Method for detecting magnetic field position of 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 operating at a low speed.

Background Art

[0002] Conventionally, a brushed DC motor has been used for small DC motors, but there are problems such as brush noise, electrical noise, and durability, and a brushless DC motor has emerged. More recently, a sensorless motor without a position sensor has attracted attention from the viewpoints of miniaturization, weight reduction, robustness, and low cost. First, it was adopted for hard disk drives and the like in the information equipment field, but with the development of vector control technology, it has also begun to be adopted in the household electric appliance and in-vehicle fields.

[0003] In sensorless drive, the rotor position is detected from the induced voltage. However, since no induced voltage is generated at rest, the rotor position cannot be determined and the motor cannot be started. In order to detect the rotor position at rest, there is a method in which a coil current sensor and a current detection circuit are provided, and a sine-wave coil current is passed through the coil by PWM drive using an inverter to estimate the position from the current response. Also, the rotor position at rest can be detected from the inductance deviation using the method represented above. Alternatively, the rotor can be rotated by forced commutation without position sensing to determine the position.

[0004] However, once the start-up begins, energization for rotation is performed, so it becomes difficult to detect the rotor position from the inductance deviation by applying a sensing pulse. For example, it is conceivable to superimpose a high-frequency current on the exciting current to detect the inductance deviation, but this would be a large-scale operation both in terms of hardware and software. Also, the effects of magnetic saturation and induced voltage must be considered, and furthermore, elements such as the inherent errors of the motor and drive circuit, which are difficult to estimate, are included. Therefore, the ramp start method, in which the rotor is forcibly positioned with fixed excitation without performing position detection and then the rotational speed is gradually increased while synchronizing, is widely used. However, this method requires a long time to position the rotor and also has a problem of reverse rotation. Also, since synchronization is achieved by open-loop control, it takes time to accelerate and is prone to losing synchronization due to load fluctuations. To avoid this, it starts with a large current, resulting in a decrease in efficiency and an increase in the size of the DC power supply. It cannot be used in applications where it goes out of tune during load fluctuations, such as reciprocating motion mechanisms or applications that rotate by an external force, or applications with viscous loads or fluctuating loads.

[0005] Therefore, as a field magnetic position detection method that achieves cost reduction with simple hardware and software and can detect the rotor position in units of excitation sections in 120° energization without generating a sensing sound during start-up, in the 120° energization drive of a three-phase brushless motor, when a voltage is applied to the motor, a method has been proposed to detect the rotor position by utilizing the fact that the induced voltage generated in the non-energized phase changes depending on the position of the permanent magnet field (rotor) (Patent Document 1; Japanese Patent Application Laid-Open No. 2019-17235).

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, when the motor is energized with 120° conduction, the induced voltage generated in the non-energized phase varies depending on the magnitude of the drive voltage applied to the motor. Particularly in the case of PWM drive, it will vary depending on the magnitude (on-time) of the duty ratio of the drive voltage. Figure 10 is a graph showing the change in the induced voltage generated in the non-energized phase according to the rotational position of the rotor when the drive voltage fluctuates at a fixed duty ratio of 20% for a certain motor. From this graph, if there is a voltage fluctuation above a certain level, such as when the battery system or the power supply voltage fluctuates, the waveform of the induced voltage may change significantly, and there is a risk that the field position cannot be detected.

[0008] In Patent Document 1, in order to solve this phenomenon, a method of multiplying a correction coefficient according to the duty ratio has been proposed. However, depending on the motor, even if a correction coefficient is multiplied by the induced voltage, it is difficult to detect the position at a low duty ratio below a certain level or a high duty ratio above a certain level. Therefore, only a limited range of duty ratios can be used.

Means for Solving the Problems

[0009] The present invention has been made to solve these problems, and its object is to provide a method for detecting the field position of an electric motor that can surely detect the field position of the electric motor and perform low-speed operation even when the drive voltage fluctuates when a three-phase brushless motor is driven sensorlessly by PWM control with 120° conduction at low speed.

