JP2001186788A - Drive circuit of brushless motor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the drive circuit of a brushless motor capable of efficiently ensuring sufficient servo pull-out torque according to the status of operation. SOLUTION: The drive circuit 1 of a brushless motor 3 energizes each phase of three-phase windings 3u, 3v, and 3w annularly connected with one another and drives the motor by magnetic fields due to the energization. The drive circuit is provided with power supply means 9, 10, and 11 for supplying each phase of the three-phase windings 3u, 3v, and 3w with power and a controlling means 2 that switches a plurality of pulse signals, different in pulse width, comprising the power and thereby controls the power supply means 9, 10, and 11 so that any windings in two phase of the three-phase windings 3u, 3v, and 3w are supplied with the power.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in a drive circuit of a brushless motor which energizes respective phases of three-phase windings connected in a ring shape and is driven by a magnetic field generated by the energization.

[0002]

2. Description of the Related Art In recent years, with the spread of computers and information terminals, disk devices for recording and / or reproducing data on, for example, disk-shaped information recording media (hereinafter simply referred to as "disks") have been developed one after another. . This disk device uses a brushless motor for rotating the disk when exchanging data with a disk-shaped information recording medium. The brushless motor may also include a VTR cylinder, a cassette deck capstan, a flexible disk drive, or a CD (Comp).
act Disc (trade name) is also used in players and the like. A brushless motor is a motor provided with an electronic commutation mechanism by removing a brush and a commutator from a DC motor. Therefore, the brushless motor is characterized in that only mechanical noise is generated, no electrical noise is generated, and no noise is generated in principle. Another characteristic of the brushless motor is that a motor having an extremely low speed or an ultra high speed, a multi-pole, and a long life can be easily manufactured.

[0003] The general structure of a brushless motor has, for example, a magnet rotor rotatable at the center and a drive coil surrounding the periphery thereof, and the rotation of these is controlled by a drive circuit. The drive circuit has a role as an electronic rectifier circuit instead of the commutator in the brushless motor. The drive circuit detects the position of the magnet rotor using a magnetic sensor such as a Hall element, and generates a magnetic field based on this signal.

The brushless motor has an operation mode in which the disk is rotated at a high speed and a low load by controlling a drive circuit under a limited power supply voltage and current capacity, and a disk is rotated at a low speed and a high load. There is an operation mode to rotate.

[0005]

However, the brushless motor satisfies the required servo escape torque in the other operation mode when trying to secure a sufficient servo escape torque at the time of high speed low load or low speed high load described above. There was a disadvantage that it could not be done.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned problems and to provide a brushless motor drive circuit which can ensure a sufficient servo escape torque efficiently according to an operation state.

[0007]

SUMMARY OF THE INVENTION According to the first aspect of the present invention, there is provided a brushless motor which energizes respective phases of three-phase windings connected in a ring shape and is driven by a magnetic field generated by the energization. A driving circuit for supplying power to each phase of the three-phase windings, and switching a plurality of pulse signals of different pulse widths constituting the power, the three-phase winding Control means for controlling the power supply means so as to supply the two-phase windings to the two-phase windings.

According to the first aspect, the control means controls the power supply means for supplying power to each of the three-phase windings. The control means controls the power supply means so that pulse signals having a plurality of different pulse widths are switched to any two-phase windings of the three-phase windings and supplied to the brushless motor. The drive circuit switches a plurality of pulse signals having different pulse widths and supplies the pulse signals to any two-phase winding of the three-phase windings. That is, the drive circuit is driven by the two-phase windings, for example, by supplying a pulse signal having a long pulse width to any two-phase windings of the three-phase windings, so that the torque can be improved. . Further, the drive circuit can increase the rotation speed by supplying a pulse signal having a short pulse width to any two-phase winding of the three-phase windings. Therefore, by switching and driving the drive circuit as needed, it is possible to drive the brushless motor so as to secure a sufficient output while efficiently consuming power.

According to a second aspect of the present invention, in the first aspect, the plurality of pulse signals are a combination of a pulse signal having an electrical angle of approximately 120 ° and a pulse signal having an electrical angle of approximately 180 °. Features.

