GB2313499A

GB2313499A - Method of driving stepping motor

Info

Publication number: GB2313499A
Application number: GB9710327A
Authority: GB
Inventors: Masashi Ogasawara; Keiji Katano
Original assignee: Alps Electric Co Ltd
Current assignee: Alps Alpine Co Ltd
Priority date: 1996-05-22
Filing date: 1997-05-21
Publication date: 1997-11-26
Anticipated expiration: 2017-05-21
Also published as: IT1291382B1; CN1068155C; ITMI971196A0; KR100217789B1; KR980012831A; GB2313499B; GB9710327D0; CN1172377A; FR2749111B1; FR2749111A1; DE19721282A1; ITMI971196A1

Description

2313499 METHOD OF DRIVING STEPPING MOTOR The present invention relates to

a method of driving stepping motor, and more particularly to a method of driving stepping motor used as a driving source of a carriage driving mechanism of a carriage of a printer or a paper f eed mechanism.

A serial type printer in which predetermined recording is performed by repeating, while a carriage on which a print head is mounted is moved along a platen, driving the print head selectively so as to print one line portion on a recording blank form, carrying the blank form by one line portion after performing printIng of the one line portion, and recording a next line, is widely used as an output apparatus of a computer or a word processor.

Further, in such a serial type printer, a stepping motor is generally used in order to control driving of a driving mechanism of a carriage or a paper feed mechanism. The stepping motor is used because of the reasons described hereunder.

1. The rotary angle of a motor is in proportion to the number of input pulses, and no accumulative error is produced.

1 2. The rotational speed of the motor is in proportion to an innut mulse speed, and precise synchronous operation is possible and a control area is wide.

3. Starting and stopping characteristics are excellent, and the operation at a constant frequency is possible at a self-starting frequency or below.

4. Responsibility is high, and the output is also high.

5. It is possible to control the position only by generating an input pulse in accordance with a target position.

6. it- is possible to control digitally.

A sr-euping motor is provided with, as illustrating the structure thereof in principle in Fig. 9, a stator 6 having a first (A), a second (B), a third (C) and a fourth (D) magnetic poles (phases) 2, 3, 4 and 5 arranged at 90 degree intervals for instance, and a rotor 7 consisting of a rotatable permanent magnet having an N pole and an S pole at 180 degree interval, and an out-out shaft not shown is connected to this rotor 7. Further, a first coil 8 is wound around the f i rs t (A) and the third (C) magnetic poles 2 and 4, and a second coil 9 is wound around the second (B) and the fourth (D) magnetic poles 3 and When an exciting current is applied to the coils 8 and 9 of resnective phases of the staLor 6 in order to drive to rotate such a stepping motor 1, a magnetic field is generated

2 with this current, and an attracting or repulsing electromagnetic force is generated between the stator 6 and the rotor 7. The electromagnetic force between the stator 6 and the rotor 7 is switched by switching this exciting current successively, which generates a torque for rotating the rotor.

Fig. 10 is a block diagram showing a motor driver IC for driving a general stepping motor. As shown in Fig. 10, motor driver IC 10 is structured of a control circuit 11, drive circuit 12 and a power supply 13. The control circuit 11 has functions for controlling the whole such as variation of input voltage, a rotational speed and direction, a distance and an angle other than an input interface, and performs the control of timing of the pulse supplied to the stepping motor i Further, the drive circuit 12 is a circuit for distributing pulse signals from the control circuit 11 and amplifying them so as to excite respective phases of the stepping motor 1 in certain sequence. As the power supply 13, two types, one for driving a stepping motor and the other for an IC circuit are required.

Further, there are a unipolar drive system and a bipolar drive system as the drive system of the stepping motor 1.

The unipolar drive system is a method of applying a current to respective coils in one direction only by connecting one piece each of transistors 21, 22, 23 and 24 to respective 3 coils, as showing an example in Fig. 11, and turning respective transistors ON. As against the above, in the bipolar drive system a plurality of transistors 25, 26, 27 and 28 are connected to respective coils as shown in Fig. 12. When descriction is made with resDect to the phase A only, a current i n the direction A is applied at the time of operation by turning the first transistor 25 and the fourth transistor 28 ON, and a current in a reverse direction B by turning the second transistor 26 and the third transistor 27 ON. in the unipolar drive system, the circuit structure is simple as compared with tI-.e bicolar drive system since the number of transistors is 1/2. On the other hand, the bipolar drive system has such an advantage that larger motor torque is obtainable than the unimolar drive svstem when the input electric power is the same. 3esides, a method of d_riving the stepping motor 1 in the present invention described later is by bipolar driving.

