JPH11146684A - Method and equipment for detecting rotating position of rotor of synchronous motor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a rotating position detection signal which can exactly indi cate the positions of magnetic poles of a rotor by obtaining a new rotating position for inverter control by caculating the rotating position of the rotor from a plurality of previous rotating positions. SOLUTION: First neutral point voltage obtained at the connection of stator windings 5a, 5b, 5c wherein induced voltage appear is applied to a noninverted input terminal of an ON amplifier 1a and second neutral point voltage obtained at the connection of resistors 6a, 6b, 6c is applied to an inverted input terminal of the ON amplifier 1a, and a difference between the two voltages is integrated to generate an integration signal ∫ VMNdt. By supplying the integration signal ∫VMNdt to a zero-cross comparator 3, a rotating position detection signal is obtained. The rotating position detection signal is supplied as an interruption signal to a microcomputer 4, which generates a new rotating position detection signal for inverter control from a plurality of previous rotating position detection signals.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a synchronous motor control method and apparatus, and more particularly, to detecting a rotational position of a rotor of a synchronous motor and applying an operating voltage to the synchronous motor based on the rotational position. The present invention relates to a synchronous motor control method and an apparatus for controlling an inverter for applying voltage.

[0002]

2. Description of the Related Art Conventionally, in order to control a brushless DC motor, a rotation position of a rotor is detected, and an inverter for supplying an operating voltage to the brushless DC motor is controlled based on a rotation position detection signal. It is known that it is necessary to: The reason is that when the phase of the output waveform does not correspond to the magnetic pole position (rotational position) of the rotor, the motor efficiency is reduced, and in the worst case, step-out is prevented from occurring. That's why. Here, as a device for detecting the rotational position of the rotor, a device using a sensor using a Hall element or the like (Japanese Patent Application Laid-Open No.
An apparatus for detecting the rotational position of a rotor from a motor induced voltage is known.

[0003]

However, the rotational position detection signal obtained by the above-described device is affected by the characteristics of the hardware that generates the rotational position detection signal, or by the characteristics of the motor induced voltage. Therefore, it is almost impossible to accurately indicate the position of the magnetic pole of the rotor. Then, the motor efficiency is reduced due to the deviation between the rotational position detection signal and the magnetic pole position of the rotor, and in the worst case, the step-out occurs.

[0004]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and a rotor position detection method for a synchronous motor capable of obtaining a rotation position detection signal capable of accurately indicating a rotor magnetic pole position. It is an object to provide a method and an apparatus thereof.

[0005]

According to a first aspect of the present invention, there is provided a method for detecting a rotational position of a rotor of a synchronous motor, comprising detecting a rotational position of a rotor of the synchronous motor and applying an operating voltage to the synchronous motor based on the rotational position. This is a method of calculating the rotational position of the rotor from a plurality of past rotational positions to obtain a new rotational position for inverter control in controlling the inverter for performing the control.

According to a second aspect of the present invention, a method of detecting a rotor rotational position of a synchronous motor includes calculating a rotational position of a rotor from a plurality of past rotational positions, calculating an average value of the calculated rotational positions, and newly rotating the rotor. It is a way to get the position. According to a third aspect of the present invention, there is provided a method for detecting a rotor rotational position of a synchronous motor, wherein a rotational position detection signal is taken in through an insulating means, a rotational position of the rotor is calculated from an even number of rotational positions in the past, and the calculated rotational position is calculated. Is a method of calculating a new rotational position by calculating an average value of.

According to a fourth aspect of the present invention, there is provided a synchronous motor rotor rotational position detecting device which detects a rotational position of a rotor of a synchronous motor and controls an inverter for applying an operating voltage to the synchronous motor based on the rotational position. The synchronous motor control device includes a rotational position correcting means for calculating a rotational position of the rotor from a plurality of past rotational positions and newly obtaining a rotational position for inverter control.

According to a fifth aspect of the present invention, in the synchronous motor rotor rotational position detecting device, the rotational position correcting means calculates a rotational position of the rotor from a plurality of past rotational positions, and calculates an average value of the calculated rotational positions. Is calculated to obtain a new rotational position. The rotor position detecting device for a synchronous motor according to claim 6, further comprising insulating means for taking in a rotational position detection signal, wherein the rotational position correcting means detects the rotational position of the rotor from an even number of past rotational positions. A method of calculating and calculating an average value of the calculated rotation positions to obtain a new rotation position is employed.

