JPH09247987A - Brushless motor control - Google Patents

Abstract

PROBLEM TO BE SOLVED: To conduct optimum control by regular position detection without any interference with rotation control even in case of being unable to detect a position of a rotor in a brushless motor. SOLUTION: The voltage of an armature winding in a brushless motor 4 is compared with the reference voltage by a position detection part 10 to obtain a position detection signal D, which is inputted into a control circuit 11 to detect the position of a rotor. Based on the position detection, current-carrying of the winding voltage is switched. The control circuit 11 counts time between position detection intervals, and based on the counted time, the time corresponding to phase angle 30 is predicted from the position detection and position detection at the predicted time is checked. In case of no position, present current-carrying is conducted based on that at the earlier time than previous switching time of current-carrying. The voltage and number of revolutions which control the brushless motor 4 are decreased for every prescribed hour by a prescribed value until position detection is conducted and the present number of revolutions is maximum in position detection to limit the number of revolutions of the brushless motor 4 to its maximum value for a prescribed time.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rotation control technology of a sensorless DC brushless motor (hereinafter referred to as a brushless motor) used in a compressor of an air conditioner, and more particularly, to a position detection of a brushless motor rotor. In addition, while making it possible to switch the energization of the armature winding current,
The present invention relates to a control method of a brushless motor that enables proper position detection and switches energization appropriately.

[0002]

2. Description of the Related Art In controlling the rotation of a brushless motor of this type, there is a method of utilizing a voltage induced in an armature winding of a brushless motor, for example, without using a Hall element as a means for detecting the position of the rotor. is there. When the position of the rotor is detected by using this induced voltage to switch the energization of the armature winding current of the brushless motor, for example, the control device shown in FIG. 7 is required.

In FIG. 7, this control apparatus rectifies and smoothes an AC power supply (commercial 100 V) 1 by a voltage doubler rectifying and smoothing circuit 2, and switches the voltage doubled rectified and smoothed DC power supply Vdc into a plurality of switching elements. Element (transistor) U,
The inverter circuit 3 in which V, W, X, Y and Z are bridge-connected is switched and applied to the armature winding of the three-phase brushless motor 4. The voltage doubler rectifying / smoothing circuit 2 includes a rectifier circuit, a capacitor circuit, and a smoothing capacitor which are already known.

The terminal voltage of the brushless motor 4 (for example, 1
20 degree phase difference voltage; including induced voltage) R, S, T
Is inputted to the position detecting section 5, and the position detecting section 5 delays and smooths the voltage waveforms of the terminal voltages R, S, T by 90 degrees and compares them with the neutral point potential Vn and compares them with the position detecting signal A. ，
B and C are output to the control circuit (microcomputer) 6. Therefore, the position detecting unit 5 includes a differentiating circuit 5a and an integrating circuit 5b, a resistance circuit 5c for obtaining a neutral point potential Vn by combining a 90 degree phase delayed voltage, the 90 degree phase delayed voltage and a neutral point potential. Comparison circuit 5d for comparing with Vn
And

The brushless motor 4 is started and operated synchronously,
After this synchronous operation is performed for a predetermined time, the position detection operation is performed by switching the energization of each armature winding based on the position detection signals A, B and C from the position detection unit 5. Therefore, the control circuit 6 outputs to the drive circuit 7 a drive signal for driving the switching elements (transistors) U, V, W, X, Y, Z of the inverter circuit 3 on and off and a chopping signal for chopping drive.

As shown in FIG. 7, when the above control device is used for controlling the compressor of the air conditioner, the power factor improving reactor 8 is provided between the AC power source 1 and the voltage doubler rectifying and smoothing circuit 2.
Is provided.

In the position detecting operation of the control apparatus having the above-mentioned structure, the position detecting section 5 sets the terminal voltages R, S, T of the armature windings shown in FIGS. , The voltage of this phase delay (FIGS. 8D to 8F)
(Shown in FIG. 8) and the neutral point potential Vn are compared, and the position detection signal changes at the intersections (shown in FIGS. 8 (g) to 8 (i)).
Outputs A, B and C.

The position detection signals A, B, C are transmitted to the control circuit 6
When input to the position detection signal A, B,
Based on the timing of C, a drive signal (shown in FIGS. 8 (j) to 8 (o)) for switching the conduction state of each transistor U, V, W, X, Y, Z of the inverter circuit 3 is generated.