[0010] ​A rotor having a permanent magnet field, a stator having a three-phase coil, an output means for bidirectionally energizing the three-phase coil via a half-bridge type inverter circuit, and a coil output is PWM-controlled according to a command from a host controller, storing energization angle information and energization direction information in units of 60° energization sections enabling continuous rotation, and a control means for switching the energization state by performing switching control of the output means based on these, and a measurement means for A / D converting a three-phase coil voltage and sending it to the control means. Using an electric motor provided with these, the control means performs 120° energization including an off-cycle periodically in an energization direction in which the position of self-excitation stop by two-phase fixed energization through the output means coincides with the start position of the 60° energization section, and detects the field position of the electric motor without using a sensor by measuring the energized phase voltage and the non-energized phase voltage at the on-cycle of PWM energization by the measurement means. When the drive voltage fluctuates when applying a drive voltage to the three-phase coil at a predetermined duty ratio for operation, assuming the inductance of the electric motor is Lm, the resistance value is Rm, the drive voltage is V, the energization time is t, and the motor current is Im,

Equation

Equation

[0011] Thus, when the drive voltage fluctuates to V2 when applying the drive voltage V1 to the three-phase coil at a predetermined duty ratio for low-speed operation,

Equation

Number

Advantages of the Invention

[0012] When a three-phase brushless motor is operated at low speed by PWM control with 120° energization, it is possible to provide a method for detecting the field position of a motor that can reliably detect the field position of the motor and perform low-speed operation by updating the duty ratio even when the drive voltage varies.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Embodiments for Carrying Out the Invention

[0014] Hereinafter, embodiments 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. As an example of the present invention, a sensorless motor having a permanent magnet field on the rotor, with windings arranged on the stator at a 120° phase difference and star-connected, and the phase terminals connected to the motor output means will be used for the description.

[0015] Hereinafter, as an example, a method for detecting the permanent magnet field position of a sensorless motor that sensorlessly drives a three-phase DC brushless motor will be described together with the configuration of a sensorless motor drive device. FIG. 9 shows an embodiment of a three-phase brushless DC motor according to the present invention. As an example, a three-phase brushless DC motor provided with a 2-pole permanent magnet rotor and a stator 4 provided with 3 slots will be exemplified. The motor may be either an inner rotor type or an outer rotor type. Also, as the permanent magnet type field, it may be either an interior permanent magnet (IPM) type motor or a surface permanent magnet (SPM) type motor.

[0016] In FIG. 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. On a stator 4, pole teeth U, V, and W are arranged to face the permanent magnet 3 with a 120° phase difference. 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 and wired to a three-phase brushless DC motor to be described later that is wired to a motor drive device. Incidentally, the common wire is omitted because it is unnecessary.

[0017] Next, an example of a drive circuit for a three-phase DC brushless motor is shown in FIG. 8. As a drive method at startup, 120° conduction bipolar rectangular wave excitation is assumed. MOTOR is a three-phase sensorless motor. MPU51 is a microcontroller (control means). MPU51 stores field position information that designates six energization directions for the three-phase coils (U, V, W) and the 120° conduction excitation switching intervals (intervals 1 to 6) corresponding to each energization direction, and switches the output means by switching control in response to a rotation command 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 anti-parallel to the switching elements, and three half-bridge type switching circuits that can be arbitrarily connected to the positive power supply line and the ground power supply line are provided. The current sensor 53 measures the coil current when performing PWM energization on the three-phase coils. Specifically, a shunt resistor r is provided between the common ground side terminal of the inverter circuit 52 and the ground. Since only a low voltage of several V corresponding to the voltage drop is applied to the shunt resistor r, it can be used even when the coil applied voltage is a high voltage of several hundred V. The operational amplifier 54 amplifies the coil voltage corresponding to the coil current and sends it to the A / D conversion circuit 55 (ADC: measurement means).

[0019] The A / D conversion circuit 55 has the coil output terminals U, V, and W connected thereto, simultaneously samples the coil voltages of each of the three phases according to the conversion start signal from the MPU 51, sequentially performs analog-to-digital conversion, and sends the conversion results to the MPU 51. Usually, the ADC 55 is built into the MPU 51. When using the built-in ADC 55, since the maximum input voltage is low, it is desirable to provide a voltage dividing circuit using resistors. Thus, the drive circuit can be configured very simply.

