JP2008061444A - Brushless dc motor, brushless dc motor drive system, hydraulic pump system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technology for easily reducing a secondary frequency component of cogging torque of a brushless DC motor. <P>SOLUTION: An angle θ which opens in circumferential direction of the tip of a gap 31 that extends toward the central part of a field magnet of a rotor 100 is selected to reduce cogging torque. The fundamental wave component of the cogging torque is reduced by control in driving a brushless DC motor. The secondary frequency component of the cogging torque is reduced by devising a structure of a brushless DC motor, especially the structure of a rotor. Thus, the pressure pulsation in an hydraulic pump system can be reduced. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a brushless DC motor, for example, a brushless DC motor applied to a hydraulic pump system.

  Reduction of torque ripple is desired in a brushless DC motor. This is because torque ripple causes undesirable phenomena such as vibration and noise. In particular, in a hydraulic pump, since a function of maintaining a constant pressure at a predetermined fixed position is required, torque ripple that causes pressure pulsation should be reduced.

  By the way, when a constant pressure is maintained at a predetermined fixed position, the load applied to the brushless DC motor is small and may be driven at a low speed. In general, the torque ripple at the low speed of the brushless DC motor is dominated by the cogging torque caused by the field magnetic flux generated by the magnet provided in the rotor, and the present invention also aims to reduce the cogging torque.

  The cogging torque has a fundamental wave component that varies with the least common multiple LCM of the number of poles Nr of the rotor and the number of slots Ns of the stator per rotation of the rotor of the brushless DC motor. For example, when the rotor has 6 poles and the stator has 36 slots, since the least common multiple of both is 36, cogging torque having 36 basic periods is generated during one rotation of the rotor. The cogging torque further has a higher-order frequency component.

In order to reduce the cogging torque, the shape of the motor has been devised. Generally, a skew is provided, the magnetomotive force of the rotor is made close to a sine wave, the change in magnetic flux in the so-called air gap between the rotor and stator is alleviated, or the frequency of cogging torque is increased by increasing the number of slots. It is generally known to increase the effect by reducing the effect, or to make the permeance uniform by devising the shape and interval of the magnetic poles. Examples of Patent Documents 1 to 6 that disclose techniques for reducing cogging torque are given below.

  Patent Document 1 discloses a technique for dividing the structure of a motor into a plurality of blocks along the rotation axis direction in order to reduce cogging torque generated in a motor in which a magnet is embedded in a magnetic body of a rotor. These blocks are arranged in the direction of the rotation axis at an angle that cancels the cogging torque.

  Patent Document 2 focuses on the angle of the magnetic pole determined by the shape of the magnetic flux barrier that prevents the field magnetic flux from being short-circuited, and reduces the cogging torque by setting the angle to a predetermined value or making the pitch unequal. Techniques to do this are disclosed.

  Patent Document 3 discloses a technique for selecting a value of a parameter that minimizes cogging torque using a magnetic pole angle or the like as a shape parameter.

  Patent Document 4 discloses a technique for selecting an angle of the width of a magnet with respect to a rotation center axis to a predetermined value so as to reduce cogging torque without performing skew.

  In Patent Document 5, torque ripple data is stored, and the torque command is corrected using the data. Thereby, the current of the sine wave flowing through the armature is corrected, and the torque ripple is reduced.

  Patent Document 6 shows that torque correction data is obtained by adding a waveform that is an integral multiple of the energization frequency to the armature current to reduce torque fluctuations.

JP-A-5-236687 JP-A-11-98731 JP 2004-343861 A JP 11-299199 A JP-A-11-55986 Japanese Patent Laid-Open No. 5-15188

  Laminating a plurality of blocks in the direction of the rotation axis as in Patent Document 1 complicates the process of assembling the rotor and deteriorates productivity.

  In the technique described in Patent Document 2, cogging torque remains, and it is unclear whether the waveform of the remaining cogging torque is smooth.