Each drive signal causes each transistor U, V,
Since W, X, Y, and Z are driven, the brushless motor 4
The armature winding current of the brushless motor 4
Is controlled to rotate.

The control circuit 6 outputs a chopping signal having a predetermined duty ratio (ON / OFF ratio) to the drive circuit 7 in addition to the drive signal, and the drive circuit 7 outputs the transistor U of the upper arm of the inverter circuit 3. , V, W drive signals are chopped on (see FIGS. 8 (j) to 8 (l)). There is also a method of chopping driving the transistors X, Y, Z of the lower arm.

As described above, the energization of each armature winding of the brushless motor 4 is switched based on the position detection signals A, B and C to control the rotation of the brushless motor 4 and the on / off ratio of chopping is changed. Then, the brushless motor 4 is controlled to a predetermined rotation speed.

[0012]

In the brushless motor control method, the position of the rotor 4a is detected using the integrating circuit 5b, and the armature winding winding is detected based on this position detection. Since the timing to switch the power supply is obtained,
The following problems occur.

First, various frequency components are mixed in the voltage waveform induced in the armature winding, and constants cannot be determined for all frequencies due to the characteristics of the integrating circuit (integrating filter) 5b. Therefore, the voltage waveform passing through the integrating circuit includes an unnecessary frequency component, and in particular, the frequency included due to the change in the rotation speed of the brushless motor 4 causes an error in the output delay of 90 degrees, resulting in the position of the rotor 4a. There is an error in detection.

Further, since the integrating circuit 5b is required, not only the cost is inevitably increased, but also the control board is large due to the increase in the number of parts and there is a drawback that a space is taken up.

Further, although the 90 ° delay obtained through the integrating circuit 5b is always constant, the 90 ° delay is not optimum depending on the rotation speed and load condition of the brushless motor 4, that is, the energization that maximizes the efficiency of the brushless motor 4. Since the switching timing varies depending on the number of revolutions and load condition,
It is not possible to switch the energization for maximum efficiency at all times.

Therefore, there is a method of controlling the brushless motor 4 by detecting the position of the rotor 4a without using the integrating circuit 5b. In the case of this control method, the voltage waveform induced in the armature winding is compared with the neutral point potential (reference voltage), and the result of this comparison is input to the control circuit (microcomputer) of the brushless motor 4, It is calculated by a computer to determine the energization switching timing.

More specifically, the terminal voltage waveform (induced voltage waveform) of the direct armature winding and the reference voltage (Vdc / 2)
The position detection signal obtained by comparing and is input to the microcomputer. This microcomputer uses the first change point of the position detection signal after switching the energization as the position detection point of the rotor 4a, and from this position detection point a phase angle of 30 degrees (electrical angle 3
The energization of the armature winding is switched when a time corresponding to (0 degree) has elapsed.

In this way, by using the integrating circuit 5b, 90
Whereas the position detection signal delayed by 90 degrees (energization switching timing at 90 degrees) is obtained, in the method which does not use the integrating circuit 5b, the switching timing is 30 degrees, which is the timing closest to the intersection of the induced voltage and the reference voltage. It becomes possible to switch the energization.

However, in the case where the integration circuit 5b is not used, the change point (detection edge) of the first position detection signal after the energization switching is normal due to the noise due to the back electromotive voltage (spike voltage k) due to the energization switching. It may not be the position detection point. For example, there are cases where the voltage width of the spike voltage k is large and is applied to the position detection point (the intersection of the induced voltage and the reference voltage), and the position detection point cannot be obtained.

As described above, when the spike voltage k is applied to the position detection point and the position detection point of the rotor 4a cannot be obtained, the rotation control of the brushless motor 4 is hindered, and the cause of the stop or step out occurs. Also, the position detection operation may not be continued.

The present invention has been made in view of the above-mentioned problems, and an object thereof is to stop the rotation of the brushless motor without any trouble even if the position of the rotor of the brushless motor cannot be detected. It is an object of the present invention to provide a brushless motor control method capable of preventing step-out and continuing position detection operation.

[0022]

In order to achieve the above-mentioned object, the present invention, when controlling the rotation of a brushless motor, detects the position obtained by comparing the voltage of the armature winding of the brushless motor with a reference voltage. A brushless motor control method for detecting the position of the rotor of the brushless motor by a signal, and switching the energization of the armature winding current of the brushless motor based on the position detection. Time is counted, at least the time after the next position detection is predicted based on the measured time, and when there is no position detection at the predicted time, the current detection is performed based on the position detection time interval before the previous time. While switching, the voltage and the rotational speed for controlling the brushless motor are lowered by a predetermined value at predetermined time intervals until the position is detected.