According to a third aspect of the present invention, in the first aspect, the plurality of pulse signals include a pulse signal having an electrical angle of approximately 120 ° and a pulse signal having an electrical angle of approximately greater than 120 ° and less than approximately 180 °. Characterized by the combination of

[0011] The above object is attained by the invention of claim 4;
A brushless motor drive circuit that energizes each phase of a three-phase winding connected to each other in a ring shape and is driven by a magnetic field generated by the energization, and supplies power to each phase of the three-phase winding. A first mode in which a first pulse signal constituting the electric power and having an electric angle of approximately 120 ° is supplied to any two of the three-phase windings; And control means for controlling the power supply means so as to switch between a second mode in which a second pulse signal of approximately 180 ° is supplied to one of the three-phase windings. This is achieved by a characteristic brushless motor drive circuit.

According to the fourth aspect of the present invention, the control means is arranged so that any one of the three-phase windings has an electric angle of about 12 or less.
Control is performed so that the power supply means supplies a pulse signal having a first pulse width of 0 °. Since the rotor of the brushless motor is driven by the two-phase windings, the drive circuit can improve the torque of the brushless motor. Further, the control means may cause the power supply means to supply a pulse signal of a second pulse width having an electrical angle of approximately 180 ° to one of the three-phase windings as necessary. Control. The drive circuit can increase the rotation speed by supplying a pulse signal having a short pulse width to any one of the three-phase windings. Therefore, by switching and driving the drive circuit according to the characteristics of the brushless motor, it is possible to drive the brushless motor so as to secure a sufficient output while efficiently consuming power.

[0013]

Preferred embodiments of the present invention will be described below in detail with reference to the accompanying drawings. Note that the embodiments described below are preferred specific examples of the present invention,
Although various technically preferable limits are given, the scope of the present invention is not limited to these modes unless otherwise specified in the following description. "Bidirectional" refers to energizing one of the two-phase windings of the brushless motor, and "one-way" refers to any of the three-phase windings of the brushless motor. Or means to energize a single-phase winding. In the following description,
“Brushless motor” is simply referred to as “motor”. Also,
In this description, the “servo escape torque” refers to a minimum load torque at which the target rotation speed cannot be maintained during speed servo.

FIG. 1 is a block diagram showing a configuration example of a brushless motor 3 and a drive circuit 1 as a first embodiment of the present invention. The motor 3 is, for example, a VTR (Video
It is used for a tape recorder (Tape Recorder) cylinder, a cassette deck capstan, a flexible disk drive, or a CD (Compact Disc: trademark) player. The motor is a motor provided with an electronic commutation mechanism by removing a brush and a commutator from a DC motor. Therefore, the motor 3 is characterized in that it generates only mechanical noise, does not generate electrical noise, and does not generate noise in principle. Motor 3
Is also characterized in that a motor having an ultra-low speed or an ultra-high speed, a multi-pole, and a long life can be easily manufactured.

The motor 3 energizes each phase of a three-phase winding (coil) connected to each other in a ring shape (hereinafter referred to as "Y connection"), and is driven by a magnetic field generated by the energization. That is, the motor 3 includes, for example, a stator in which three-phase coils are connected to each other in a ring shape, and a rotor including a permanent magnet provided to surround the stator. Needless to say, the configurations of the rotor and the stator may be reversed.

The drive circuit 1 has a control circuit 2 (control means) and a power supply section 4 (power supply means). Control circuit 2
Are a motor position detecting element 23, a first-stage amplifier 25, a gain switching section 31, a waveform synthesizing section 27, a current control signal generating section 2
1, a comparator 13 and a differential circuit 31. The motor position detecting element 23 detects the relative phase between the stator and the rotor of the motor 3 in order to switch the current flowing through the coils 3u, 3v, 3w of the motor 3, respectively. The first stage amplifier 25
The three-phase signal obtained from the motor position detecting element 23 is amplified. The waveform synthesizing unit 27 further combines the amplified three-phase signals to form the coil 3 of the motor 3.
u, 3v, and 3w are combined to form a basic three-phase waveform that is energized. Hereinafter, the three-phase composite waveform is referred to as a “three-phase composite waveform”.