[0020] The change in inductance (spatial harmonic) due to the rotor angle θ is approximated as ΔL = -cos(2θ), and it is known to have a two-periodicity per electrical angle. On the other hand, when two-phase energization is performed with rectangular-wave PWM energization in the three-phase coils, it is known that voltage fluctuations with a two-periodicity are observed in the non-energized phase around the neutral point potential according to θ.

[0021] Fig. 1 shows the voltage change waveform ΔVw of the non-energized phase, and the theoretical value waveforms of the inductance changes (ΔLu, ΔLv) of the U and V phases and the combined inductance change ΔLu-v of the two phases when rotating by one electrical angle while exciting U-V with PWM energization. Note that the voltage change waveform is obtained by inverting the polarity of the combined inductance change waveform and oscillates positively and negatively around the neutral point potential which is half of the coil applied voltage.

[0022] Fig. 7A shows the measured waveform of the non-energized phase coil voltage using an inner-rotor type motor. The theoretical value of the voltage waveform of the non-energized phase is assumed to have an inverse polarity reflecting the inductance, and it can be seen that the waveforms are approximately the same, indicating that the assumption is correct. Also, in the case of rectangular-wave energization, ringing occurs in the induced voltage. However, as a result of the measurement, the ringing time is very short and converges within the range of measurement error in various motors, being several μs to several tens of μs, and the induced voltage can be accurately detected even with the rectangular-wave PWM energization pulses for motor drive.

[0023] When a large current flows through the three-phase coil, magnetic saturation occurs and the inductance no longer changes, which is particularly prominent in small outer-rotor motors. When magnetic saturation occurs, the two-periodic inductance change waveform has peaks and bottoms adjacent to the setup position that self-excites and stops due to two-phase fixed current conduction remaining, while the other peaks and bottoms disappear and become one-periodic. Fig. 7B shows an example of the inductance waveform that becomes one-periodic due to magnetic saturation. The motor used for measurement is a small outer-rotor motor, which is different from the motor used in Fig. 7A. The setup position during U-V current conduction is 150°, and in the ΔVw waveform, only the peaks and bottoms adjacent to the setup position are clearly observed.

[0024] The setup position that self-excites and stops due to two-phase fixed current conduction is both the inductance zero-crossing point and the induced voltage zero-crossing point, and the setup point and the adjacent peaks and bottoms are also stable against magnetic saturation. As can be seen from Fig. 7A and Fig. 7B, the voltage fluctuation of the non-powered phase reflects the rotor angle θ and monotonicity is ensured within the interval. Therefore, even when the induced voltage at rest is not generated, the rotor position can be estimated by flowing the excitation current. The voltage fluctuation amplitude is more than 10% of the coil applied voltage and reaches the order of several volts, which is overwhelmingly advantageous considering that the conventional method detects the induced voltage on the order of millivolts at startup.

[0025] As described above, by using rectangular wave PWM control for motor drive to detect the inductance change of the non-powered phase coil and using only the inductance change near the setup position, stable position detection is possible from the stationary state to the low-speed rotation range. This simplifies the sensing procedure, improves efficiency without requiring power for sensing, and eliminates the sensing noise and achieves silent operation.

[0026] The angles and energization directions for each energization interval of 120° energization are summarized in the following table. In the table, CW energization refers to the energization direction that rotates in the direction of increasing angle, and CCW energization refers to the energization direction that rotates in the direction of decreasing angle. The setup energization is the energization direction that self-excites and stops at the angle described in () within the frame in the table, and both the start point and the end point are described for each interval. Each energization direction is described with the phase connected to the + power supply side first and the phase connected to the GND side after the hyphen.