Further, the application system of an exciting current _cludes 1-phase excitation, 1-2 phase excitation, 2-2 phase excitation and so on.

The above-mentioned method of driving a stepping motor 1 bv!-chase excitation is a most basic driving method of exciting respective phases chase by phase consecutively so as to rotate the stepping motor at a basic step angle. Although the angular precision is high, this method has such drawbacks 4 that the driving torque is small and the power efficiency is low. Therefore, this method is not used so often. Besides, noarticular, one s ter) angle when driven by 1-phase excitation is referred to as a basic step angle.

The above-mentioned method of driving a stepping motor 1 by 2-2 phase excitation is a method of always exciting mutually adjacent two phases at the same time and switching excitation of one phase at a time. Since twophases are always excited, utilization efficiency of power is high and high output is obtainable for the same motor power supply voltage, and moreover, the stepping motor works advantageously against vibration such as overshooting of the rotor. Therefore, this method is used widely as a method of driving the stepping motor 1.

Furthermore, the above-mentioned method of driving a stepping motor 1 by 1-2 phase excitation is a method of repeating 1-phase excitation and 22 phase excitation alternately. Because of such a reason tha t a stop position of the rotor by lphase excitation and a stop position by 2-2 phase excitation are shif ted f rom each other by 1/2 of the basic step angle, the output by a step angle 1/2 of the step angle of 1-phase excitation and 2-2 phase excitation drive is obtainable by repeating these two excitation states alternately. Thus, resolution is redoubled as compared with other driving method, thus making ir- possible to perform fine s tem feed. Further, the s temping motor can be driven wi th low noise and stable driving can be performed at a high speed. Therefore, this method is used when it is reauired to obtain accurate rotational quantity.

in such a method of driving a stepping motor 1, however, when input power is increased in order to secure the torque at the time of high-speed operation, excessive torque is generated in a low-speed area and vibration and noises are,i.aused.

In oider to solve such a trouble, a driving metnod called microstep drivingbya constant -current chopper system inwhich a stem angle determined mechanically from the structure of the szemping moter 1 is split more finely further by means of an electronic circuit and the rotor of the stepping motor 1 is driven to rotate smoothly is being carried out. Here, a case that microsteo driving has been performed by bipolar driving with 2-2 mhase excitation will be described.

States of charge of an exciting current at the time of stem driving and at the time of microstep driving are shown in Fig. 13. When the torcrue angle characteristics of the stemming motor 1 have a sinusoidal wave configuration, smooth rotation with small torque fluctuation becomes possible by applying a sinusoidal wave exciting current such as shown in 6 Fig. 13. This sinusoidal wave exciting current is formed by splitting one period into a plurality of periods by means of a control circuit. Fig. 13 shows a case that one period has been split into 40 portions, but, since it means that the basic step angle has been split into 10 portions, the resolution becomes 10 times as high as before. Besides, the number of split can be set optionally.

Now, in a conventional cons tant- current chopper system in microstep driving, a constant -current is obtained by using either of the methods described hereunder. Here, the constant -current chopper driver in use is structured, as showing the current waveform in Fig. 14, so as to maintain a constant-current by providing a current OFF state for a nredetermined ueriod when the supply current value reaches a set value, and bv bringing into an ON state again thereafter so that the supply current value shows a set value.

Further, according to a first method for obtaining the constant -current, in a drive circuit shown in Fig. 12, a first transistor 25 and a fourth transistor 28 are turned ON in an ON state of the power supply, and, when the supply current value reaches a ser- value, the f i_-st transistor 25 is turned OFF in a state that the fourth transistor 28 is held ON. Then, although the exciting current is decreased gradually, such an operation that the first transistor 25 is brought into an ON 7 state, the current is increased to a set value, and the first transistor 25 is turned OFF again is repeated when a predetermined period of time elapses. Further, according to a second method, in a drive circuit shown in Fig. 12, such an oDeration that the first transistor 25 and the fourth transistor 28 are turned ON, the first transistor 25 is turned OFF and the fourth transistor 28 is also turnedOFF concurrently when the supply current value reaches the set value, the current value is reduced drastically, the first transistor 25 and the f ourth transistor 2 8 are turned ON so as to increase the current to the set value, and the first transistor 25 and the fourth transistor 28 are turned OFF again when a predetermined period of time has elansed is repeated.