The comparator 13 calculates the difference between the torque command 14 (torque command value) and the value of the current flowing through the motor, and outputs the result to the current control signal generator 21. Further, the current control signal generator 21 outputs the output of the comparator 13 and the differential circuit 3
A predetermined operation is performed by an input from 1 to generate a current control signal 21 a and output it to the differential circuit 31. Differential circuit 3
1, the current control signal 21a is used as the total current, and the upper output transistor 33 of each phase is obtained from the three-phase composite waveform.
And a base current 34a for the lower output transistor 34. Although FIG. 1 shows only the U-phase output stage 9, the same applies to the V-phase output stage 10 and the W-phase output stage 11.

The power supply unit 4 is supplied with power from a driving power supply having a predetermined power. The power supply unit 4
The base currents 33a and 34a are supplied to the upper output transistor 33 and the lower output transistor 34 of the U-phase output stage 9, respectively, to drive the motor 3. The driving system of the motor according to the configuration of the driving circuit 1 described above is the same as a general motor driving system used so far.

What is characteristic in the present invention is that the drive circuit 3 switches a plurality of pulse signals having different pulse widths so that any one of the two-phase coils 3u, 3v, and 3w of the motor 3 is driven. Is to be supplied. As will be described later, the plurality of pulse signals are preferably a combination of a pulse signal having a pulse width of, for example, an electrical angle of approximately 120 ° and a pulse signal having an electrical angle of approximately 180 ° (hereinafter referred to as “three-phase pulse signals,” respectively). 120-degree current in both directions "and" 180-degree current in three-phase two directions "). As described later, the plurality of pulse signals are preferably a combination of a pulse signal having a pulse width of, for example, an electrical angle of approximately 120 ° and a pulse signal having an electrical angle of approximately greater than 120 ° and less than approximately 180 °. There may be.

The switching of the plurality of pulse signals is performed, for example, by changing the power supply width (pulse width) in the drive circuit 1 by changing the gain of the first-stage amplifier 25.
The gain switching unit 31 is connected to the first-stage amplifier 25, and a signal generated by inputting the conduction width switching signal 21 to the gain switching unit 31 is input. The power supply width switching signal 21 is generated, for example, when it is necessary to switch the power supply width in an electronic device on which the motor 3 is mounted. A specific driving method will be described below with reference to the waveform output from each block.

FIG. 2A shows an example of a signal waveform output from the first-stage amplifier 25 when the motor 3 is driven by the drive circuit 1 of FIG. 1, and FIG. 2B shows the signal waveform of FIG. Waveform synthesizer 2 when motor 3 is driven by circuit 1
7 shows an example of a signal waveform output from the signal generator 7 shown in FIG.
3 shows an example of a signal waveform output from the differential circuit 31 when the motor 3 is driven by the drive circuit 1 in FIG.

In FIG. 2A, "U-phase" indicates an output signal 25a of the first-stage amplifier 25, "V-phase" indicates an output signal 25b of the first-stage amplifier 25, and "W-phase"
"Phase" indicates the output signal 25c of the first-stage amplifier 25. In FIG. 2B, “U-phase” refers to the waveform synthesizing unit 27.
"V-phase" indicates an output signal 27b of the waveform synthesizing unit 27, and "W-phase" indicates an output signal 27c of the waveform synthesizing unit 27. FIG.
In (C), “U-phase” indicates a drive signal 33 a as an output of the differential circuit 31, “V-phase” indicates a drive signal 33 b as an output of the differential circuit 31,
The “W phase” is a drive signal 33 as an output of the differential circuit 31.
c is shown. Therefore, the motor 3 outputs the drive signal 33a.
Are supplied to the coils 3u, 3v, 3w of the respective phases, whereby the rotor is rotationally driven.

FIG. 2A shows the first stage amplifier 2 as described above.
5, which is a waveform close to a smooth sine wave by reducing the gain of the first-stage amplifier 25. FIG. 2 (B) combines these waveforms to form coils 3u, 3v,
It has a waveform synthesized so as to energize 3w, and has an energization width of approximately 180 ° in electrical angle. FIG. 2 (C)
Means that the drive circuit 2 is configured to determine that the windings 3u, 3v, 3
drive signal 33 whose electrical angle is approximately 180 ° with respect to w
a, 33b and 33c are supplied. Next, FIG. 2C shows the waveform of the voltage for actually driving the motor 3 as described above. Since the drive circuit 1 in FIG. 1 is, for example, a current drive, the upper output transistor is usually used. The output voltage waveform when the saturation of the upper output transistor 33 is increased is shown, with a difference between the saturation of the lower output transistor 33 and the saturation of the lower output transistor 34 shown.

