with a specified potential difference from the neutral point potential are set in advance, and the non-energized phase coil voltage ΔVw is compared with the threshold value Vth for each measurement, and if the threshold value is exceeded, it can be detected that the end point of the section has been exceeded. Refer to Figure 4, as VU current is applied during CCW rotation. The rotor rotates from the 90° electrical angle side towards the 30° electrical angle side. Therefore, the end point of the interval is at an electrical angle of 30°. Because the setup point is at an electrical angle of 330°, the bottom part on the 30° electrical angle side has a stable phase and can be used for position detection. Therefore, just as with CW rotation, the end point of the interval can be detected by comparing the voltage of the non-energized phase W with the threshold value Vth.

[0030] In section 2, which is from an electrical angle of 90° to an electrical angle of 150°, UW excitation is selected. Figure 2 shows the change in inductance and the change in the coil voltage of the non-energized phase during UW excitation. The waveform is shifted 60° and the polarity is inverted from that of Figure 1, the non-energized phase is the V phase, and the setup position, point C, is at an electrical angle of 210°. When the rotor is in section 2 and rotating in the clockwise direction, the non-energized phase coil voltage will always pass through point B, so at that point the rotor position is 150 electrical degrees. If point B is detected, switching to section 3 will enable continuous rotation.

[0031] In section 3, which is from electrical angle 150° to electrical angle 210°, VW excitation is selected. Figure 3 shows the change in inductance and the change in the coil voltage of the non-energized phase during VW excitation. The waveform is shifted by 60° and the polarity is inverted from that of Figure 2, the non-energized phase is the U phase, and the setup position, point C, is at an electrical angle of 270°. When the rotor is in section 3 and rotating in the clockwise direction, the non-energized phase coil voltage will always pass through point B, so at that point the rotor position is 210 electrical degrees. If point B is detected, switching to section 4 will enable continuous rotation.

[0032] In section 4, which is from electrical angle 210° to electrical angle 270°, VU excitation is selected. Figure 4 shows the change in inductance and the change in the coil voltage of the non-energized phase during VU excitation. The waveform is shifted by 60° and the polarity is inverted from that of Figure 3, the non-energized phase is the W phase, and the setup position, point C, is at an electrical angle of 330°. When the rotor is in section 4 and rotating in the clockwise direction, the non-energized phase coil voltage will always pass through point B, so at that point the rotor position is 270 electrical degrees. If point B is detected, switching to section 5 will enable continuous rotation.

[0033] In section 5, which is from electrical angle 270° to electrical angle 330°, WU excitation is selected. Figure 5 shows the change in inductance and the change in the coil voltage of the non-energized phase during WU excitation. The waveform is shifted by 60° and the polarity is inverted from that of Figure 4, the non-energized phase is the V phase, and the setup position, point C, is at an electrical angle of 30°. When the rotor is in section 5 and rotating in the clockwise direction, the non-energized phase coil voltage will always pass through point B, so at that point the rotor position is 330 electrical degrees. If point B is detected, switching to section 6 will enable continuous rotation.

[0034] In section 6, which is from electrical angle 330° to electrical angle 30°, WV excitation is selected. Figure 6 shows the change in inductance and the change in the coil voltage of the non-energized phase during WV excitation. The waveform is shifted by 60° and the polarity is reversed from that of Figure 5, the non-energized phase is the U phase, and the setup position, point C, is at an electrical angle of 90°. When the rotor is in section 6 and rotating in the clockwise direction, the non-energized phase coil voltage will always pass through point B, so at that point the rotor position is 30 electrical degrees. If point B is detected, switching to section 1 will enable continuous rotation.

[0035] In this way, the section end point located at the peak or bottom portion adjacent to the setup position can be detected using a preset threshold. When the de-energized phase coil voltage exceeds the threshold, the section number is increased by +1 for clockwise rotation and decreased by -1 for counterclockwise rotation, allowing for continuous rotation.

[0036] As before, the rotation direction in which the section number increases is defined as CW, and the rotation direction in which the section number decreases is defined as CCW. The section start position is the boundary point with the adjacent - (negative) side section when CW, and the + (positive) side section when CCW. For example, in the case of energized section 1, the boundary point with energized section 6 is 30° when CW, and the boundary point with energized section 2 is 90° when CCW.

[0037] In Figure 1, the start point of the section when rotating in the CW direction with UV energization is shown as point A. During normal operation, the motor rotates in the desired direction, so there is no need to detect the start point of the section, but when an external force is causing the motor to rotate at low speed in the opposite direction to the desired direction, start point detection is required to switch excitation correctly. When rotating at high speeds, it is necessary to apply the brakes to slow down the motor, so it is thought that detection of the start point position is only necessary when rotating at low speeds. When rotating in the opposite direction, induced voltage becomes a problem. In Figure 1, the induced voltage of the non-energized phase W has a zero-cross point at the electrical angle of 60° in the middle of the interval, and when rotating in the forward direction, the gradient due to the inductance change and the gradient of the induced voltage match, making it possible to reliably detect the end of the interval. However, when rotating in the reverse direction, the gradients of the two are opposite, and the waveform due to the inductance change cancels out, making it difficult to detect the start of the interval. Furthermore, in the case of a motor that has a periodicity due to magnetic saturation, it is nearly impossible to detect the start of the interval.

[0038] Therefore, if we focus on point C, the setup position for WU excitation in Figure 5, point C passes through an electrical angle of 30°, so by comparing the neutral point potential with the non-energized phase V-phase voltage ΔVv, if ΔVv becomes smaller than the neutral point potential, it can be detected that the electrical angle has exceeded 30° and that the rotor has rotated toward section 6. Therefore, if UV excitation is performed in section 1 and the non-energized phase V-phase voltage is measured by momentarily switching to WU excitation, it is possible to determine whether the electrical angle is just before or has passed 30°. If measurements are repeated periodically until the electrical angle passes 30°, the start of the section, i.e., the excitation switching position, can be detected.

[0039] When detecting the section start point electrical angle of 90° during CCW rotation, the 90° electrical angle position can be detected by performing WV excitation with reference to Figure 6 and measuring the voltage of the non-energized U phase, just as when rotating in the CW direction. Around the setup positions of 30° electrical angle or 90° electrical angle, the voltage change gradient is steep, making it easy to determine whether the voltage is positive or negative, and there is little phase shift, so position detection can be performed reliably. Although it is small, some power is consumed for sensing, so it is desirable to lengthen the sensing cycle.

[0040] Similarly, for current conduction sections 2 to 6, the start of the section can be detected by selecting the current conduction direction that serves as the setup point and periodically detecting the inductance zero crossing points. If the start point is detected, it means that the motor is rotating in the reverse direction, so if the section number is returned to the reverse direction, continuous rotation will be possible.

[0041] If the rotor is rotating in the opposite direction to the desired direction at an extremely low speed due to an external force, it is necessary to detect the start of the interval and switch excitation in order to return to normal rotation. The start of the interval can be detected by setting a start threshold. For example, in Figure 1, if the motor is located in section 1, the potential at point A can be set as the start-point threshold. Similarly, for sections 2 to 6 in Figures 2 to 6, the potential at point A for each section can be set as the start-point threshold. Therefore, a start-point threshold Vth2 with a predetermined potential difference from the neutral point potential is set in advance, and the non-energized phase coil voltage ΔV is compared with the start-point threshold Vth2 for each measurement. If the start-point threshold is exceeded, it can be detected that the section start point has been exceeded. The gradient of the non-energized phase coil voltage ΔV can also be determined, and if the gradient is opposite to that during forward rotation, it can be detected that the motor is in a reverse rotation state.

[0042] Therefore, if the start point is detected while the rotor is in reverse rotation, the excitation interval can be reversed by one interval, generating forward rotation torque, i.e., braking, to suppress reverse rotation and return to forward rotation. However, because the polarity of the induced voltage during reverse rotation is opposite to that during forward rotation, the de-energized phase coil voltage ΔV at the start of the interval is smaller than during forward rotation and does not exceed the start-point threshold Vth2. In this case, it is possible to estimate the induced voltage through calculation and correct the start-point threshold Vth2. Alternatively, start-point detection may be limited to extremely low rotation speeds where errors due to induced voltage can be ignored.

[0043] This method allows the start point to be detected in the drive excitation state without any special field position detection excitation. Therefore, there is no reduction in current conduction efficiency and no electromagnetic noise caused by sensing current conduction. Furthermore, by detecting the start point, it is possible to apply the brakes from the reverse state and return to forward rotation.

[0044] The voltage of the non-energized phase coils changes depending on the duty ratio of the drive voltage. When a motor is driven at a specified drive voltage and a constant duty ratio, if voltage fluctuations exceeding a certain level occur due to fluctuations in the battery system or power supply voltage, the waveform of the induced voltage in the non-energized phase coils may change significantly, as shown in Figure 10, making it impossible to detect the field position. Therefore, as described below, a method is adopted in which the duty ratio of the drive voltage applied to the three-phase coils is updated based on the PWM cycle and the drive voltage is applied from the output means, making it possible to detect the field position.

[0045] When the drive voltage fluctuates, if the motor inductance is Lm, resistance is Rm, drive voltage is V, and current flow time is t, the motor current Im is:

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[0046] For example, in the waveform diagram of the induced voltage induced in the non-energized phase coil when the drive voltage varies with a fixed duty ratio of 20% in FIG. 10 , the graph of a drive voltage of 18V and a duty ratio of 20% shows that the duty ratio for this motor calculated using Equation 2 is approximately 32% for a drive voltage of 12V, approximately 26% for a drive voltage of 14V, approximately 16% for a drive voltage of 22V, and approximately 15% for a drive voltage of 24V. In the example of FIG. 10 , the drive voltage is set to 18V because the optimal duty ratio (energization time) was determined for the motor shown in FIG. 10 at a drive voltage of 18V. The duty ratio was set to 20% because the change in the positive and negative induced voltages generated in the non-energized phase in the motor shown in FIG. 10 was significant and easy to measure. However, the drive voltage and duty ratio are not limited to these values ​​in the present invention. The waveform diagram of the induced voltage induced in the non-energized phase coil when the duty ratio is corrected in accordance with these voltage fluctuations is shown in Fig. 11. The graph in Fig. 11 shows that when the duty ratio is corrected in accordance with fluctuations in the drive voltage, the range of voltage fluctuation is small for all drive voltages, and controllability is improved.

[0047] In this way, when the drive voltage is applied to the three-phase coil at a predetermined duty ratio and the motor is operated at low speed, if the drive voltage fluctuates,

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[0048] 1 Rotor shaft 2 Rotor 3 Permanent magnet 4 Stator 50 Host controller 51 MPU 52 Inverter circuit (INV) 53 Current sensor 54 Operational amplifier 55 A / D conversion circuit (ADC)

Claims

[Claim 1] an electric motor including a rotor having a permanent magnet field, a stator having three-phase coils, output means for bidirectionally energizing the three-phase coils via a half-bridge inverter circuit, control means for PWM-controlling the coil output in response to a command from a host controller, storing energization angle information and energization direction information for each 60° energization section that allows continuous rotation, and switching-controlling the output means based on the stored information to switch the energization state; and measurement means for A / D-converting the three-phase coil voltages and sending them to the control means, the control means periodically performs 120° energization including off cycles in a current direction in which the position at which self-excitation is stopped by two-phase fixed energization coincides with the start position of the 60° energization section through the output means, and the measurement means measures energized phase voltages and non-energized phase voltages in on cycles of PWM energization to detect the field position of the motor, thereby operating the motor in a sensorless manner, When a driving voltage is applied to the three-phase coil at a predetermined duty ratio for operation, if the driving voltage fluctuates, If the inductance of the motor is Lm, the resistance is Rm, the drive voltage is V, the current flow time is t, and the motor current is Im, then [Equation 1] The control means substitutes the predetermined drive voltage V1, the PWM period, and the current application time t1 calculated from the predetermined duty ratio into (Equation 1) to calculate the motor current Im, and solves (Equation 1) for t. [Equation 2] and a drive voltage V2 that has changed from the drive voltage V1, calculated by substituting the motor current Im calculated by (Equation 1) and the drive voltage V2 that has changed from the drive voltage V1 into (Equation 2), and then calculating a current flow time t2 by dividing the current flow time t2 by a PWM period to calculate and update a duty ratio, and the control means applies the drive voltage V2 from the output means at the updated duty ratio to operate the motor.

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