In the brushless motor control method of the present invention, the time of the position detection interval is clocked, at least the time after the next position detection is predicted based on the clocked time, and the predicted time is set to the predicted time. When the position is not detected, the current is switched based on the position detection time interval before the previous time, while when the position is not detected a predetermined number of times or more, the brushless motor is rotationally controlled until the same voltage is detected. The voltage and the rotational speed for reducing the rotational speed of the brushless motor when the position is detected, and the rotational speed of the brushless motor does not exceed the obtained rotational speed. It is characterized by having done.

Further, in the brushless motor control method of the present invention, the time of the position detection interval is clocked, at least the time after the next position detection is predicted based on the clocked time, and the predicted time is set to the predicted time. When the position is not detected, the current is switched based on the position detection time interval before the previous time, while when the position is not detected a predetermined number of times or more, the brushless motor is rotationally controlled until the same voltage is detected. The voltage and the rotation speed for reducing the rotation speed of the brushless motor when the position is detected, and the rotation speed of the brushless motor does not exceed the obtained rotation speed. After that, the limitation on the number of revolutions is released after a predetermined time has elapsed.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to FIGS. FIG.
In FIG. 7, those parts which are the same as or corresponding to those in FIG.

In FIG. 1, a control device to which this brushless motor control method is applied is a terminal voltage (including an induced voltage) R of an armature winding of a brushless motor 4 and a reference voltage V.
dc is compared with the position detection unit 10, and the result is output to the control circuit 11 as a position detection signal D. Control circuit 11
Detects the position of the rotor 4a from the first change point of the position detection signal after switching the energization, measures the time of this position detection interval, and based on the position detection time and the time of the position detection interval. Then, the energization switching timing is calculated, and a drive signal for driving the transistor of the inverter circuit 3 is output according to the calculated energization switching timing. The control unit 11 has the function of the control circuit 6 shown in FIG.

The position detecting section 10 compares the reference voltage generating circuit 10a for generating the reference voltage Vdc / 2 with the DC power supply Vdc of the inverter circuit 3 being 1/2, and the reference voltage and the terminal voltage R of the brushless motor 4. And position detection signal D
And a comparison circuit 10b for outputting

The control circuit 11 temporarily stores the calculated energization switching timing (energization switching time). On the other hand, when the current position cannot be detected by a predetermined time (predicted time), the control circuit 11 already stores the energization switching timing. Then, the energization of the armature winding current of the brushless motor 4 is switched.

When the position is not detected at the predicted time, the content of the position detection register is incremented,
When the position is detected, the contents of the position detection register are decremented. Further, when there is no position detection, the energization is switched based on the energization switching timing before the previous time (while the synchronous operation is performed), and if the content of the position detection register is equal to or more than a predetermined value, the brushless motor 4 is controlled. The voltage (LOW width) and the rotation speed (the rotation speed of the synchronous operation) are reduced by a predetermined ratio value (that is, V / f is changed),
This operation is repeated until the position is detected.

Incidentally, lowering the LOW width of the voltage by a predetermined value means that the transistors U, V, W, and
Shorten off time of X, Y, Z (longer on time),
The applied voltage of the brushless motor 4 is lowered by a predetermined value.

When the content of the position detection register is equal to or greater than a predetermined value, the number of revolutions when the position is detected is set to the maximum value, and the rotation of the brushless motor 4 is prevented from exceeding the maximum value for a predetermined time. The number may be controlled.

Next, the operation of the control device will be described in detail with reference to the time chart of FIG. 2 and the flow charts of FIGS. 3 to 6. First, the rotation control of the brushless motor 3 enters the position detection operation mode. It is assumed that

In this case, the position detector 10 compares the terminal voltage R of the brushless motor 3 (see FIG. 2A) with the reference voltage Vdc / 2 from the reference voltage generation circuit 10a.
0b is compared, and a position detection signal D that changes at the intersection of the comparison results is output (see FIG. 2B).