  In the techniques described in Patent Document 3 and Patent Document 4, it is considered that higher-order components of cogging torque remain.

  In the methods disclosed in Patent Documents 1 to 4, although attention is paid to the cogging torque amplitude and peak-to-peak values, the frequency distribution is not considered. Therefore, a high-order frequency component of cogging torque remains.

  In Patent Documents 5 and 6, it is necessary to accurately detect rotor position information. In the case where an error occurs in the position information in the correction for reducing the fundamental wave component of the cogging torque, the higher-order component of the cogging torque may be deteriorated.

  For example, the position information may be detected with an error of about 1 degree in the mechanical angle. In the cogging torque having 36 fundamental periods during one rotation of the rotor, the fundamental period corresponds to a mechanical angle of 10 degrees. Therefore, an error of 1 degree in the mechanical angle corresponds to 2π / 10 of the phase of the basic period of the cogging torque.

  In the correction of the cogging torque, a correction torque obtained by shifting the waveform of the cogging torque by π is added to the DC torque to be output. Therefore, when the phase of the waveform of the correction torque is shifted by 2π / 10 corresponding to the error of 1 degree mechanical angle, sin (α) −sin (α + 2π / 10) = 0.62 cos for the fundamental wave component of the cogging torque. (Α + π / 10), and an amplitude of about 60% remains without being canceled. On the other hand, for the secondary component of cogging torque (frequency component twice the fundamental wave component), an error of 1 degree in the mechanical angle corresponds to 2π / 5 of the phase. Therefore, corresponding to the error of 1 degree mechanical angle, the secondary component of the cogging torque is sin (α) −sin (α + 2π / 5) = 1.2 cos (α + π / 5). That is, when an error of 1 degree mechanical angle occurs in the secondary component, an amplitude of about 20% is added, and the torque ripple is deteriorated. Therefore, it is not easy to reduce the secondary component of the cogging torque by correcting the DC torque to be output.

  It is not easy to reduce the error in the mechanical angle. When a motor is installed on an appliance to which the motor is applied, the stator is held on the motor frame by, for example, shrink fitting, press fitting, or fastening with bolts. The rotor is held by, for example, shrinking and press-fitting the rotating shaft. The rotating shaft is held by a bearing incorporated in a bracket on the load side and the opposite side of the motor, and the bracket is attached to the frame of the motor.

  A rotor position detector such as an encoder is attached to the rotating shaft protruding from the motor frame. It is necessary to calibrate the relationship between the rotor position and the rotor position signal obtained from the rotor position detector and indicating the rotor position. By rotating the rotating shaft with an external force, the waveform of the induced voltage generated in the motor is measured, and a deviation between the zero cross point and the rotor position signal is obtained as an offset amount. Since the motor attached to the frame has a unique offset amount, the offset amount is recorded on the motor characteristic table or the nameplate of the frame. When the motor is driven, the controller performs control corrected using the offset amount.

  The measurement of the zero-cross point is affected by noise generated in the waveform of the induced voltage, and thus causes an error. Although it is one method to measure the waveform of the induced voltage several times and average it, it is not practical from the viewpoint of productivity.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a technique for easily reducing the secondary frequency component of the cogging torque of the brushless DC motor. The fundamental wave component of the cogging torque can be reduced by control from the outside of the brushless DC motor by a known method as described in Patent Documents 5 and 6.

  A first aspect of the brushless DC motor according to the present invention includes a peripheral edge (10), a plurality of field magnets (6) arranged annularly in the circumferential direction, each having a pair of end portions in the circumferential direction, A gap (31, 32) is provided at each end and extends toward the center of the field magnet at a predetermined distance (L1) from the periphery on the periphery side of the field magnet. And a rotor (100) rotatable in the circumferential direction and a stator (200) facing the rotor while being spaced apart from each other. A tip of the air gap provided at the end of the other field magnet adjacent to the one field magnet on the one field magnet side, and the other of the one field magnet. The angle (θ) at which the tip of the gap provided at the end on the field magnet side extends in the circumferential direction is the number of poles (Nr) of the rotor and the stator per one rotation of the rotor. For the original angle (θ1) that gives the minimum value of the fundamental component of the cogging torque that fluctuates with the number of least common multiple (LCM) with the number of slots (Ns), the least common multiple of 90 degrees is divided. The selected angle is selected by adding or subtracting the calculated angle.

  A second aspect of the brushless DC motor according to the present invention is the first aspect, wherein the original angle is obtained as the angle giving the minimum value of the amplitude of the cogging torque.

  The brushless DC motor driving system according to the present invention includes a brushless DC motor (M) according to the present invention, a position measuring unit (501) for determining the position (φ) of the rotor, and a current (iM) flowing through the brushless DC motor. ), A cancellation signal (Z) for canceling the fundamental wave component of the cogging torque from the data (D) of the fundamental wave component of the cogging torque and the position of the rotor. ) And a motor drive unit (506) for driving the brushless DC motor based on the position of the rotor, the current, and the cancellation signal.

  The hydraulic pump system according to the present invention includes the brushless DC motor drive system according to the present invention and a hydraulic pump (36) driven by the brushless DC motor. Desirably, the hydraulic pump system drives the injection molding machine (34).

  According to the first aspect of the brushless DC motor according to the present invention, the secondary frequency component of the cogging torque can be easily reduced. Moreover, the fundamental wave component can be reduced by control from the outside of the brushless DC motor by a known method.

  According to the second aspect of the brushless DC motor according to the present invention, the original angle can be easily estimated.

  According to the brushless DC motor drive system according to the present invention, the fundamental wave component of the cogging torque can be reduced.

  According to the hydraulic pump system of the present invention, the pressure pulsation is small. Moreover, the pressure pulsation at the low speed can be reduced, and the pressure pulsation in the injection molding machine that keeps the piston in a predetermined position can be reduced.

  FIG. 1 is a block diagram showing a hydraulic pump system that employs a hydraulic pump and performs injection molding. The injection molding machine 34 has a molding machine 34a and a hydraulic cylinder 34b. The hydraulic cylinder 34 b is hydraulically driven by a hydraulic pump 36. For example, a gear pump, a vane pump, or a piston pump is used as the hydraulic pump 32. A hydraulic meter 33 is provided between the hydraulic pump 32 and the hydraulic cylinder 34b to monitor the hydraulic pressure.

  The hydraulic pump 32 is driven by a brushless DC motor drive system 35. The brushless DC motor drive system 35 includes a brushless DC motor 311, a rotational position detector 312 that detects the rotational position of the rotor, and a controller 313 that controls the rotation of the brushless DC motor 311. The controller 313 supplies an appropriate armature current to the brushless DC motor 311 based on the rotational position of the rotor. The controller 313 is supplied with a DC voltage or an AC voltage from the power supply 30.

  The speed of the piston J of the hydraulic cylinder 34b is determined by a value obtained by dividing the flow rate of oil used for hydraulic drive by the cross-sectional area of the hydraulic cylinder 34b. The force that can be generated in the piston J is determined by the product of the hydraulic pressure and the cylinder cross-sectional area. When the piston J is displaced, a flow rate of oil is required. Therefore, the brushless DC motor 311 needs to rotate to flow oil against the hydraulic pressure.

  On the other hand, a hydraulic pump is required to have a function of maintaining a constant pressure at a predetermined fixed position. For example, in the molding machine 34a, although not shown in detail, a plastic resin is injected into the mold. After filling the mold with the plastic resin, a constant pressure is maintained without injecting the plastic resin. In this case, the piston J does not need to be displaced, but leaks from the hydraulic pump 32, for example. Therefore, since some oil flow is generated in the hydraulic pump 32, the brushless DC motor 311 rotates at a slightly lower rotational speed in order to drive the hydraulic pump 32.