FIG. 4A shows an example of a signal waveform output from the first-stage amplifier 25 when the motor 3 is driven by the driving circuit 1 in FIG. 1, and FIG. 4B shows the signal waveform in FIG. Waveform synthesizer 2 when motor 3 is driven by circuit 1
7 shows an example of a signal waveform output from the signal generator 7 shown in FIG.
3 shows an example of a signal waveform output from the differential circuit 31 when the motor 3 is driven by the drive circuit 1 in FIG. The “U phase”, “V phase”, and “W phase” in FIGS. 4A, 4B, and 4C respectively correspond to FIG.
(A), FIG. 3 (B) and FIG. 3 (C) have the same meanings as those in FIG.

FIG. 4A shows the first-stage amplifier 2 as described above.
5, which has a waveform close to the comparator output by increasing the gain of the first-stage amplifier 25.
FIG. 4B combines these waveforms to form coils 3u and 3u.
v, 3w is a synthesized waveform to energize,
The electric angle has a conduction width of about 120 °. FIG.
(C) shows the case where the drive circuit 2 is configured such that the windings 3u, 3
This shows that drive signals 33a, 33b, and 33c having an electrical angle of approximately 180 ° with respect to v and 3w are supplied. Next, FIG. 4 (C) shows the actual state of the motor 3 as described above.
The drive circuit 1 in FIG. 1 is, for example, a current drive. Therefore, as a usual method, a difference is set between the saturation levels of the upper output transistor 33 and the lower output transistor 34. The output voltage waveform when the saturation of the output transistor 33 is increased is shown.

The reason why the flat waveform of the intermediate potential in the waveform of FIG. 4B has a slope in the output voltage waveform of FIG. 4C is that the waveform of the intermediate potential in the waveform of FIG. This is because the point became high impedance, and as a result, the back electromotive voltage waveform of the motor became visible. In the waveform of FIG. 4B, the point at the intermediate potential becomes high impedance in each of the output stages 9, 10, and 11 in the differential circuit 31 of the drive circuit 1 in FIG. This is because a dead zone is provided so that neither the upper output transistor 33 nor the lower output transistor 34 reacts at the intermediate potential of the normal output waveform so that a through current does not flow through each of the upper transistor 33 and the lower transistor 34. .

By the above-described operation, the gain of the first-stage amplifier 25 is reduced at the time of a high-speed rotation load to make the current approximately 180 ° as shown in FIG. 2C, and the gain of the first-stage amplifier 25 is increased at the time of a low-speed rotation and a high load. Almost 12 like 3 (C)
By applying 0 ° current, the servo escape torque can be increased in each rotation range as shown in FIG. Also,
By adjusting the first-stage amplifier 25, it is possible to set an appropriate conduction width between approximately 120 ° and approximately 180 °. The necessity of this is that when the current is substantially 180 ° energized, a reverse torque may be generated near the switching of the energized phase due to a displacement of the mounting position of the motor position detecting element 23 or the like. This is because an excessively large instantaneous current may flow due to the increase, and as a countermeasure against this, it is effective to set an energization narrower than approximately 180 °.

FIG. 4 is a diagram showing an example of a torque-current characteristic and a torque-speed characteristic of the motor 3 driven by the drive circuit 1. In FIG. That is, in FIG. 4, both the torque-current characteristics of the motor 3 and the torque-speed characteristics of the motor 3 are shown in the same drawing so that they can be compared. The motor 3 has, for example, a high-speed mode and a low-speed mode, and the rotation speed RT is between a servo escape torque TH in the high-speed mode and a servo escape torque TL in the low-speed mode.
Can be changed between the rotation speed RTH in the high-speed mode and the rotation speed RTL in the low-speed mode. Here, it is assumed that the current I flowing through the motor 3 is limited to the limit current Ix. Therefore, the illustrated torque-current characteristic is a characteristic that does not exceed the limit current Ix.