Besides, only the exciting current in the phase A has been explained in the previous description, but similar control is also made while staggering the exciting hours with respect to the other uhase coils.

According to the first method, there is such a nonconformity that, although the current ripple can be reduced as shown in Fig. 15, the exciting current is distorted, and generation of heat in the stepping motor is increased.

Further, in the second method, there is such a nonconformity that the current ripple is increased as shown n Fig. 16 thereby to increase the loss of the motor, and the 8 torque is reduced.

Furthermore, since it is required for microste.p driving at the time of high-speed rotation to split one step (pulse) finely further in giving a driving pulse at a high frequency, there is such a nonconformity that the drive circuit and control thereof become commlicated.

As a method of driving a stepping motor for overcoming -the problems in such a conventional method, a method of driving a stepping motor in which the motor is driven by a normal exciting system at the time of high- speed rotation, microstep driving is performed at the time of low-speed rotation, and the current at OFF time at the time of chopping operation for a constant -current supplied at the time of microstep driving J s reduced by combining high-speed attenuation with low-speed attenuation has been proposed.

According to such a driving method, it is possible to make the current ripple smaller thereby to check generation of heat and vibration at the time of low-speed rotation of the motor without complicating the control circuit.

in such a method of driving a stepping motor, however, the reference voltage corresponding to a first coil 8 wound around the phase A and the phase C and a second coil 9 wound around the phase B and the chase D of a motor driver IC 10 are sinusoidal waves as showing the waveforms in Fig. 17A and 9 Fig. 17B at the time of microstep driving, and the exciting currents supplied to respective coils 8 and 9 also show sinusoidal waves as shown in Fig. 18A and Fig. 18B based an the reference voltage. When the sum of the exciting currents in the first coil 8 and the second coil 9 is examined, it is found that current sum ripple has been produced as shown in Fig. 18C. When the enlarged view of Fig. 18C is shown in Fig. 19, it is seen that the current sum has a peak every time when the exciting current of the first coil 8 and the exciting current of the second coil 9 overlap each other. Such a current sum ripple becomes a torque ripple of the stepping motor, and has been one cause of generating vibration at the time of microstep driving.

It is an object of the present invention to provide a method of driving a stepping motor capable of controlling vibration at the time of microstep driving without complexing a control circuit.

It is another object of the present invention to provide a method of driving a stepping motor in which vibration in acceleration/ deceleration areas at t,L-.e time of low-sDeed rotation or at the time of high-speed rotation is controlled.

It- is another object of -the present invention to provide a method of driving a stepping ino'tor for controlling so as to perform microstep driving in the low-frequency acceleration/ decelerat ion area at the time of low-siDeed rotation and at the time of high-speed rotation of the motor during one stem of switching the phase by bipolar driving, thereby to smooth the rotation of the rotor core of a stem ping motor in the low-frequency acceleration/ deceleration area at the time of low- speed rotation and at the time of high-speed rotation so as to suppress vibration to the minimum.

it is another object of the present invention to provide a method of driving a stepping motor capable of preventing a current sum riumle from generating and suppressing vibration at the time of microstep driving without complexing a control circuit by forming an exciting current supplied to respective phases into a chopping wave current by microstep, driving.

It is another object of the present invention to provide a method of driving a stepping motor in which the vibration level of the motor has been improved by applying an exciting current composed of a chomping wave current obtained by applying bias to respective coils of the stepping motor.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1A is a diagram showing a speed change of a stepping 11 motor at the time of low-speed rotation according to the present Fig. 1B is a diagram showing a change of voltage applied to each phase of a motor at the time of low-speed rotation (the Dartion shown with a saw- tooth wave shows driving positions by microstep driving); Fig. 2A is a diagram showing a speed change of a stepping motor at the time of high-speed rotation according to the nresent invention; Fig. 2B is a diagram showing a change of voltage applied to each phase of a motor at the time of high-speed rotation (the portion shown with a saw- tooth wave shows driving positions by microstep driving); Fig. 3 is a waveform diagram of a coil current by an embodiment of a method of driving a stepping motor according to the uresent invention; Fig. 4A is a waveform diagram showing reference voltage corresponding to a first coil of a motor driver IC according to an embodiment of a method of driving a stepping motor of the present invention; Fig. 4B is a waveform diagram showing reference voltage corresponding to a second coil of a motor driver IC similarly to the above; Fig. SA is a waveform diagram showing an exciting current 12 of the first coil according to an embodiment of a method of driving a stepping motor of the present invention; Fig. 5B is a waveform diagram showing an exciting current of the second coil similarly to the above; Fig. 555C is an explanatory diagram showing a current sum of exciting currents of the first coil and the second coil; Fig. C' is an enlarged diagram of Fig. SC; Fig. 7A is a waveform diagram showing reference voltage applied with bias corresponding to the first coil of the motor driver IC according to another embodiment of a method of driving a stepping motor of the present invention; Fig. 7B is a waveform diagram showing reference voltage applied with bias corresponding to the second coil of the motor driver IC similarly to the above; Fig. 8A is a waveform diagram showing an exciting current applied with bias of the first coil according to another embodiment of a method of driving a stepping motor of the present invention; Fig. 8B is a waveform diagram showing an exciting current applied with bias of the second coil similarly to the above; Fig. SC is an explanatory diagram of a current sum of exciting currents applied with bias of the first coil and the second coil; Fig. 9 is a principle diagram for explaining a structure 13 of a stepping motor; Fig. 10 is a block diagram showing a driver of a stepping motor; Fig. 11 is an explanatory diagram showing a drive circuit of a stepping motor of a unipolar system; P ig. 12 is an explanatory diagram showing a drive circuit of a stepping motor o:E a bipolar system; Fig. 13 is an explanatory diagram for explaining the change of an exciting current at the time of full stelD driving and at the time of microstep driving; Pig. 14 is a waveform diagram of an exciting current by a constant- current chopper system; Fig. 15 is a waveform diagram showing an exciting current at the time of low-speed attenuation which is a conventional driving method; Fig. 16 is a waveform diagram showing an exciting current at the time of high-soeed attenuation which is a conventional driving method; Fig. 17A is a waveform diagram showing reference voltage corresponding to the f irst coil of the motor driver IC according to a conventional method of driving a stepping motor; Fig. 17B is a waveform diagram showing reference voltage corresponding to the second coil of the motor driver IC similarly to the above; 14 Fig. 18A is a waveform diagram showing an exciting current of the first coil according to a conventional method of driving a stepping motor; Fig. 18B is a waveform diagram showing an exciting current of the second coil similarly to the above; Fig. 18C is an explanatory diagram showing a current sum of exciting currents of the first coil and the second coil; and Fig. 19 is an enlarged diagram of Fig. 18C. 5.

Embodiments of a method of driving a stepping motor according to the present invention will be explained hereunder with reference to the drawings.

A method of driving a stepping motor according to the Dresent invention is premised on chopping driving by the above-mentioned bipolar drive circuit. In a f irst embodiment of the present invention, driving is performed by normal 1-2 phase excitation or 2-2 phase excitation at the time of high-speed rotation of the stepping motor, and microstep driving is performed in a low-frequency acceleration/deceleration area at the time of low-speed rotation and at the time of high-speed rotation of the stepping motor 1.

Further, in a second embodiment of the present invention, driving is perf ormed by normal 1-2 phase excitation or 2-2 phase excitation at the time of high-speed rotation and microstep driving is performed at the time of low-speed rotation, and a chopping wave current is used for the exciting current supplied to each phase.

Furthermore, in a third embodiment of the uresent invention, a stepping motor is driven by supplying an exciting current with bias applied thereto in the second embodiment.

Besides, the time of low-speed rotation mentioned here means the time of driving when a driving pulse width per one ster) is from approximately 650 microseconds to 10 milliseconds.

Fig. 1 and Fig. 2 are for explaining the speed of a motor and the voltage applied to each phase at that time in a method of driving a stepping motor in the first embodiment of the present invention.

Fig. 1A shows the speed of the stepping motor 1 at the time of low-speed rotation, and Fig- IB shows the voltage applied to each phase at that time. Further, Fig. 2A shows the speed of the stepping motor 1 at the time of high-speed rotation, and Fig. 2B shows the voltage applied to each phase at that time. Besides, in Fig.!B and Fig. 2B, the portion shown with a sawtooth shaped line shows the area where driving is performed by microstep driving.