Then, the control circuit 11 detects the position of the rotor 4a at the first change point (edge) of the position detection signal after switching the energization (after the spike voltage k), and based on the position detection interval time. Phase angle 30, 90 degrees and 1
Calculate the time of 50 degrees. The energization switching timing (time) is calculated based on this calculated time, and energization of the armature winding current of the brushless motor 4 is switched at this calculated time.

Therefore, as shown in FIGS. 2C to 2H, the transistors U, V, W of the inverter circuit 3 are
A drive signal for driving X, Y, Z is output, and a chopping signal and a switching signal for chopping a predetermined section of the drive signal according to the instructed rotation speed of the brushless motor 4 are output.

At this time, the control circuit 11 measures the time T1 between position detections (1 cycle) for each position detection, and
The next position detection point time is calculated based on this time T1, and a predetermined time (predicted time) T corresponding to a phase angle of 30 degrees is calculated from the next position detection point. The predetermined time T may be between the next position detection time and the time corresponding to the phase angle 30.

For example, as shown in FIG. 2, if the current position detection is point a, a predetermined time T at point c is calculated (the time corresponding to a phase angle of 30 degrees is predicted from the next position detection). . In calculating the predetermined time T, the time of the speed variation α (degree) of the brushless motor 4 may be taken into consideration.
Also, (the time of the previous 30-degree energization switching) + (180
(Time corresponding to the degree), the predetermined time T may be calculated, and the time of the speed variation α (degree) may be added to the calculated time.

When the position is detected at the predetermined time T, the routine shown in FIG. 3 is executed, and when the position is not detected, the routine shown in FIG. 5 is executed.

In the routine shown in FIG. 3, since the position is first detected, the internal position detection flag is set (step ST1), and the content of the internal position detection register (the number of times without position detection) is 3 or more. It is determined whether there is any (step ST2).

Subsequently, it is determined whether or not the content of the position detection register (the number of times without position detection) is 1 or more (step ST2). When the content of the position detection register is 1 or more, the content of the position detection register is decremented by a predetermined value (for example, 1) (step ST3). When the content of the position detection register is not 1 or more, that is, when the position detection has been performed so far, the routine is ended.

The following processing may be added between steps ST1 and ST2. That is, when the position detection register has three or more contents and the position is detected this time even though the position is not detected many times, the current rotation speed of the brushless motor 4 (the rotation speed instructed by the control circuit 11).
Is stored as the maximum value. In this case, when the content of the position detection register is not 3 or more, that is, when the number of times of no position detection is small, the maximum value is not stored and the process proceeds to step ST2.

After a predetermined time from the position detection point, the routine shown in FIG. 4 is executed to judge whether the position detection flag is set (step ST4). When the position detection flag is set, the position detection signal D
Since the normal position detection is being performed by, the routine ends.

When the position detection flag is not set, since the normal position detection cannot be obtained by the position detection signal D, an interrupt flag without position detection is set (step ST5), and the routine ends.

In the routine shown in FIG. 5, since the interrupt flag without position detection is set, the content of the position detection register is increased by a predetermined value (for example, 1) (step ST10), and the position detection register is increased. It is determined whether or not the content of is greater than or equal to a predetermined value (for example, 3) (step ST11).

When the position detection is above the predetermined value (3), the content of the position detection register is returned to the predetermined value (3) (step ST12), and there is a possibility that the position detection cannot be performed thereafter. Lowering the LOW width of the brushless motor 4 by a predetermined value (for example, 1 μsec) (step ST13), and the energization time interval of the brushless motor 4 by a predetermined value (for example, 1 μsec).
Lower (step ST14).

That is, the applied voltage of the brushless motor 4 is lowered by a predetermined value, and the rotation speed of the brushless motor 4 is lowered by a predetermined value (that is, V / f is changed). Further, this routine is repeatedly executed every predetermined time until the position is detected.

When the position detection is not more than the predetermined value, the above process is not necessary, and the routine is finished. That is, this is because the number of times that the position has not been detected is small, and there is a possibility that the position can be properly detected by the synchronous operation described later.

On the other hand, the routine shown in FIG. 6 is executed at every predetermined time T, and it is determined whether or not the position detection flag is set (step ST20). Step S in FIG.
At T1, when the position detection flag is not set, the process proceeds from step ST20 to ST21, and the energization switching process with the phase angle of 30 degrees is immediately performed. That is, the predetermined time T corresponds to the energization switching timing of the phase angle of 30 degrees. When the predetermined time T is not the time corresponding to the phase angle of 30 degrees, the time corresponding to the phase angle of 30 degrees is also set in the internal timer as described above.

Then, the time corresponding to the phase angle of 90 degrees is set in the internal timer (step ST22). For example, T90 = (previous 90-degree energization switching time) +
The time corresponding to the phase angle of 90 degrees is calculated using the formula (time corresponding to 180 degrees).

Then, the time corresponding to the phase angle of 150 degrees is set in the internal timer (step ST23). For example, T150 = (the time of the previous 150-degree energization switching)
+ (Time corresponding to 180 degrees) phase angle 15
Calculate the time corresponding to 0 degrees.

Then, the position detection flag is cleared and the routine is finished. The above step ST20
When it is determined that the position detection flag is set, the position detection flag is cleared and the routine ends.

For example, as shown by the broken line in FIG.
When the spike voltage k is applied to the position detection point b (broken line arrow in the figure) and there is no position detection, energization is switched based on the previous energization switching time (previous data), that is, the brushless motor 4 is synchronously operated. Like. In addition,
When the position detection is not continuous, the energization is switched based on the energization switching time two times before.

As a result, even if the position cannot be detected, the rotation control can be prevented, stoppage and step-out can be prevented, and the position detection operation can be continued.

Further, in the synchronous operation of the brushless motor 4, the voltage for controlling the brushless motor 4 and the rotation speed (V / f ratio) are lowered every predetermined time (in other words, the energization time interval is changed). As a result, the position detection point is delayed, so that the spike voltage k is not applied to the position detection point, and when the position is detected, the position detection flag is set (step ST1 in FIG. 3).

When this position detection flag is set, energization can be switched again by regular position detection,
Optimal rotation control is possible, and of course, the position detection operation can be optimally continued without stopping or step-out.

Further, when the maximum value of the instructed rotation speed is set in the routine of FIG. 3, after the brushless motor 4 is returned to the position detecting operation, the rotation speed of the brushless motor 4 does not exceed the maximum value. The brushless motor 4 is rotationally controlled.

In this case, the number of revolutions may not exceed the maximum value for a predetermined time after returning to the position detecting operation. That is, while the position detection may be temporarily lost depending on the load state, even if the rotation speed of the brushless motor 4 is made larger than the maximum value, the position detection may not be impossible again.

As described above, when the position of the rotor 4a cannot be detected, the current energization switching time (phase angle 30 degrees, 90 degrees and 150 degrees) is obtained based on the energization switching time before the previous time. Therefore, the rotation control of the brushless motor 4 is not hindered, stoppage and step-out can be prevented, and the position detection operation can be continued.

On the other hand, even if the position detection is stopped, the spike voltage k is reduced to the position detection point (the induced voltage and the reference voltage Vdc) by lowering the ratio (V / f ratio) of the applied voltage to the brushless motor 4. Since the intersection is not applied, the regular position can be detected, and the rotational speed at the time of detecting the regular position is set to the maximum value so that the rotational speed of the brushless motor 4 is limited not to exceed the maximum value.

Therefore, the normal position detection can be surely performed, that is, the optimum rotation control can be performed, and the position detection operation can be optimally continued without any stop or step out.

Further, after the predetermined time, the limitation of the rotational speed of the brushless motor 4 is released, so that the rotational speed of the brushless motor 4 is not limited and the normal rotational speed control can be restored. When the motor 4 is used as a compressor motor of an air conditioner, it is possible to suppress a decrease in operating efficiency of the compressor.

[0062]

As described above, according to claim 1 of the brushless motor control method of the present invention, the position of the rotor is detected, and the armature winding of the brushless motor is detected based on this position detection. When switching the energization of the current, the time after the next position detection is predicted, and when there is no position detection at this predicted time, the current energization is performed based on the previous energization switching time (synchronous operation and The same), when the position is not detected a predetermined number of times or more, the voltage is decreased by a predetermined value and the rotation speed is decreased by a predetermined value, and the voltage and the rotation speed are decreased by a predetermined value every predetermined time until the position is detected. So, even if the position is not detected,
It is possible to switch the energization of the brushless motor, to prevent the rotation control from being disturbed, to prevent stoppage and step out, to continue the position detection operation, and to enable regular position detection. The effect is that you can.

According to the second aspect of the present invention, in addition to the first aspect, the control is made so as not to increase the rotational speed of the brushless motor beyond the current rotational speed when the position can be detected. In addition to the same effect as 1, there is an effect that regular position detection can be surely performed, that is, optimum rotation control is possible, and position detection operation can be continued.

According to the third aspect of the present invention, in addition to the second aspect, the limitation of the maximum value of the rotational speed is released after a predetermined time has passed from the position detection, so that the same effect as the second aspect is obtained. In addition, it is possible to return to normal rotation speed control, and for example, when a brushless motor is used as a compressor motor of an air conditioner, there is an effect that it is possible to suppress a decrease in operating efficiency of the compressor.

[Brief description of drawings]

FIG. 1 is a schematic block diagram of a control device to which an embodiment of the present invention is applied, to which a control method of a brushless motor is applied.

FIG. 2 is a schematic time chart diagram for explaining the operation of the controller for the brushless motor shown in FIG.

3 is a schematic flowchart for explaining the operation of the brushless motor control device shown in FIG. 1. FIG.

FIG. 4 is a schematic flowchart for explaining the operation of the brushless motor control device shown in FIG. 1.

FIG. 5 is a schematic flowchart for explaining the operation of the brushless motor control device shown in FIG.

FIG. 6 is a schematic flowchart for explaining the operation of the brushless motor control device shown in FIG. 1.

FIG. 7 is a schematic block diagram of a conventional control device for a brushless motor.

8 is a schematic time chart diagram for explaining the operation of the brushless motor control device shown in FIG. 7. FIG.

[Explanation of symbols]

1 AC power supply 2 Double voltage rectifying / smoothing circuit 3 Inverter circuit 4 Brushless motor (sensorless DC brushless motor) 4a Rotor (of brushless motor 4) 6,11 Control circuit (microcomputer) 10 Position detection unit 10a Reference voltage generation circuit 10b Comparison Circuit D Position detection signal k Spike voltage R Armature winding terminal voltage Vdc Reference voltage

Claims (3)

[Claims] 1. When controlling the rotation of a brushless motor, the position of the rotor of the brushless motor is detected by a position detection signal obtained by comparing a voltage of an armature winding of the brushless motor with a reference voltage, A method of controlling a brushless motor that switches energization of armature winding current of the brushless motor based on position detection, wherein the time of the position detection interval is timed, and at least the following is based on the timed time. The time after the position detection is predicted, and when there is no position detection at the predicted time, the current is switched based on the position detection time interval before the previous time, while the voltage for controlling the brushless motor and A method of controlling a brushless motor, characterized in that the rotational speed is lowered by a predetermined value every predetermined time until the position is detected. 2. When controlling the rotation of the brushless motor, the position of the rotor of the brushless motor is detected by a position detection signal obtained by comparing the voltage of the armature winding of the brushless motor with a reference voltage, A method of controlling a brushless motor that switches energization of armature winding current of the brushless motor based on position detection, wherein the time of the position detection interval is timed, and at least the following is based on the timed time. When the time after the position detection is predicted, and when there is no position detection at the predicted time, the current switching is performed based on the position detection time interval before the previous time, while the position detection is not performed a predetermined number of times or more. Until the voltage equal position is detected, the voltage and the rotational speed for controlling the rotation of the brushless motor are reduced by a predetermined value at predetermined time intervals,
A method for controlling a brushless motor, wherein the number of rotations of the brushless motor is obtained when the position is detected, and the number of rotations of the brushless motor does not exceed the obtained number of rotations. 3. When controlling the rotation of the brushless motor, the position of the rotor of the brushless motor is detected by a position detection signal obtained by comparing the voltage of the armature winding of the brushless motor with a reference voltage, A method of controlling a brushless motor that switches energization of armature winding current of the brushless motor based on position detection, wherein the time of the position detection interval is timed, and at least the following is based on the timed time. When the time after the position detection is predicted, and when there is no position detection at the predicted time, the current switching is performed based on the position detection time interval before the previous time, while the position detection is not performed a predetermined number of times or more. Until the voltage equal position is detected, the voltage and the rotational speed for controlling the rotation of the brushless motor are reduced by a predetermined value at predetermined time intervals,
When the position is detected, the number of rotations of the brushless motor is obtained, and the number of rotations of the brushless motor is prevented from exceeding the obtained number of rotations, and thereafter, the limitation of the number of rotations is released after a predetermined time has elapsed. A method for controlling a brushless motor characterized by the above.