As can be seen by referring to the torque-rotation speed characteristics when the motor 3 is energized at 120 ° in both three-phase directions, the torque T is equal to the servo escape torque TL in the low-speed mode even at the rotation speed RTL in the low-speed mode. It can be seen that the output is up to. In this case, if the motor 3 is set to the high-speed mode while the three-phase bidirectional 120 ° energization is performed, the torque T does not become the servo escape torque TH in the high-speed mode, but the motor 3 is switched to the three-phase bidirectional 180 ° energization at a predetermined timing. Even when the rotation speed RT is the rotation speed RTH in the high-speed mode, the torque T can output up to the servo escape torque TH in the high-speed mode.

Here, the timing of this switching is as follows.
For example, if the motor 3 is used in a disk device or the like, it may be when the type of disk is detected by the disk device. The drive circuit 1 further increases the servo escape torque in each of the operating rotation ranges as shown in FIG. 4 by switching the energization width (pulse width). That is, the servo escape torque TH in the high-speed mode, and the servo escape torque in the low-speed mode. TL can be obtained.

According to the first embodiment of the present invention, when only a limited power supply voltage is applied, at the time of high speed and low load, it becomes difficult for the current to flow due to the back electromotive force of the motor 3. By supplying as much current as possible, a sufficient servo escape torque can be secured. When the same motor 3 is to be used at a low speed and a high load, the drive constant is increased by setting the conduction width to approximately 120 ° even in the closed current capacity, and the current pulsation due to the phase of the motor 3 is reduced. Therefore, a sufficient servo escape torque can be secured without flowing an excessive current.

Second Embodiment FIG. 5 is a block diagram showing a configuration example of a brushless motor 3 and a drive circuit 1a according to a second embodiment of the present invention. In the drive circuit 1a of the brushless motor 3a according to the second embodiment, the portions denoted by the same reference numerals in FIG. 1 as those of the drive circuit 1 of the brushless motor 3 according to the first embodiment have the same configuration. explain. The motor 3a has substantially the same overall configuration as the motor 3 of the first embodiment. However, in the driving method, the three-phase bidirectional energization is performed simultaneously when the energization width is reduced, and the energization width is simultaneously increased when the energization width is increased. The difference is that three-phase one-way current is applied. Specifically, the driving circuit 1a
Sets the output of the output stage upper transistor 33 to the OFF state of the differential circuit 31 in the previous stage in a configuration substantially similar to that of the drive circuit 1.
By doing so, a high impedance state is established, and the drive power supply 5 and the midpoint of the Y-connected coils 3u, 3v, 3w of the motor 3 are connected by the emitter and collector of the changeover switch 9, respectively. The drive circuit 1a changes the energization method by turning on the changeover switch 29.

The setting of the energization width is the same as that described in the first embodiment. For example, in three-phase two-way energization, the electrical angle is approximately 120 ° (hereinafter referred to as “three-phase two-way 120 °”), three-phase one-way When conducting electricity, the electrical angle is approximately 180
゜ (hereinafter referred to as “three-phase one-way 180 °”). Regarding the setting of the energization width, the place where the electric angle is approximately 180 ° in the above-described three-phase one-way energization is set, for example, even if the electric angle is set to be larger than approximately 120 ° and smaller than approximately 180 ° in the three-phase one-way energization. Good.

FIG. 6 is a diagram showing an example of a torque-current characteristic and a torque-speed characteristic of the motor 3 driven by the drive circuit 1a. That is, in FIG. 6, both the torque-current characteristics of the motor 3 and the torque-speed characteristics of the motor 3 are illustrated in the same drawing so that they can be compared. The motor 3 has, for example, a high-speed mode and a low-speed mode, and the rotation speed R is between a servo escape torque TH in the high-speed mode and a servo escape torque TL in the low-speed mode.
It is preferable that T can change between the rotation speed RTH in the high-speed mode and the rotation speed RTL in the low-speed mode.
Here, it is assumed that the current I flowing through the motor 3 is limited to the limit current Ix. Therefore, the illustrated torque-current characteristic is a characteristic that does not exceed the limit current Ix.

As can be seen by referring to the torque-rotational speed characteristics when the motor 3 is energized at 120 ° in both three-phase directions, even if the rotation speed RTL is in the low-speed mode, the torque T is equal to the servo escape torque TL in the low-speed mode. It can be seen that the output is up to. In this case, if the motor 3 is set to the high-speed mode while the three-phase bidirectional 120 ° energization is performed, the torque T does not become the servo escape torque TH in the high-speed mode, but the motor 3 is switched to the three-phase bidirectional 180 ° energization at a predetermined timing. Even when the rotation speed RT is the rotation speed RTH in the high-speed mode, the torque T can output up to the servo escape torque TH in the high-speed mode.