16 As shown in Fig. 1, when the stepping motor 1 is rotated at a low-speed, it is controlled so as to drive it by microstep driving over the whole area from the acceleration area (time tO to tl) to the constant speed area (time tl to t2) and the deceleration area (time t2 to t3). on the other hand, as shown in Fig. 2, it is controlled so that, when the stepping motor 1 is rotated at a high-speed, driving is performed by microstep driving only in an area where the driving pulse width per one step is from three times of the self-starting frequency of the stepping motor 1 (approximately 650 microseconds) to 10 milliseconds among the area where the rotation is accelerated to a predetermined constant sr)eed and the deceleration area f rom the predetermined constant speed until it stops, that is, only in the time tO to tl and the time t4 to tS, and driving is performed by a normal exciting method in the other high-speed acceleration/deceleration areas and low-speed areas.

At this time, the current is supplied by a constantcurrent chopper system, and a first transistor 25 is turned OFF when a current value reaches a set value in a drive circuit shown in Fig. 12 at respective split time, but it is made so that the ON state and the OFF state of a fourth transistor 28 may be selected in the state that the first transistor 25 is turned OFF, and the fourth transistor 28 is also turned OFF together with the f irst transistor 25 when the supplied current value reaches a set value. Thereupon, the exciting current is decreased drastically (Ihigh-speed attenuation). Then, when the exciting current is decreased to a predetermined value (predetermined period), the f ourth transistor 28 is turned ON. Then, the reduction of the exciting current becomes slow (low-speed attenuation). Further, when the current value is decreased to a second set value (a predetermined period elaoses), the f irst transistor 25 is turned ON again, thereby to increase the current value. When the current value increases to the set value, the above- mentioned control is made for the first transistor 25 and the fourth transistor 28. The chopping operation is controlled at one split time by repeating such control in a plurality of times.

A current waveform obtained when control is made as described above is shown in Fig. 3. By repeating such control at respective split times, the exciting current shows a smooth waveform with distortion or ripple supressed, thus making it possible to control generation of heat of the stepping motor 1 and also to make the r)ower loss of the stepping motor 1 small. Thus, the torque is not reduced, and the rotation of a rotor 7 of the stepping motor 1 also becomes smooth without having vibration. Besides, ON and OFF operations of this transistor is controlled by means of a CPU of a control circuit 14.

18 Next, a second embodiment of the present invention will be explained. This second embodiment differs from the above-mentioned f irst embodiment in the fact that a chopping wave current is adonted as the exciting current supplied to each phase in microstep driving at the time of low-speed rotation.

The waveforms of the reference voltage supplied to the motor driver IC 10 and the exciting currents of respective coils 8 and 9 when such control is made are shown in Fig. 4 and Fig. 5. Fig. 4A shows the reference voltage corresponding to the first coil 8 wound around the phase A and the phase C which is supplied to the motor driver IC 10, and Fig. 4B shows the reference voltage corresponding to the second coil 9 wound around the phase B and the phase D similarly to the above. Further, Fig. SA shows an exciting current applied to the f irst coil 8 based on the reference voltage shown in Fig. 4A, and Fig. 5B shows an exciting current applied to the second coil 9 based on the reference voltage shown in Fig. 4B. As shown in these figures, by forming the waveform of the reference voltage supplied to the motor driver IC 10 into a chopping wave configuration increasing or decreasing rectilinearly, it is possible to form the exciting currents supplied to the first coil 8 and the second coil 9 also into a chomping wave configuration increasing or decreasing rectilinearly.

19 Further, a graph obtained by summing up respective exciting currents of the first coil 8 and the second coil 9 is shown in Fig. SC, and a diagram obtained by enlarging Fig. SC is also shown in Fig. 6. As shown in these diagrams, the current sum of the exciting currents applied to the f irst coil 8 and the second coil 9 becomes constant. In other words, no current sum ripple is generated.

Accordingly, no torque ripple is produced, and vibration of the stepping motor 1 can be controlled still more effectively.

Next, a third em-bodiment of the present invention is shown in Fig. 7 and Fig. 8.

In the nresent embodiment, bias is applied when the exciting current controlled in microstep driving in the second embodiment described above is supplied to the first coil 8 and the second coil 9. To apply bias means that the zero point 0E the voltage is deflected toward the plus or minus side by applying D.C. voltage to A.C. voltage or that the zero point of the current is deflected toward the plus or minus side by applying D.C. current to A.C. current in order to obtain a mredetermined oneration point.

It is assumed that, in order to apply a bias current to the exciting current, bias is applied to the reference voltage to the motor driver IC 10 as showing the waveform thereof in Fig. 7.

Fig. 7A and Fig. 7B show reference voltage waveforms of the motor driver IC 10 corresponding to the first coil 8 and the second coil 9, respectively, and show that bias is applied to the reference voltage shown in Fig. 2. With this, as shown in Fig. 8A and Fig. 8B, bias is also applied to the exciting currents supplied to the first coil 8 and the second coil 9, and, when these exciting currents are summed up, the current sum becomes constant and no current sum ripple is generated as shown in Fig. 8C.

Accordingly, when the exciting current is supplied with bias applied, the vibration level of the stepping motor 1 is improved and stabilized rotary driving can be obtained.

Besides, the present invention is not limited to the above-mentioned embodiments, but may be modified in various manners when occasion demands.

For example, microsten driving is controlled so as to be performed at the time of low-speed rotation. Even in the case of high-speed rotation, however, a large effect is produced f or preventing vibration and so on in the acceleration area when microstep driving is used in the above-mentioned constant-current chopper system.

Further, similar ef f ect is produced not only in 2-2 phase excitation driving, but also in the case of 1-2 phase excitation 21 and so on.

As described above, according to the present invention, such an excellent ef f ect that vibration at the time of microstep driving can be checked without complexing a control circuit is produced.

22

Claims

Claim

1. A method of driving a stepping motor in which a stator having a plurality of phases wound aroundwith coils is disposed around a rotor, an electromagnetic force for attraction or repulsion is generated between said stator and said rotor by applying an exciting current to the coils, and said electromagnetic force is switched by switching the exciting currents supplied to said respective phases successively, thereby to rotate said rotor, wherein said exciting current is controlled with microstep driving in a low frequency acceleration/deceleration area at the time of low-speed rotation and at the time of high-speed rotation of the motor.

2. A method of driving a stepping motor according to Claim 1, wherein driving is performed by a normal exciting system in a high frequency acceleration/ deceleration area and a constant speed area at the time of high-speed rotation.

3. A method of driving a stepping motor according to Claim 2, wherein said exciting current is supplied with a bias current applied thereto.

IL 7569GB

4. A method of driving a stepping motor substantially as hereinbefo're described with reference to the accompanying drawings is

3. A method of driving a stepping motor in which a stator having a plurality of phases wound around with coils is disposed around a rotor, an electromagnetic force for attraction or repulsion is generated between said stator and said rotor by applying an exciting current to the coils, and said electromagnetic force is switched by switching the exciting currents supplied to said respective phases successively, 23 7569GB thereby to rotate said rotor, wherein control of said exciting current is made with microstep driving and said exciting current is formed into a chopping wave current at the tine of low-speed rotation of a motor.

4. A method of driving a stepping motor according to Claim 3, wherein said exciting current is supplied with a bias current applied thereto.

5. A method of driving a stepping motor substantially as hereinbefore described with reference to the accompanying drawings S Amendments to the claims have been filed as follows CLAIMS 1. A method of driving a stepping motor in which a stator having a plurality of phases wound around with coils is disposed around a rotor, an electromagnetic force for attraction or repulsion is generated between said stator and said rotor by applying an exciting current to the coils, and said electromagnetic force is switched by switching the exciting currents supplied to said respective phases successively, thereby to rotate said rotor, wherein said exciting current is controlled with microstep driving in a low frequency acceleration/deceleration area at the time of low-speed rotation and at the time of high-speed rotation of the motor; and wherein driving is performed by a normal exciting system in a high frequency acceleration/deceleration area and a constant speed area at the time of high-speed rotation.

2. A method of driving a stepping motor in which a stator having a plurality of phases wound around with coils is disposed around a rotor, an electromagnetic force for attraction or repulsion is generated between said stator and said rotor by applying an exciting current to the coils, and said electromagnetic force is switched by switching the exciting currents supplied to said respective phases successively, thereby to rotate said rotor, wherein control of said exciting current is made with microstep driving and said exciting current is formed into a chopping wave current at the time of low-speed rotation of a motor.