According to a seventh aspect of the present invention, a rotor rotational position detecting device for a synchronous motor employs a photocoupler as an insulating means.

[0010]

According to the first aspect of the present invention, there is provided a method for detecting a rotational position of a rotor of a synchronous motor.
In controlling the inverter for applying the operating voltage to the synchronous motor based on the rotational position, the rotational position of the rotor is calculated from a plurality of past rotational positions and a new rotational position for inverter control is calculated. Since the position is obtained, even if the actually detected rotational position does not indicate the magnetic pole position of the rotor, the newly obtained rotational position indicates the magnetic pole position of the rotor, and the motor efficiency decreases. It is possible to prevent inconveniences such as step-out and step-out.

According to a second aspect of the present invention, a method of detecting a rotor rotational position of a synchronous motor calculates a rotational position of the rotor from a plurality of past rotational positions, and calculates an average value of the calculated rotational positions. Therefore, the same operation as in the first aspect can be achieved. According to the method for detecting a rotor rotational position of a synchronous motor according to claim 3, the rotational position detection signal is taken in through an insulating means, and the rotational position of the rotor is calculated from an even number of rotational positions in the past. Since a new rotational position is obtained by calculating the average value of the rotational position, the deviation of the rotational position detection signal caused by the characteristics of the photocoupler can be corrected, and the same effect as in claim 1 can be achieved. .

According to a fourth aspect of the present invention, there is provided an inverter for detecting a rotational position of a rotor of a synchronous motor, and applying an operating voltage to the synchronous motor based on the rotational position. Is controlled by the rotational position correcting means, the rotational position of the rotor is calculated from a plurality of past rotational positions, and a new rotational position for inverter control can be obtained.

Therefore, even if the actually detected rotational position does not indicate the magnetic pole position of the rotor, the newly obtained rotational position indicates the magnetic pole position of the rotor. Inconvenience such as tone can be prevented from occurring. In the synchronous motor rotor rotational position detecting device according to claim 5, as the rotational position correcting means, a rotational position of the rotor is calculated from a plurality of past rotational positions, and an average value of the calculated rotational positions is calculated. Since the one that calculates and newly obtains the rotational position is adopted, the same operation as the fourth aspect can be achieved.

According to a sixth aspect of the present invention, there is provided a synchronous motor rotor rotational position detecting device, further comprising an insulating means for taking in a rotational position detecting signal, wherein the rotational position correcting means rotates from an even number of past rotational positions. Calculate the child's rotational position,
A method of calculating an average value of the calculated rotational positions to obtain a new rotational position is employed, so that an error of the rotational position detection signal caused by the characteristics of the insulating means is corrected.
The same operation as described above can be achieved.

According to the seventh aspect of the present invention, since the photocoupler is used as the insulating means, an error of the rotational position detection signal caused by the characteristics of the photocoupler is corrected. The same operation as the sixth aspect can be achieved.

[0016]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a synchronous motor rotor position detecting method according to an embodiment of the present invention; FIG. 1 is an electric circuit diagram showing an embodiment of a synchronous motor control device incorporating a rotor rotational position detecting device of the present invention. This device employs a brushless DC motor as a synchronous motor.

This synchronous motor control device comprises an integrator 1
And a zero-cross comparator 3, a microcomputer 4, a brushless DC motor 5, a Y-connected resistor circuit 6, and a three-phase inverter 7 controlled by the microcomputer 4. This will be described in more detail. The stator windings 5a, 5b, 5c of the brushless DC motor 5 are Y-connected, and the resistors 6a, 6b,
6c is connected in parallel with the stator windings 5a, 5b, 5c. The output voltage of the three-phase inverter 7 is applied to the stator windings 5a, 5b, 5c and the resistors 6a, 6b, 6c. In addition, 5d is a rotor.

The integrator 1 has an operational amplifier 1a and a resistor 1b and a capacitor 1c connected in parallel between the inverting input terminal and the output terminal of the operational amplifier 1a. Then, the first neutral point voltage VN obtained at the connection point between the stator windings 5a, 5b, 5c is applied to the non-inverting input terminal of the operational amplifier 1a, and the first neutral voltage VN obtained at the connection point between the resistors 6a, 6b, 6c is obtained. 2 neutral point voltage VM is applied to the inverting input terminal of the operational amplifier 1a. Therefore,
The difference voltage VMN (= VN−VM) between the two neutral point voltages is integrated by the integrator 1, and an integrated signal ∫VMNdt can be obtained.

The zero-cross comparator 3 includes an operational amplifier 3a, a resistor 3b and a light emitting diode 3c connected in series between a non-inverting input terminal and an output terminal of the operational amplifier 3a, a non-inverting input terminal of the operational amplifier 3a and a ground terminal. And a pull-up resistor 3 connected to a connection point between the resistor 3b and the light emitting diode 3c.
e, a phototransistor 3f for receiving light from the light emitting diode 3c, and a resistor 3g connected between a collector terminal of the phototransistor 3f and an operation power supply terminal. Then, the emitter terminal of the phototransistor 3f is directly connected to the ground,
Output the rotation position detection signal SINT from the collector terminal of the control circuit. However, a photocoupler as an insulating means including the light emitting diode 3c and the phototransistor 3f may be omitted, and the rotational position detection signal SINT may be directly output from the output terminal of the operational amplifier 3a. Further, as the insulating means, an isolation amplifier or the like can be employed instead of the photocoupler. Therefore, the rotation position detection signal SI whose level is inverted every time a zero cross of the integration signal ∫VMNdt occurs.
NT can be output. Then, the rotational position detection signal SINT is supplied to the microcomputer 4.

The comparison section 2 sets a comparison level based on a comparison level setting signal output from the microcomputer 4.
a, the integrated signal 積分 VMNdt is supplied to perform a level determination, and the level determination result signal is supplied to the flip-flop circuit 2b.
And the data (level detection signal) held by the flip-flop circuit 2 b is supplied to the microcomputer 4. The reset signal output from the microcomputer 4 is supplied to the flip-flop circuit 2b.

The microcomputer 4 outputs a rotational position detection signal SI
With NT as an input, a new rotation position detection signal for inverter control is generated, and based on the newly generated rotation position detection signal, for example, speed control and efficiency control are performed to generate a voltage amplitude command and a phase command. Then, pulse width modulation based on the voltage amplitude command and the phase command is performed to generate an inverter waveform signal, which is supplied to the inverter 7 via the drive circuit 7a. Since the speed control, efficiency control, and the like in the microcomputer 4 are conventionally known, detailed description will be omitted. The overcurrent detection circuit 7 b detects that the input current of the inverter 7 has become overcurrent, generates an overcurrent detection signal, and supplies the signal to the microcomputer 4.

Next, the operation of the rotational position detecting device having the above configuration will be described. The first neutral point voltage VN obtained at the connection point of the stator windings 5a, 5b, 5c where the induced voltages shown in (A), (B), and (C) shown in FIG. 2 are generated is the operational amplifier 1a.
Of the resistors 6a, 6a
b, 6c, the second neutral point voltage V obtained at the connection point
When M is applied to the inverting input terminal of the operational amplifier 1a, the difference voltage VMN shown in FIG. 2D is integrated, and an integrated signal 積分 VMNdt shown in FIG. 2E can be obtained. Then, by supplying the integration signal ∫VMNdt to the zero-cross comparator 3, a rotation position detection signal shown in FIG. 2 (F) can be obtained.

This rotation position detection signal is supplied to the microcomputer 4 as an interrupt signal, and a rotation position detection signal for inverter control is newly generated from a plurality of past rotation position detection signals. Here, the number of rotation position detection signals for generating a new rotation position detection signal may be set to an integral multiple of the fluctuation period of the rotation position detection signal. FIGS. 3 and 4 are schematic diagrams for explaining a process of generating a new rotation position detection signal for inverter control from a plurality of past rotation position detection signals.

FIG. 3 shows a case where six rotation position detection signals are generated during one rotation, and the six rotation position detection signals have different phase differences from each other. Here, since the phase difference between the rotation position detection signals should be 60 °, in FIG. 3, by adding a phase difference of an integral multiple of 60 ° from the actually obtained rotation position detection signal, The rotation position detection signal can be estimated (see arrows indicated by 1, 2, 3, 4, and 5). Therefore, for example, by calculating the average value of a plurality of estimated rotational position detection signals, a rotational position detection signal with less error can be obtained (see the broken line in FIG. 4). The rotation position detection signal having a small error has a phase difference of approximately 60 °.
It has become.

However, in this state, there is no guarantee that the rotation position detection signal matches the magnetic pole position of the rotor.
However, for example, the generated torque of the reluctance DC motor is determined by the voltage amplitude and the voltage phase, and the output waveform phase is a value obtained by adding the phase correction amount to the magnetic pole position (phase) of the rotor. By eliminating the short-term variation such as the variation of the rotation position detection signal as described above and controlling the phase correction amount in this state, the rotation position detection signal can be made to substantially match the magnetic pole position of the rotor. .

FIG. 5 is a flowchart for explaining an example of a process for newly generating a rotational position detection signal for inverter control from a plurality of past rotational position detection signals.
In step SP1, the time (cycle) from the latest rotation position detection signal interruption to the latest rotation position detection signal interruption.
And the calculated value of the time corresponding to 0 ° (60 ° × 0) calculated from the average rotation speed, and in step SP2, the latest rotation position detection signal interruption from the previous rotation position detection signal interruption is calculated. The difference between the time (period) up to and the calculated value corresponding to 60 ° (60 ° × 1) calculated from the average rotation speed is calculated, and in step SP3, the rotation position detection signal interrupted two times before is calculated. 120 ° (6 °) calculated from the time (cycle) at the latest rotation position detection signal interruption and the average rotation speed.
0 ° × 2), and the difference from the calculated value of the time corresponding to 0 ° × 2) is calculated, and in step SP4, the time (cycle) from the rotation position detection signal interruption three times before to the latest rotation position detection signal interruption and the average The difference from the calculated value of the time corresponding to 180 ° (60 ° × 3) calculated from the rotation speed is calculated, and in step SP5, the latest rotation position detection signal is interrupted from the rotation position detection signal interrupted four times before. And the calculated value of the time corresponding to 240 ° (60 ° × 4) calculated from the average rotational speed is calculated from the average rotational speed, and in step SP6, the latest from the rotational position detection signal interrupt five times before is calculated. 300 ° calculated from the time (period) at which the rotation position detection signal is interrupted and the average rotation speed
The difference from the calculated value of the time corresponding to (60 ° × 5) is calculated, and in step SP7, steps SP1 to SP
An average value of the six values calculated in P6 is calculated, and in step SP8, the above average value is added to the latest rotational position detection signal interruption time to obtain a new time. The time is set to the corrected rotational position detection signal interrupt position, and the series of processing ends.

FIG. 6 is a flowchart for explaining another example of a process for generating a new rotational position detection signal for inverter control from a plurality of past rotational position detection signals. In step SP1, the time (cycle) at the latest rotation position detection signal interruption is calculated from the latest rotation position detection signal interruption, and in step SP2, the latest rotation position detection signal from the previous rotation position detection signal interruption. The time (cycle) at the interruption is calculated, and at step SP3, the time (cycle) at the latest rotation position detection signal interruption is calculated from the rotation position detection signal interruption two times before. At step SP4, 3 is calculated. The time (period) at the latest rotation position detection signal interruption is calculated from the previous rotation position detection signal interruption, and at step SP5, the latest rotation position detection signal interruption is calculated from the four previous rotation position detection signal interruptions. In step SP6, the time (cycle) at the latest rotation position detection signal interruption is calculated from the rotation position detection signal interruption five times before, and step S6 is performed. In 7, step SP
Six times (cycles) calculated from 1 in step SP6
Is calculated, and in step SP8, a calculated value of a time (cycle) corresponding to 0 ° (60 ° × 0) calculated from the average rotation speed is calculated. In step SP9, the calculated value of 60 ° calculated from the average rotation speed is calculated. A calculated value for a time (cycle) corresponding to (60 ° × 1) is calculated, and in step SP10, 120 ° (60 ° × 2) calculated from the average rotation speed.
A calculated value for a considerable time (cycle) is calculated, and step SP1
1, 180 ° (60 °) calculated from the average rotation speed.
° × 3) A calculated value for a time (cycle) corresponding to 240 ° calculated from the average rotation speed in step SP12.
A calculated value of a time (period) corresponding to ° (60 ° × 4) is calculated, and in step SP13, a calculated value of a time corresponding to 300 ° (60 ° × 5) calculated from the average rotation speed is calculated. In step SP8, the total value of the six times (cycles) calculated in step SP8 to step SP13 is calculated.
The difference between the total value calculated in step 7 and the total value calculated in step SP14 is calculated. In step SP16, the difference calculated in step SP15 is divided by 6 to calculate an average value of the correction values. In SP17, the average value of the correction values is added to the latest rotation position detection signal interruption time to obtain a new time, and the obtained time is set as the corrected rotation position detection signal interruption position. Then, the series of processing ends.

FIG. 7 illustrates the deviation of the finally obtained rotational position detection signal (secondary SINT) when the output signal (primary SINT) from the zero cross comparator 3 is output through a photocoupler for insulation. FIG.
As shown in FIGS. 7A and 7B, the integration signal and the output signal (SINT primary) from the zero-cross comparator 3 are equal to the signal shown in FIG. However, as shown in FIG. 7 (C), the rise time and fall time of the output signal of the photocoupler are not equal to each other, so that there is a difference between the high level time and the low level time of the output signal of the zero cross comparator 3. As a result, the rotational position detection signal (SINT secondary) is shifted as shown in FIG.

As a result, as shown in FIG. 8, in the rotational position detection signal, a state where the phase difference is small and a state where the phase difference is large occur alternately. In this case, the phase of the rotational position detection signal may be newly determined from the phases of the past two rotational position detection signals, and as shown by a broken line in FIG.
The phase difference between the rotational position detection signals can be made constant by eliminating the influence of the rise time and the fall time of the output signal of the photocoupler. However, the present invention is not limited to the past two rotational position detection signals, and it is possible to employ a rotational position detection signal that is an integral multiple of the fluctuation period.

FIG. 10 is a diagram for explaining a shift generated in the rotational position detection signal under the influence of the motor winding current. In FIG. 10, a broken line indicates a signal waveform when no load is applied, and a solid line indicates a signal waveform affected by an error voltage. In the inverter for the reluctance DC motor, the difference signal VMN detected from the motor terminal voltage and the neutral point potential is integrated to obtain an integrated signal ∫VMNdt. Then, by detecting the zero-cross point of the integrated signal ∫VMNdt by a zero-cross comparator, the rotational position detection signal SINT is detected.
Get. However, the actual difference signal VMN is superimposed with a voltage due to a voltage drop generated due to the influence of the motor winding current, as shown in FIG. Due to this influence, as shown in FIGS. 10B and 10C, the integrated signal ΔVMNdt and the rotational position detection signal SINT are shifted.

FIG. 11 is a diagram for explaining an example of a deviation of the rotational position detection signal SINT. FIG. 11 shows a state in which the rotational position detection signal indicated by the broken line is shifted in the direction indicated by the arrow. Then, the rotational position detection signal S shown in FIG.
The shift of the INT phase is determined by comparing the past six rotational position detection signals S.
By performing the correction based on the phase of the INT, it is possible to greatly reduce the deviation of the rotational position detection signal SINT, as indicated by the chain line in FIG.

In the above, a case has been described in which a rotational position detection signal for newly controlling the inverter is generated based on the past two rotational position detection signals and the past six rotational position detection signals. Can generate a new rotation position detection signal for controlling the inverter based on the past three to five rotation position detection signals and the past seven or more rotation position detection signals. Various design changes can be made without departing from the spirit of the invention.

[0033]

According to the first aspect of the present invention, even if the actually detected rotational position does not indicate the magnetic pole position of the rotor, the newly obtained rotational position indicates the magnetic pole position of the rotor. In addition, it is possible to suppress the noise and sudden fluctuation of the rotation position detection signal, and to achieve a unique effect that it is possible to prevent the occurrence of inconveniences such as a decrease in motor efficiency and step-out.

The second aspect of the invention has the same effect as the first aspect. According to the third aspect of the invention, the displacement of the rotational position detection signal caused by the characteristics of the insulating means is corrected, and the same effects as those of the first aspect are obtained. According to the invention of claim 4, even if the actually detected rotational position does not indicate the magnetic pole position of the rotor, the newly obtained rotational position indicates the magnetic pole position of the rotor. Noise and sudden fluctuations can be suppressed, and a disadvantage such as a reduction in motor efficiency or step-out can be prevented beforehand.

The fifth aspect of the invention has the same effect as the fourth aspect. According to the sixth aspect of the invention, the same effect as the fourth aspect is obtained by correcting the deviation of the rotational position detection signal caused by the characteristics of the insulating means. According to the seventh aspect of the invention, the displacement of the rotational position detection signal caused by the characteristics of the photocoupler is corrected, and the same effect as that of the sixth aspect is obtained.

[Brief description of the drawings]

FIG. 1 is an electric circuit diagram showing an embodiment of a synchronous motor control device incorporating a rotor rotational position detection device of the present invention.

FIG. 2 is a diagram showing signal waveforms at various parts of the device of FIG.

FIG. 3 is a diagram illustrating a rotational position detection signal estimated from six rotational position detection signals actually obtained.

FIG. 4 is a diagram showing actually obtained six rotational position detection signals and six corrected rotational position detection signals.

FIG. 5 is a flowchart illustrating an example of a process of generating a new rotation position detection signal for inverter control from a plurality of past rotation position detection signals.

FIG. 6 is a flowchart illustrating another example of a process of newly generating a rotational position detection signal for inverter control from a plurality of past rotational position detection signals.

FIG. 7 shows an output signal from the zero-cross comparator 3,
FIG. 9 is a diagram illustrating a deviation of a rotational position detection signal finally obtained when an output is performed through a photocoupler for insulation.

FIG. 8 is a diagram showing a rotational position detection signal in which a shift has occurred.

9 is a diagram showing a state in which the rotational position detection signal of FIG. 8 has been corrected.

FIG. 10 is a diagram illustrating a shift generated in a rotational position detection signal under the influence of a motor winding current.

FIG. 11 is a diagram showing a rotational position detection signal in which a shift has occurred.

FIG. 12 is a diagram showing a state in which the rotational position detection signal of FIG. 11 has been corrected.

[Explanation of symbols]

 3c light emitting diode 3f phototransistor 4 microcomputer 5 brushless DC motor 5d rotor 7 inverter

Claims (7)

[Claims] An inverter (7) for detecting a rotational position of a rotor (5d) of a synchronous motor (5) and applying an operating voltage to the synchronous motor (5) based on the rotational position.
The synchronous motor control method for controlling the synchronous motor according to claim 1, wherein a rotational position of the rotor (5d) is calculated from a plurality of past rotational positions to newly obtain a rotational position for inverter control. Rotation position detection method. 2. A rotor (5) from a plurality of past rotational positions.
2. The method according to claim 1, wherein the rotational position of d) is calculated, and an average value of the calculated rotational positions is calculated to obtain a new rotational position. 3. A rotational position detection signal is isolated by an insulating means (3c).
(3f), a rotation position of the rotor (5d) is calculated from an even number of rotation positions in the past, and an average value of the calculated rotation positions is calculated to obtain a new rotation position. 3. The method of detecting a rotor rotational position of a synchronous motor according to claim 1. 4. An inverter (7) for detecting a rotational position of a rotor (5d) of the synchronous motor (5) and applying an operating voltage to the synchronous motor (5) based on the rotational position.
A rotational position correcting means (4) for calculating a rotational position of the rotor (5d) from a plurality of past rotational positions and newly obtaining a rotational position for inverter control in the synchronous motor control device for controlling A rotor position detecting device for a synchronous motor. 5. The rotation position correcting means (4) calculates a rotation position of the rotor (5d) from a plurality of past rotation positions, calculates an average value of the calculated rotation positions, and newly rotates the rotation position. 5. The rotor position detecting device for a synchronous motor according to claim 4, wherein the position is obtained. 6. An insulated means (3c) (3f) for taking in a rotational position detection signal, wherein said rotational position correcting means (4) is configured to output the rotor (5) from an even number of rotational positions in the past.
5. The synchronous motor rotor rotational position detecting device according to claim 4, wherein the rotational position of d) is calculated, and an average value of the calculated rotational positions is calculated to newly obtain a rotational position. 7. The synchronous motor rotor rotational position detecting device according to claim 6, wherein the insulating means (3c) (3f) are photocouplers (3c) (3f).