[0027] (The following is blank space)

Table 1

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

[0029] In Fig. 1, when rotating in the CW direction with U-V excitation, the start point of the energization interval is shown as point A, and the end point of the energization interval is shown as point B. The setup point is point C, and the bottom part where point B is located has a stable phase and can be used for position detection. Therefore, positive and negative threshold values Vth with a predetermined potential difference with respect to the neutral point potential are set in advance, and for each measurement, the voltage ΔVw of the non-energized phase coil is compared with the threshold value Vth. If it exceeds the threshold value, it can be detected that the end point of the interval has been exceeded. During CCW direction rotation, since it is V-U energization, refer to Fig. 4. The rotor rotates from the electrical angle 90° side to the electrical angle 30° side. Therefore, the end point of the interval is the electrical angle 30°. Since the setup point is the electrical angle 330°, the bottom part on the electrical angle 30° side has a stable phase and can be used for position detection. Therefore, similar to the CW case, 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 interval 2 from the electrical angle 90° to the electrical angle 150°, U-W excitation is selected. Fig. 2 shows the inductance change and the change in the non-energized phase coil voltage during U-W excitation. The waveform is obtained by shifting the waveform in Fig. 1 by 60° and reversing the polarity. The non-energized phase is the V phase, and the setup position C point is at an electrical angle of 210°. When located in section 2 and rotating in the CW direction, since the non-energized phase coil voltage always passes through point B, the rotor position at that time is at an electrical angle of 150°. If point B is detected and switched to section 3, continuous rotation can be achieved.

[0031] In section 3 from an electrical angle of 150° to an electrical angle of 210°, V-W excitation is selected. Fig. 3 shows the inductance change and the change in the non-energized phase coil voltage during V-W excitation. The waveform is obtained by shifting the waveform in Fig. 2 by 60° and reversing the polarity. The non-energized phase is the U phase, and the setup position C point is at an electrical angle of 270°. When located in section 3 and rotating in the CW direction, since the non-energized phase coil voltage always passes through point B, the rotor position at that time is at an electrical angle of 210°. If point B is detected and switched to section 4, continuous rotation can be achieved.

[0032] In section 4 from an electrical angle of 210° to an electrical angle of 270°, V-U excitation is selected. Fig. 4 shows the inductance change and the change in the non-energized phase coil voltage during V-U excitation. The waveform is obtained by shifting the waveform in Fig. 3 by 60° and reversing the polarity. The non-energized phase is the W phase, and the setup position C point is at an electrical angle of 330°. When located in section 4 and rotating in the CW direction, since the non-energized phase coil voltage always passes through point B, the rotor position at that time is at an electrical angle of 270°. If point B is detected and switched to section 5, continuous rotation can be achieved.

[0033] In section 5 from an electrical angle of 270° to an electrical angle of 330°, W-U excitation is selected. Fig. 5 shows the inductance change and the change in the non-energized phase coil voltage during W-U excitation. The waveform is obtained by shifting the waveform in Fig. 4 by 60° and reversing the polarity. The non-energized phase is the V phase, and the setup position C point is at an electrical angle of 30°. When it is located in section 5 and rotating in the CW direction, since the non-energized phase coil voltage always passes through point B, the rotor position at that time is 330° electrical angle. If point B is detected and switched to section 6, continuous rotation can be achieved.

[0034] In section 6 from 330° electrical angle to 30° electrical angle, W-V excitation is selected. Fig. 6 shows the inductance change and the non-energized phase coil voltage change during W-V excitation. The waveform is the waveform of Fig. 5 shifted by 60° and the polarity inverted. The non-energized phase is the U phase, and the setup position C point is 90° electrical angle. When it is located in section 6 and rotating in the CW direction, since the non-energized phase coil voltage always passes through point B, the rotor position at that time is 30° electrical angle. If point B is detected and switched to section 1, continuous rotation can be achieved.

[0035] In this way, the section end points located at the peak or bottom adjacent to the setup position can be detected with a preset threshold value. When the non-energized phase coil voltage exceeds the threshold value, the section number can be incremented by 1 during CW rotation and decremented by 1 during CCW rotation to achieve continuous rotation.

[0036] Similar to the above, 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 -(minus) side section during CW rotation and the adjacent +(plus) side section during CCW rotation. For example, in the case of energized section 1, it is the boundary point of 30° with energized section 6 during CW rotation and the boundary point of 90° with energized section 2 during CCW rotation.

[0037] In Fig. 1, the section start point during CW rotation with U-V energization is shown as point A. In normal operation, detection of the section start point is not required for rotation in the desired direction. However, when rotated slowly in the opposite direction to the desired rotation direction by an external force, start point detection is necessary for correct excitation switching. During high-speed rotation, it is necessary to apply a brake to decelerate, and the detection of the start point position is considered to be only during low-speed rotation. When rotating in the opposite direction, the induced voltage becomes a problem. In Fig. 1, for the non-energized phase W, the induced voltage has a zero-crossing point at the electrical angle of 60° at the center of the interval. When rotating in the forward direction, the gradient due to the inductance change and the gradient of the induced voltage coincide, and the end point of the interval can be reliably detected. However, when rotating in the reverse direction, the gradients of the two are opposite, and the waveform due to the inductance change is canceled out, making it difficult to detect the start point of the interval. Furthermore, in the case of a motor with a 1-periodicity due to magnetic saturation, it is almost impossible to detect the start point of the interval.

[0038] Therefore, focusing on point C, the setup position of W-U excitation in Fig. 5, since point C passes through an electrical angle of 30°, by comparing the neutral point potential and the non-energized phase V-phase voltage ΔVv, when ΔVv becomes smaller than the neutral point potential, it can be detected that the rotation has passed beyond the electrical angle of 30° and entered section 6. Therefore, when exciting U-V in section 1, by momentarily switching to W-U excitation and measuring the non-energized phase V-phase voltage, it can be determined whether it is before or after the electrical angle of 30°. By repeating the measurement periodically until passing through the electrical angle of 30°, the start point of the interval, that is, the excitation switching position, can be detected.

[0039] Regarding the detection of the start point electrical angle of 90° during CCW rotation, similar to CW rotation, referring to Fig. 6, by performing W-V excitation and measuring the non-energized phase U-phase voltage, the position of the electrical angle of 90° can be detected. Around the setup positions of the electrical angle of 30° or 90°, the voltage change gradient is steep, the positive / negative determination is easy, and the phase shift is small, so the position can be reliably detected. Although it consumes a small amount of power for sensing, it is desirable to increase the sensing period.

[0040] Regarding the energized intervals 2 - 6 as well, by selecting the energization direction as the setup point and periodically detecting the inductance zero-crossing point, the start point of the interval can be detected. When the start point is detected, it means rotating in the reverse direction, so if the interval number is reversed, continuous rotation can be achieved.

[0041] When the rotor reverses at an extremely low speed in the direction opposite to the desired rotation direction due to an external force, etc., it is necessary to detect the start point of the section and switch the excitation in order to return to the forward rotation. The start point of the section can be detected by setting a start point threshold value. For example, in FIG. 1, when located in section 1, the potential at point A may be set as the start point threshold value. Similarly, for sections 2 to 6 in FIGS. 2 to 6, the potential at point A in each section may be set as the start point threshold value. Therefore, a start point threshold value Vth2 having a predetermined potential difference with respect to the neutral point potential is set in advance, and for each measurement, the non-energized phase coil voltage ΔV and the start point threshold value Vth2 are compared in magnitude. When the start point threshold value is exceeded, it can be detected that the start point of the section has been exceeded. Also, the gradient of the non-energized phase coil voltage ΔV can be determined, and if the gradient is opposite to that during forward rotation, it can be detected that the rotor is in the reverse rotation state.

[0042] Therefore, when the rotor is in the reverse rotation state and the start point is detected, if the excitation section is returned by one section and excitation is performed, a forward rotation torque can be generated, that is, a brake can be applied to suppress the reverse rotation and return to the forward rotation. However, since the polarity of the induced voltage during reverse rotation is opposite to that during forward rotation, the non-energized phase coil voltage ΔV at the start point of the section becomes smaller than that during forward rotation and does not exceed the start point threshold value Vth2. In that case, it is also possible to estimate the induced voltage by calculation and correct the start point threshold value Vth2. Alternatively, the start point detection may be limited only to the extremely low speed rotation at which the error due to the induced voltage can be ignored.

[0043] According to this method, the start point can be detected in the drive excitation state without performing special field position detection excitation. Therefore, the energization efficiency is not reduced, and no electromagnetic noise is generated due to sensing energization. Also, by detecting the start point, a brake can be applied from the reverse rotation state to return to the forward rotation.

[0044] The change in the non - energized phase coil voltage varies according to the duty ratio of the drive voltage. When the motor is driven at a constant duty ratio with a predetermined drive voltage, if there is a voltage fluctuation of a certain level or more, such as fluctuations in the battery system or power supply voltage, as shown in Fig. 10, the waveform of the induced voltage induced in the non - energized phase coil changes significantly, and there is a possibility that the field position cannot be detected. Therefore, as described below, a method is adopted to enable field position detection by updating the duty ratio of the drive voltage applied to the three - phase coil from the PWM period and applying the drive voltage from the output means.

[0045] When the drive voltage fluctuates, assuming the inductance of the motor is Lm, the resistance value is Rm, the drive voltage is V, and the energization time is t, the motor current Im is

Equation

Equation

[0046] For example, in the induced voltage waveform diagram induced in the non-conducting phase coil when the drive voltage fluctuates with a fixed duty ratio of 20% in FIG. 10, from the graph of the drive voltage of 18V and a duty ratio of 20%, for this motor, according to (Equation 2), the duty ratio of the drive voltage of 12V is approximately 32%, the duty ratio at a drive voltage of 14V is approximately 26%, the duty ratio at a drive voltage of 22V is approximately 16%, and the duty ratio at a drive voltage of 24V is approximately 15%. In the example of FIG. 10, the drive voltage was set to 18V because the optimal duty ratio (energization time) was obtained with a drive voltage of 18V for the motor shown in FIG. 10. The duty ratio was set to 20% because in the motor shown in FIG. 10, the changes in the positive and negative induced voltages generated in the non-energized phase were significant and easy to measure, and it does not limit the drive voltage and duty ratio in the present invention. FIG. 11 shows the induced voltage waveform diagram induced in the non-conducting phase coil when the duty ratio is corrected according to these voltage fluctuations. From the graph of FIG. 11, it can be seen that when the duty ratio is corrected according to the fluctuation of the drive voltage, the range of voltage fluctuation is small at any drive voltage and the controllability is improved.

[0047] Thus, when the drive voltage fluctuates when applying the drive voltage to the three-phase coil at a predetermined duty ratio for low-speed operation,

Number

Number

Explanation of symbols

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

Claims

【Claim 1】 A rotor having a permanent magnet field, a stator having a three-phase coil, output means for applying bidirectional power to the three-phase coil via a half-bridge type inverter circuit, and controlling the coil output by PWM according to a command from a host controller, storing energization angle information and energization direction information in units of 60° energization sections enabling continuous rotation, and switching the energization state by performing switching control on the output means based on these, and measurement means for A / D converting the three-phase coil voltage and sending it to the control means. Using an electric motor provided with: The control means performs 120° energization including an off-cycle periodically in the energization direction in which the position where self-excitation stops by two-phase fixed energization through the output means coincides with the starting point position of the 60° energization section, and measures the energized phase voltage and the non-energized phase voltage at the on-cycle of PWM energization by the measurement means to detect the field position of the motor while operating sensorlessly. A method for detecting the field position of an electric motor, characterized in that: When the drive voltage fluctuates when applying a drive voltage to the three-phase coil at a predetermined duty ratio for operation, Assuming the inductance of the motor is Lm, the resistance value is Rm, the drive voltage is V, the energization time is t, and the motor current is Im, 【Number 1】 is established, and the control means substitutes the energization time t1 obtained from a predetermined drive voltage V1, the PWM period, and a predetermined duty ratio into (Equation 1) to obtain the motor current Im, and solves (Equation 1) for t 【Number 2】 is used to substitute the motor current Im obtained by (Equation 1) and the changed drive voltage V2 from the drive voltage V1 into (Equation 2) to calculate the energization time t2, divides the energization time t2 by the PWM period to calculate and update the duty ratio, and the control means applies the drive voltage V2 at the updated duty ratio from the output means for operation. A method for detecting the field position of an electric motor, characterized in that.

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