  The oil leakage tends to increase as the hydraulic pressure increases. The higher the hydraulic pressure, the higher the rotation speed of the brushless DC motor 311. In other words, hydraulic pumps are used at higher speeds as torque load increases. Therefore, although the influence of the torque ripple on the hydraulic pressure fluctuation is small due to the inertia of the motor rotor and the hydraulic pump at high speed, the influence is large at low speed and torque ripple correction is important. Since the load is light at low speed, the current supplied to the brushless DC motor 311 is small, and the cogging torque is dominant in the torque ripple. Therefore, it is important to reduce the cogging torque in order to reduce the pressure pulsation in the hydraulic pump system.

  FIG. 2 is a plan view illustrating the structure of the brushless DC motor 311. The brushless DC motor 311 includes a rotor 100 and a stator 200 that faces the rotor 100 while being spaced apart. A plane perpendicular to the rotation axis of the rotor 100 is shown in FIG. 2 as a plan view.

  In the stator 200, teeth 201 and slots 202 are alternately arranged in the circumferential direction. Actually, the armature winding that winds around the teeth 201 is housed in the slot 202, but in FIG. 2, the drawing of the armature winding is omitted in order to avoid complication of the drawing. Here, a case where the number of slots 202 is 36 is illustrated.

  FIG. 3 is a plan view showing the structure of the rotor 100 in detail. The rotor 100 is rotatable around a rotation axis Z0 and has a peripheral edge 10 on the outer peripheral side thereof. A plurality of field magnets 6 that are annularly arranged in the circumferential direction and each have a pair of end portions in the circumferential direction are embedded in a direction perpendicular to the rotation axis Z0. Each of the field magnets 6 is provided with a gap 31 for preventing a magnetic flux short circuit. The air gap 31 extends toward the center of the field magnet 6 at a predetermined distance L <b> 1 on the periphery 10 side of the field magnet 6.

  The rotor 100 is provided with a through hole 5 through which a rotary shaft (not shown) passes, and a plurality of fastening holes 4 are provided on the outer peripheral side thereof. The fastening hole 4 is used when the core of the rotor 100 in which the field magnet 6 is embedded is formed by laminating a plurality of steel plates. For example, when these steel plates are laminated and fixed with bolts and nuts, the bolts penetrate through the fastening holes 4. Other methods for laminating and fixing steel plates without using the fastening holes 4 are also well known. In this case, of course, the fastening holes 4 are unnecessary.

  In FIG. 3, six field magnets 6 are provided and the number of poles is six. The number of fastening holes 4 is also six, but it is not necessary to match the number with the number of field magnets.

  The tips of the air gaps 31 provided at the ends of different field magnets adjacent to each other are directed to opposite sides in the circumferential direction. The angle at which the pair of tips expands in the circumferential direction is depicted as an angle θ in FIG.

  4 to 7 are graphs showing cogging torque at electrical angles of 0 to 60 degrees when the configuration shown in FIG. 3 is adopted for the rotor, and the angles θ are different from each other. The electric angle is taken on the horizontal axis, and here the case where the number of poles of the rotor is set to 6 is illustrated, so the electric angle is 3 (= 6/2) times the mechanical angle. Since the fundamental wave component of cogging torque is equal to the number of the least common multiple LCM for the mechanical angle of 360 degrees, here is 36 periods, one period corresponds to a mechanical angle of 10 degrees. Therefore, one period of the fundamental wave component of the cogging torque corresponds to an electrical angle of 30 degrees.

  4, 5, 6, and 7 show cases where the angle θ is 17 degrees, 20 degrees, 22 degrees, and 25 degrees, respectively. In FIG. 6, it can be seen that the cogging torque has the smallest amplitude, but its higher order components remain.

  FIG. 8 is a graph showing the dependence of the amplitude of the fundamental component of the cogging torque on the angle θ. Since the fundamental wave component of the cogging torque is dominant, it can be seen that the amplitude of the cogging torque is minimized when the angle θ is around 22 degrees corresponding to FIGS. The value of the angle θ that gives the minimum value of the amplitude of the fundamental component of the cogging torque is defined as the original angle θ1.

  For example, Patent Document 3 suggests that an integer multiple of a value obtained by dividing 360 degrees by the least common multiple LCM is adopted as the angle θ for reducing the cogging torque. In other words, the angle θ1 appears every 360 degrees / 36 = 10 degrees. Also in FIG. 8, since one of the original angles θ1 appears in the vicinity of 22 degrees, the other values of the original angle θ1 are estimated to be 12 degrees and 32 degrees, and the prediction is reasonable from the shape of the graph of FIG. Presumed to be.

  FIG. 9 is a graph showing the dependence of the secondary component amplitude of the cogging torque on the angle θ. The angle θ that minimizes the amplitude of the secondary component appears at intervals of 15 degrees, 20 degrees, 25 degrees, 30 degrees, and 5 degrees.

  That is, it can be seen that there are a plurality of angles θ at which the amplitude of the secondary component of the cogging torque is minimized, with a minimum interval of 360 degrees / LCM of the plurality of original angles θ1 and an interval of 360 degrees / LCM / 2. In other words, by selecting the angle θ by adding or subtracting an angle obtained by dividing 90 ° by the least common multiple LCM to the original angle θ1, which is the angle θ that gives the minimum value of the fundamental component of the cogging torque, The amplitude of the secondary component of the cogging torque can be minimized.

  FIG. 10 is a graph showing the dependence of the cogging torque PP (peak-to-peak) value, the amplitude value of the primary component (fundamental wave component), and the amplitude value of the secondary component on the angle θ. It is. The original angle θ1 is an angle θ that gives the minimum value of the amplitude of the fundamental wave component of the cogging torque, but can be easily obtained as an angle that gives the minimum value of the amplitude of the cogging torque. This is because the fundamental wave component of the cogging torque is dominant as described above. Note that the third and higher harmonics of the cogging torque are small enough not to cause a problem.

  In this embodiment, the amplitude of the secondary component of the cogging torque is minimized by devising one of the parameters of the motor shape such as the angle θ. Unlike the angle θ1, the angle θ does not minimize the amplitude of the fundamental component of the cogging torque. However, since the angle θ does not maximize the amplitude of the fundamental component of the cogging torque, the well-known technique exemplified in Patent Documents 5 and 6 is adopted, and the secondary of the cogging torque is corrected by correcting the armature current. It is easy to reduce the fundamental wave component, not the component.

  FIG. 11 is a block diagram illustrating the configuration of a brushless DC motor drive system. The motor M corresponds to the brushless DC motor 311 in FIG. The rotational position detector 501a corresponds to the rotational position detector 312 in FIG. The rotational position detector 501 detects the position of the rotor of the motor M as a position signal φ0, which is corrected by the offset value φ1 of the angular position, and the position φ is measured. That is, the rotational position detector 501a and the adder / subtractor 501b for adding / subtracting the offset value φ1 can be combined to be understood as a position measuring unit 501 for measuring the position φ.

  The torque ripple primary component data D is given to the torque ripple primary component cancellation signal generator 505 together with the position φ. The generation unit 505 outputs a cancellation signal Z based on the data D and the position φ. The cancellation signal Z is a signal for canceling the fundamental wave component of the cogging torque.

  The data D of the primary component of torque ripple can be obtained in advance by driving the motor M, for example, with no load. As described above, when the motor is applied to the hydraulic pump, since the speed is low when the load is light, the data D is collected at a low speed. Thereby, as the data D, the fundamental wave component of the cogging torque is almost obtained.

  The current detector 502 obtains a current value iM of the current flowing through the motor M. Torque command calculator 506a calculates torque command τ * based on position φ and current iM. The motor drive waveform generation unit 506b generates a motor drive waveform based on the position φ, the current value iM, and the torque command τ *, and supplies a current that reflects the waveform to the motor M. Therefore, the torque command calculation unit 506a and the motor drive waveform generation unit 506b can be grasped as a motor drive unit 506 that drives the motor M based on the rotor position φ, the current value iM, and the cancellation signal Z.