Here, the timing of this switching is as follows.
For example, if the motor 3 is used in a disk device or the like, it may be when the type of disk is detected by the disk device.

In the drive circuit 1a, the method of switching the energization method between the three-phase two-way and the three-phase one-way has been used in various applications for a long time. As described above, the servo escape torque can be further increased in each of the used rotation ranges, that is, the servo escape torque TH can be obtained in the high-speed mode, and the servo escape torque TL can be obtained in the low-speed mode.

According to the second embodiment of the present invention, substantially the same effects as those of the first embodiment can be obtained. In addition, when the motor is started from the stopped state to the high-speed mode, the three-phase two-way Start at 120 ° energization, and at a rotational speed near the intersection of the torque vs. rotation speed characteristic at 120 ° energization in the three-phase two-way direction and the 180 ° torque / rotational speed characteristic during three-phase one-way energization, three-phase one-way 180 ° energization. By switching, the start-up time can be greatly shortened as compared with the case where the three-phase one-way 180 ° energization is started from the beginning.

The present invention is not limited to the above embodiment. Although the above-described brushless motor has been described for the case of current driving, it is needless to say that the same effect can be obtained by using voltage driving. In the above description, the motor 3 is driven by switching between two pulse signals. However, the present invention is not limited to this, and the motor 3 may be driven by switching between a plurality of pulse signals.

[0040]

As described above, according to the present invention,
It is possible to provide a brushless motor drive circuit that can efficiently secure a sufficient servo escape torque in accordance with an operation state.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration example of a brushless motor and a drive circuit as a first embodiment of the present invention.

FIG. 2 is a diagram illustrating an example of signal waveforms output from a first-stage amplifier, a waveform synthesis unit, and a differential circuit when a motor is driven by the drive circuit in FIG. 1;

FIG. 3 is a diagram showing an example of signal waveforms output by a first-stage amplifier, a waveform synthesis unit, and a differential circuit when a motor is driven by the drive circuit of FIG. 1;

FIG. 4 is a diagram illustrating an example of a torque-current characteristic and a torque-revolution speed characteristic of a motor driven by the drive circuit of FIG. 1;

FIG. 5 is a block diagram showing a configuration example of a brushless motor and a drive circuit according to a second embodiment of the present invention.

FIG. 6 is a diagram illustrating an example of torque-current characteristics and torque-speed characteristics of a motor driven by the drive circuit of FIG. 5;

[Explanation of symbols]

1 ... Drive circuit, 2 ... Control circuit (control means), 3
... Motors (brushless motors), 3u, 3v, 3w
... Coil (winding), 4 ... Power supply unit (power supply means)

Claims (4)

[Claims] 1. A brushless motor drive circuit for energizing each phase of a three-phase winding connected to each other in a ring shape and driving by a magnetic field generated by the energization. Power supply means for supplying electric power, switching a plurality of pulse signals having different pulse widths constituting the electric power, and supplying the pulse signal to any two-phase winding of the three-phase windings. A drive circuit for a brushless motor, comprising: control means for controlling power supply means. 2. The brushless motor drive circuit according to claim 1, wherein the plurality of pulse signals are a combination of a pulse signal having an electrical angle of approximately 120 ° and a pulse signal having an electrical angle of approximately 180 °. 3. The brushless motor according to claim 1, wherein the plurality of pulse signals are a combination of a pulse signal having an electrical angle of about 120 ° and a pulse signal having an electrical angle of more than about 120 ° and less than about 180 °. Drive circuit. 4. A brushless motor drive circuit for energizing each phase of three-phase windings connected to each other in a ring and driving by a magnetic field by energization, wherein each phase of the three-phase windings is Power supply means for supplying power, and a first pulse signal having an electric angle of about 120 °, which constitutes the power, is supplied to any two of the three-phase windings. Control means for controlling the power supply means to switch between a mode and a second mode in which a second pulse signal having an electrical angle of about 180 ° is supplied to one of the three-phase windings A driving circuit for a brushless motor, comprising: