JP3296636B2 - Driving method of brushless DC motor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for driving a brushless DC motor, and more particularly to a so-called sensorless brushless DC motor capable of detecting the position of a rotor without using magnetic detecting means such as a Hall element. It relates to a driving method.

[0002]

2. Description of the Related Art In general, drive control of a brushless DC motor needs to be performed by closely relating a position of a magnetic pole of a rotor to a position of a winding to be passed. The output torque of the motor is generated by the interaction between the magnetic flux of the magnetic poles of the rotor and the current flowing through the windings of the stator. For this reason, the brushless DC motor should be driven so that the maximum torque is generated by applying current to the windings of the phases that exist near the point where the magnetic flux generated from the magnetic poles of the rotor is at a maximum, thereby rotating the motor. is necessary.

[0003] Further, the drive control of the brushless DC motor is performed by switching the phase through which the current flows at every moment according to the rotation of the magnetic pole position of the rotor.
If the timing of the commutation, which is the phase switching, is significantly shifted from the magnetic pole maximum position, the generated torque decreases, and in the worst case, the motor loses synchronism and stops.

Accordingly, the drive control of the brushless motor is
It is necessary to detect the magnetic pole position of the rotor by some means, and to perform control based on this.

As a prior art relating to a rotor magnetic pole position detecting circuit of this type of sensorless brushless motor, for example, a technique described in Japanese Patent Publication No. 58-25038 is known.

In this prior art, an induced voltage of a winding of a brushless motor is delayed by using a band-pass filter, and a phase delay waveform of the filter is compared with a neutral point corresponding to three phases of a filter output voltage. A commutation timing of the motor is created, and the commutation is performed by controlling an inverter circuit that drives the motor based on the timing.

[0007]

In the above prior art, a phase delay waveform of an induced voltage of a brushless motor is created by using a filter. However, according to this conventional technique, when rotational pulsation occurs in the rotor due to load fluctuation or the like, the phase delay of the filter becomes inaccurate, and when the motor return current flows under the condition that the load of the motor is large, There is a problem that the time when the terminal voltage of the stator becomes equal to the DC voltage of the inverter increases, the waveform of the filter output voltage after filtering the induced voltage of the motor is distorted, and accurate commutation timing cannot be obtained. are doing.

Further, according to the prior art, when the load torque repeatedly fluctuates during one rotation of the motor, if the control is performed so as to keep the output torque of the motor constant, a speed fluctuation occurs in the rotation of the motor. There is a problem that the driving of the motor becomes unstable due to vibration, and in the worst case, the motor stops.

In order to solve the above-mentioned problems, the current flowing through each phase is controlled by a so-called Pulse Width Modulation (hereinafter, referred to as PWM) control in which an inverter circuit is used to change the pulse width to control the output AC voltage variably. A drive method that changes the duty ratio for each period to match the motor torque and the load torque is considered. In this case, too, the determination of the commutation timing is performed using a conventional technique using a filter. If this is done, the difference in the duty ratio during each conduction period will be reflected in the output voltage of the filter, and the phase delay will be inaccurate, thereby making the drive of the motor unstable as in the previous case, and in the worst case This causes a problem that the motor stops.

In other words, the prior art for determining the commutation timing using a filter cannot detect the correct rotor position and cannot match the commutation timing with the magnetic pole position and speed. As a result, there is a problem that the torque generated by the motor is reduced and the motor may lose synchronism.

Therefore, in the above-mentioned prior art, the commutation timing caused by the inaccuracy of the phase delay of the filter output is somehow introduced, for example, by introducing another parameter such as a DC current value in addition to the induced voltage of the stator winding. It is necessary to correct the error and determine the commutation timing so that the current flows through the winding of the phase that exists near the maximum of the magnetic flux generated by the magnetic field of the rotor. This causes a problem of complexity.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems of the prior art, and to carry out control for changing the duty ratio for each conduction period so that the load on the motor fluctuates and the motor torque fluctuates accordingly. Even when the motor is running, the rotor magnetic pole position is estimated correctly, commutation is performed at an accurate timing without stopping the motor, speed fluctuation during one rotation of the motor is reduced, and vibration and vibration caused by the speed fluctuation are reduced. It is an object of the present invention to provide a brushless DC motor driving method capable of suppressing noise.

[0013]

According to the present invention, there is provided an A / D converter for directly sampling and detecting a motor terminal voltage of a non-energized phase of a motor without using a filter. The rate of change of the detected voltage with respect to time is obtained, and extrapolated interpolation is used to obtain the induced voltage information. Based on the induced voltage information, the position of the rotor is estimated, and from the result, the advance, delay, etc. of the commutation phase are calculated. And a controller that drives the brushless DC motor so as to perform commutation at an appropriate time by estimating the commutation timing of the motor current including the above.

[0014]

The A / D converter samples and detects the motor terminal voltage of the non-energized phase of the motor.
Based on the rate of change of the voltage value discretely taken by the D converter with respect to time, extrapolated interpolation is performed on the detected voltage value to calculate induced voltage information, and the commutation timing is calculated based on the information. Then, the driver of the inverter that drives the motor is driven, and finally the motor is rotated. Further, the control unit has a memory function of storing the acquired detected voltage value and time, and a function of calculating commutation timing and the like from the information and the rate of change of the voltage value with respect to time.

As described above, according to the present invention, high-quality drive control of a DC brushless motor using an inverter circuit can be performed.

[0016]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing an embodiment of a brushless DC motor driving method according to the present invention.

FIG. 1 is a block diagram showing the system configuration of a brushless DC motor to which the driving method according to the present invention is applied. FIG. FIG. 3 schematically shows the waveforms of the induced voltage of the motor, the detection terminal voltage, and the processing voltage in the state where the motor rotates, FIG. 3 is an enlarged view of the detection terminal voltage waveform, and FIG. FIG. 9 is a diagram showing a processing voltage waveform for explaining the operation of the first embodiment. In FIG. 1, 1 is a DC power supply, 2 is an inverter circuit, 3 is a selector, 4 is A /
A D converter, 5 is a control unit, 6 is a driver circuit unit, 7 is a stator, 8 is a rotor, and 9 is a load.

As shown in FIG. 1, a brushless DC motor system to which the present invention is applied has a DC power supply 1, an inverter circuit 2, a selector 3 for selecting one of the non-energized phases among three-phase windings, A to convert the analog value of the terminal voltage of the selected terminal into a digital value so that it can be calculated by a computer
A / D converter 4, a control unit 5 that determines a rate of change and a commutation time from the detected voltage, and outputs a signal to a driver that controls the inverter circuit 2, a driver circuit unit 6, U, V of the inverter circuit 2. , A three-phase DC brushless motor having one end connected to a W-phase winding, a rotor 8 having magnetic poles using permanent magnets, and a load 9 coupled to the motor.

In the following description, a motor driving method according to an embodiment of the present invention is a 120-degree conduction type driving capable of obtaining a high output torque. Is applied to the motor via the inverter circuit 2 to control the duty ratio of the DC voltage, and the switching element for performing chopping is connected to the positive voltage side of the DC power supply 1. .

A switching element for performing chopping may be connected to the negative voltage side of the DC power supply 1, and the inverter circuit 2 is not shown in the figure but corresponds to each phase winding. As a result, three switching elements are provided.

Generally, in a DC motor driven full-wave by the inverter circuit 2, effective induced voltage information from the windings is always induced from one of the three-phase windings that is not energized. Only the voltage, and the induced voltage information during the period in which chopping by the inverter circuit is not performed is effective. For this reason, the selector 3 has a switching function of switching terminals of phases detected according to the conduction mode,
A / D converter 4 is a so-called A / D converter that converts a voltage value that is an analog value into a digital value in synchronization with the PWM control signal.
It has a conversion function.

The control unit 5 has a function of calculating the rate of change of the voltage with respect to time, that is, the slope of the voltage, based on the detected value of the induced voltage, and the time required for the slope to reach the target voltage value. , A function of outputting a signal for controlling the driver circuit 6 for controlling the switching element of the inverter circuit 2, and a function of storing the above-described detected value and inclination.

The driver circuit section 6 has a function of driving a switching element constituting the inverter circuit 2 in accordance with a signal from the control section 5.

In the above-described embodiment of the present invention, the control section 5 and the driver circuit section 6 of the switching element of the inverter circuit 2 are constituted by separate circuits, but these functions are combined into one. A microcomputer may be used as the control unit.

The stator 7 constituting the motor has three-phase windings U, V and W which are magnetized by passing a current, and one ends of these windings are connected to each other. ing. Further, the rotor 8 is a permanent magnet having a magnetic pole, and rotates according to a proper change in the magnetic pole position of the stator described above.

Next, in the brushless DC motor configured as described above, the flow of each winding of the stator 7 is controlled at an ideal commutation timing, and the operation is performed on the assumption that the motor is rotating at a constant speed. The state will be described with reference to FIG.

FIG. 2A shows an induced voltage waveform of each phase generated as a physical phenomenon in this state, and FIG. 2B shows a terminal voltage waveform of each phase winding detected in full-wave driving. 2
(C) is a signal waveform that can be obtained by detecting the terminal voltage waveform in synchronization with the PWM signal and extrapolating the undetectable point from the amount of change in the detected value with respect to time in the present invention.

As shown in FIG. 2, when the motor is commutated at an ideal commutation timing and is rotating at a constant speed, the motor is rotated as shown in FIG. A reverse voltage is induced. This induced voltage is induced by the magnetic pole of the rotor.
The to Peak value is proportional to the number of revolutions of the motor, and the transient value is determined in correlation with the magnetic pole position of the rotor. Usually, the commutation timing of the motor can be determined based on this induced voltage information.

That is, the ideal commutation timing of the motor (the output can be extracted to the maximum) is determined by points tp1 and tp shown in FIG.
In one embodiment of the present invention, the voltage at this time is defined as a maximum value e (tp1) of a voltage that is an upwardly convex inflection point, and a minimum value e of a voltage that is a downwardly convex inflection point. (tp2).
When the motor is rotating at a constant speed, the above-described maximum value and minimum value of the voltage at that time are substantially equal in any adjacent phase.

On the other hand, what can be detected as an actual system is the terminal voltage of the stator winding of the motor, and has a waveform as shown in FIG. This detection waveform indicates the voltage between the potential of the midpoint where the phase windings are combined and the terminal of each phase winding, which is obtained by the calculation, and the drive voltage applied to rotate the motor and This is a mixture of induced voltage.
In the 120-degree conduction type driving of the motor, the effective induced voltage information is the period Ts shown in FIG. 2B, and only one of the three phases is always present.

Further, even during the period Ts of the effective induced voltage information, in a section in which a current flows through the free-wheeling diode during commutation, the potential is fixed to a positive or negative value of the DC voltage. When the circuit performs PWM control, chopping is performed, and therefore, the circuit does not become continuous as shown in FIG. Therefore, it is necessary to detect the induced voltage information by some method and determine the commutation timing by performing an arithmetic process based on the information.

Therefore, according to one embodiment of the present invention, the effective induced voltage information is stored in the 120-degree conduction type drive during the period Ts shown in FIG. Selector 3
, One of the three-phase windings of the motor, which is not energized, is switched and selected according to the conduction mode, and as shown in the enlarged view of the detected waveform shown in FIG. Terminal currents e (t1) and e (t2) when no current is flowing through the freewheeling diode at chopping off
And the like, so that the induced voltage information is discretely picked up directly from the terminal voltage.

Further, in one embodiment of the present invention, the control unit 5
Then, the rate of change Δv / Δt of the voltage with respect to time is calculated from the induced voltage information obtained discretely, and the induced voltage between discrete values (shown by broken lines) is obtained by extrapolation, which is calculated for three phases. By continuous connection, a processed waveform having a triangular wave signal waveform as shown in FIG. 2C is obtained.

As described with reference to FIG. 2, the flow to the windings of each phase is controlled at an ideal commutation timing, and when the motor is rotating at a constant speed, as shown in FIG. The voltage at the point where the induced voltages of adjacent phases intersect (the maximum value of the voltage that is an upwardly convex inflection point and the minimum value that is the downwardly convex inflection point) is Is also substantially constant, and emax and emin of the processed waveform can be obtained from the voltage value at each inflection point of this signal at this time. From this, in order to obtain the optimal commutation timing, the induced voltage in each phase is detected, and the induced voltage obtained from the rate of change with respect to time at that time is emax and
You just need to find the time that is emin.

When the phase of the commutation timing is delayed or advanced, as shown in FIGS. 4B and 4C, the voltage at the pre-rotational flow point based on the current rate of change of the induced voltage is obtained. Values e (t'41) and e (t'42) and the voltage value during pre-rotational flow
The difference between e (t41) and e (t42) is taken, and the optimal value of the voltage value e (tpb) at the previous commutation time t41 and t42 is estimated and compared based on the current or previous induced voltage change rate information. By estimating the delay and advance time Δtbd of the previous commutation time,
The correction is performed at the next commutation point without accumulating the commutation time error.

In one embodiment of the present invention, by repeating the above-described correction, the commutation timing can be finally brought closer to the optimal commutation time. In addition, since the induced voltage information depends on the number of rotations and the magnetic pole position of the rotor, the embodiment of the present invention is not affected by the rotation pulsation even if a rotation pulsation occurs in the motor due to the above-described correction. Since the induced voltage also changes, an appropriate commutation timing can always be estimated.

Hereinafter, the processing for obtaining the commutation time will be described in more detail.

Now, it is assumed that the load 9 on the motor is a constant load having no periodic load fluctuation, and the motor is controlled to rotate at a constant speed. In this case, as the induced voltage,
A voltage waveform as shown in FIG. 3 is detected. Sampling for obtaining the induced voltage information from this voltage waveform is synchronized with the ON period of the PWM output of the inverter circuit 2, and data valid only during the period when no current flows through the free-wheeling diode during commutation is validated. Done.

The invalid period data is calculated by extrapolation from the detected voltage value and time using the change rate Δv / Δt. As described above, by obtaining the voltage value in the invalid period by extrapolation, it is possible to continuously estimate the rotor position for each conduction period from the time or voltage based on the result.

The commutation time is estimated from the change rate Δv / Δt of the induced voltage obtained with reference to the current time, to the maximum value, which is the inflection point convex upward during the previous one rotation, or to the lower value. The time until the minimum value of the voltage, which is a point of inflection convex to, is calculated as the reference time tbm, and the commutation phase is advanced and the delay time tbd is estimated based on the information of the pre-rotational flow point. The reference time tbm of the previous commutation time is advanced and the delay is
This is performed by a method of correcting with.

In practice, the voltage value in the detectable area is treated as being expressed by an approximate expression of a linear function with respect to time, and the rate of change K with respect to time K = Δv / Δt is obtained as shown in FIG. From the values e (t1) and e (t2), equation (1)
Ask by

K = Δv / Δt = {e (t2) −e (t1)} / (t2−t1) (1) Next, the current time tn is used as a reference to determine the value during the last rotation. Based on the rate of change K and the voltage value e (tn) at the current time, the next rotation flow reference time tbm, which is the time required to reach the maximum value of the voltage at the inflection point that is upwardly convex, is calculated by the equation (2). ). At this time, it is desirable that e (tn) be a voltage value at a position of 1/5 to 4/5 where the linear approximation is most effective in the time width from emax to emin.

Equation (2) describes the case of the maximum value, but the same applies to the case of the minimum value. Then, as shown in FIG. 4A, the phase proceeds to the phase of the commutation timing, and if there is no delay, the voltage values of the adjacent phases match at the commutation time, and the rotor position is continuously determined based on the change rate K described above. And the commutation timing can be determined.

Tbm = {emax−e (tn)} / K (2) Also, as shown in FIGS. 4B and 4C, the commutation phase generated during the previous commutation If there is a delay or advance, the correction time tbd is calculated based on the current change rate K and the change rate Kb from which the pre-rotation flow time was obtained, estimating the voltage value at the previous rotation flow point, and calculating the time when the voltage values match. And That is, first, the voltage e (t′41) at the previous commutation time during which the current is flowing through the free-wheeling diode is estimated by Expression (3).

E (t′41) = e (tn) − (tn−t41) · K (3) Furthermore, it is determined in advance from the change rate Kb used for the previous commutation time calculation. The difference from the estimated voltage e (t41) at the time of the pre-rotational flow is taken, and the half of the point is assumed to be the voltage value when the commutation is optimally performed. The time obtained from the obtained voltage e (t'41) is equal to the previous phase time t.
The next rotational flow correction time tbd (n) is calculated by adding bd (n-1) as shown in Expression (4), and the result is set as the phase delay time at the previous rotational flow point. The voltage value assumed at this time is reflected at the next rotation flow as the maximum voltage value at the ideal commutation timing.

Tbd (n) = [{e (t′41) −e (tpb)} / K] + tbd (n−1) (4) Finally, until the next rotational flow time The time tb is a time obtained by correcting the next rotation flow reference time tbm with the phase delay time tbd according to the equation (5).

Tb = tbm + tbd (n) (5) The above explanation is for the case of the commutation phase delay shown in FIG. 4B, but the phase advance shown in FIG. The same applies to the case where the rate of change is negative.

In the above description, the commutation time is obtained from the detected rate of change of the induced voltage of the motor, and the commutation is performed by performing time management at that time. , Voltage and time information, the detected voltage or extrapolated voltage is compared with the voltage value estimated to be the commutation timing, and when it matches or exceeds that voltage It is also effective to control the driving of the motor by controlling the voltage so that commutation occurs.

That is, the rate of change of the voltage with respect to time Δv
Since the use of / Δt has information on both the voltage and the time, it is possible to manage the rotor position and the commutation timing using either the voltage or the time.

Next, the characteristics when the motor is driven according to the embodiment of the present invention will be described with reference to the drawings.

FIG. 5 is a graph showing the estimated rotational output of a motor provided with a position detection sensor so that the rotational position of the rotor can be determined at a constant load of 2000 rpm by a method according to an embodiment of the present invention. It is the figure which converted and converted the time difference with the flowing time into an angle difference. Three-winding DC motor
Since there are 12 commutation points in rotation, the measurement points shown in FIG. 5 are 12 points in one rotation, and the measurement results for several rotations are shown in the figure.

According to FIG. 5, the time difference between the sensor output and the commutation time according to one embodiment of the present invention is 1 in angle.
Degrees, and the position detection accuracy of the commonly used Hall element is about ± 6 degrees. Therefore, the driving method according to the embodiment of the present invention is capable of detecting a good motor position. It can be seen that this is a good driving method that can be performed.

FIG. 6 is a diagram showing a case where driving is performed under the same conditions as in FIG. 5 described above so that the load is periodically changed, and FIG. 7 shows a change in the speed of the motor in this case. FIG.

As can be seen from FIG. 6 showing the result, in the embodiment of the present invention, even when the load of the motor fluctuates greatly, as in the case of FIG. Detection can be performed, and the speed fluctuation can be reduced as shown in FIG.

FIG. 8 shows a so-called learning control in which a load having the same fluctuation as in FIG. 6 described above is connected to the motor, and a deviation of the commutation phase is determined for each rotation, and the result is reflected after one rotation. By calculating the acceleration by differentiating the conduction time twice during the conduction period, the conduction ratio is reduced during the acceleration period, and the conduction ratio is increased during the deceleration period.
Diagram showing the result when control is performed such that the duty ratio is changed for each flow period and the motor torque is output so as to match the load torque by reflecting the acceleration as a parameter in the duty ratio. FIG. 9 is a diagram showing the speed fluctuation of the motor in this case.

As can be seen from the results shown in FIGS. 8 and 9, when the driving method according to the embodiment of the present invention is used, the position of the rotor can be accurately determined even when the conduction ratio is changed for each conduction period. Therefore, even when control is performed such that the motor torque approaches the load torque using the learning control, the speed fluctuation during one rotation of the motor can be reduced, and the speed of the motor can be reduced. Vibration and noise due to fluctuation can be suppressed.

In this case, since the rate of change of the induced voltage of the motor with respect to time is speed information, it is possible to use a method of controlling the duty ratio based on the average rate of change for each conduction period. It is.

The present invention is not limited to the method of the above-described embodiment, but since the amount of change of the induced voltage and the maximum value of the intersection point are proportional to the speed, the maximum value and the minimum value of the induced voltage are respectively adjacent to each other. By controlling the same phase to be the same, it is possible to control the motor to rotate at a constant speed.

FIG. 10 is a diagram for explaining a motor driving method in this case.

The method shown in FIG. 10 calculates an average value (emax and emin) of the maximum voltage value and the minimum voltage value of the induced voltage in one rotation of the motor, and calculates an ideal value assumed from the change rate of the induced voltage. The difference (Δv1 and Δv2) between the voltage at the time of commutation and the calculated average value is used as a parameter of the flow rate. The ideal voltage at the time of commutation assumed from the rate of change of the induced voltage is the average. If it is lower than the value, the conduction ratio is increased, and if it is higher than the average value, the conduction ratio is decreased, etc., so that the maximum and minimum values of the induced voltage are the same in each adjacent phase in each conduction period. Is controlled as follows. Also in this case, the same learning control as described above can be performed.

In the above-described embodiment of the present invention, the stator winding of the motor is described as having three phases. However, the present invention can be applied to a case where the stator has more phases. In this case, the inverter circuit for driving is also configured in multiple phases according to the number of phases of the motor.

[0062]

As described above, according to the present invention, it is possible to accurately estimate the position of the rotor even when the conduction ratio differs for each conduction period. Thus, it is possible to control the motor torque to be equal to the load torque by changing the flow rate for each flow period, thereby suppressing motor speed fluctuations and accompanying vibration and noise.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a system configuration of a brushless DC motor to which a driving method according to the present invention is applied.

FIG. 2 schematically shows waveforms of an induced voltage, a detection terminal voltage, and a processing voltage of the motor in a state where the commutation is controlled at an ideal commutation timing with respect to the motor and the motor is rotating at a constant speed. FIG.

FIG. 3 is an enlarged view of a detection terminal voltage waveform.

FIG. 4 is a diagram showing processing voltage waveforms for explaining the advance and delay of the phase of commutation timing.

FIG. 5 shows a sensor output when a motor having a position detection sensor attached so that the rotational position of the rotor can be known is driven at a constant load at 2000 rpm by a method according to an embodiment of the present invention, and an estimated commutation time. FIG. 7 is a diagram showing a time difference between the two and converted into an angle difference.

FIG. 6 is a diagram showing a case where driving is performed under the same conditions as in FIG. 5 so that the load is periodically changed.

FIG. 7 is a diagram showing speed fluctuations of the motor in the case shown in FIG. 6;

FIG. 8 shows that the load having the same variation as in FIG. 6 is connected to the motor, and the duty ratio is changed for each of the duty periods by reflecting the acceleration as a parameter in the duty ratio so that the load torque matches the load torque. FIG. 7 is a diagram showing a result of a case where control is performed to output a motor torque so as to output a motor torque.

FIG. 9 is a diagram showing the speed fluctuation of the motor in the case shown in FIG. 8;

FIG. 10 is a diagram illustrating another example of a method of driving a motor according to an embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 DC power supply 2 Inverter circuit 3 Selector 4 A / D converter 5 Control part 6 Driver circuit part 7 Stator 8 Rotor 9 Load

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H02P 6/18

Claims (8)

(57) [Claims] 1. A method for driving a brushless DC motor, comprising a permanent magnet and an armature coil, wherein one end of each phase of the armature coil is connected in common. A method for driving a brushless DC motor, comprising detecting a terminal voltage and determining a commutation timing based on a rate of change of the detected terminal voltage with respect to time. 2. The brushless DC motor is driven by using an inverter circuit.
A driving method for the brushless DC motor according to the above. 3. The detection of the terminal voltage is performed in synchronization with a PWM signal for controlling an inverter circuit, and extrapolation is performed based on a change rate of the detection voltage with respect to time to continuously detect the rotor position when detection is impossible. The method for driving a brushless DC motor according to claim 2, wherein the estimation is performed dynamically. 4. A method for driving a brushless DC motor according to claim 3, wherein a commutation timing is determined by using a point at which the interpolated value intersects with the detected value of the terminal voltage of each adjacent phase as a commutation time. 5. The motor output torque is controlled such that the maximum value and the minimum value of the voltage value at the intersection of the voltage interpolation values of adjacent phases calculated based on the rate of change of the terminal voltage with respect to time become constant. 5. The method for driving a brushless DC motor according to claim 3, wherein: 6. The brushless DC motor driving method according to claim 3, wherein the motor drive control is performed by treating the rate of change of the interpolation value with respect to time as continuous motor speed change information. 7. The brushless DC motor driving method according to claim 4, wherein the voltage value at the commutation timing estimated based on the interpolation value is reflected at the next rotation. 8. The brushless DC motor driving method according to claim 3, wherein the extrapolation is performed based on a rate of change of a portion of the detected voltage having good linearity.