  As described above, an error occurs in the offset value φ1, and this may cause an error in the position φ. However, in the brushless DC motor drive system, the fundamental wave component of the cogging torque is reduced by controlling the current value iM, and the secondary component of the cogging torque is not reduced by controlling the current value iM. Therefore, as described above, a situation in which the secondary component of the cogging torque increases is not caused. Since the secondary component of the cogging torque is reduced by devising the shape of the motor as described above, the cogging torque can be reduced in consideration of the frequency distribution.

  Thus, by reducing the fundamental wave component of the cogging torque by the control at the time of driving the brushless DC motor 311, and by reducing the secondary frequency component by the structure of the brushless DC motor 311 (in particular, the structure of the rotor 100), Pressure pulsation in the hydraulic pump system can be reduced.

It is a block diagram which shows a hydraulic pump system. It is a top view which illustrates the structure of a brushless DC motor. It is a top view which shows the structure of a rotor in detail. It is a graph which shows a cogging torque. It is a graph which shows a cogging torque. It is a graph which shows a cogging torque. It is a graph which shows a cogging torque. It is a graph which shows the dependence with respect to angle (theta) of the amplitude of the fundamental wave component of cogging torque. It is a graph which shows the dependence with respect to angle (theta) of the amplitude of the secondary component of cogging torque. It is a graph which shows the dependence with respect to angle (theta) of cogging torque. It is a block diagram which illustrates the composition of a brushless DC motor drive system.

Explanation of symbols

6 Field Magnet 10 Periphery 100 Rotor 200 Stator 31, 32 Gap 311, M Brushless DC Motor 34 Injection Molding Machine 35 Brushless DC Motor Drive System 36 Hydraulic Pump 501 Position Measurement Unit 502 Current Detector 506 Motor Drive Unit L1 Predetermined Distance θ angle θ1 Original angle φ Rotor position

Claims (5)

The periphery (10);
A plurality of field magnets (6) arranged annularly in the circumferential direction, each having a pair of ends in the circumferential direction;
A gap (31, 32) provided at each of the end portions and extending toward the center of the field magnet by being separated from the periphery by a predetermined distance (L1) on the periphery side of the field magnet. And a rotor (100) rotatable in the circumferential direction,
A stator (200) facing the rotor while being spaced apart;
A tip of the air gap provided at the end of the other field magnet adjacent to the one field magnet on the one field magnet side, and the other of the one field magnet. The angle (θ) at which the tip of the air gap provided at the end on the field magnet side extends in the circumferential direction is:
The minimum value of the fundamental component of the cogging torque that varies with the number of least common multiples (LCM) of the number of poles of the rotor (Nr) and the number of slots of the stator (Ns) per rotation of the rotor is given. A brushless DC motor (311; M) selected as an angle obtained by adding or subtracting an angle obtained by dividing 90 degrees by the least common multiple to the original angle (θ1) that is the angle.   The brushless DC motor according to claim 1, wherein the original angle is obtained as the angle that gives a minimum value of the amplitude of the cogging torque. The brushless DC motor (M) according to any one of claims 1 to 2,
A position measuring unit (501) for determining the position (φ) of the rotor;
A current detector (502) for determining a current (iM) flowing through the brushless DC motor;
A cancellation signal generator (505) for outputting a cancellation signal (Z) for canceling the fundamental wave component of the cogging torque from the fundamental wave component data (D) of the cogging torque and the position of the rotor. When,
A brushless DC motor drive system (35) comprising: a motor drive unit (506) for driving the brushless DC motor based on the position of the rotor, the current, and the cancellation signal. A brushless DC motor drive system according to claim 3;
A hydraulic pump system comprising a hydraulic pump (36) driven by the brushless DC motor. The hydraulic pump system according to claim 4, which drives an injection molding machine (34).

Images