Research on Brushless DC Motor with Passive Magnetic Bearing
Because of no mechanical wear and noise, non-contact magnetic bearings are widely used in high-speed motors and axial fund projects such as artificial heart rotary blood pumps: National Natural Science Foundation of China (59977014).
Maintenance-free special application areas. There are many types of non-contact rotary motors, of which the most attractive is a bearingless motor structure, in which the rotor's rotational torque and levitation force are generated by the stator of the motor itself, without any need for any conventional or magnetic bearings. Unfortunately, however, the complexity of the structure and control technology limits the practical application of bearingless motors. Although the motor with active magnetic bearings can realize the decoupling control of electromagnetic torque and levitation force, its control technology is relatively simpler than the bearingless motor, but it still needs the dynamic detection of multiple degrees of freedom of the rotor position and the real-time levitation force. Control, its control technology is still more complex, and the active magnetic bearing is larger. If magnetic levitation can be realized without complicated control technology, it will greatly promote the practical application of the magnetic levitation technology in the motor.
This paper proposes a new type of brushless DC motor with passive magnetic bearings. The rotor can realize magnetic levitation without dynamic detection of air gap and real-time control of levitation force. Torque control also does not require rotor position detection, but adopts no position. Sensorless Brushless DC Motor Speed ​​Control System. Theoretical analysis and prototype. The motor adopts a rotor composed of radial magnetizing tile permanent magnets and is sleeved on the rotating shaft of the non-magnetic material. The passive magnetic bearings at both ends of the rotor are composed of two groups of permanent magnet rings that are axially magnetized. As shown in the figure, the magnetization directions of the two groups of permanent magnet rings are opposite, and they are symmetrical to the permanent magnet rotor to facilitate the axial force of the motor. balance.
The suspension of the motor rotor is achieved by the radial magnetic levitation force generated by the passive magnetic bearings. The motor rotor will generate a displacement in the direction of the external force under the effect of radial external force, so that the air gap between the two permanent magnetic rings of the magnetic bearing decreases in the direction and the air gap in the opposite direction increases, and two permanent magnet rings will generate The magnetic force recovered to the uniform air gap, and the larger the air gap between the two permanent magnet rings, the greater the generated magnetic force. Therefore, as long as the design is reasonable, the passive magnetic bearing can generate a sufficient levitation force within the allowable range of rotor eccentricity, and the contact between the two magnetic rings and the stator and rotor of the motor will not occur. Since the passive magnetic bearing itself has the function of automatic adjustment of the levitation force, there is no need to control the levitation force. The torque control of the motor can be achieved by changing the size and frequency of the armature current of the brushless DC motor. Due to the use of a position sensorless control strategy, the structure of the motor is very simple.
3 Passive magnetic bearing design 3.1 Passive magnetic bearing radial levitation force According to the above analysis, the passive magnetic bearing is the core component of the new magnetic levitation brushless DC motor. The rational design of this part is the basis for ensuring the normal operation of the motor. The design of passive magnetic bearings must be based on the precise calculation of radial and axial magnetic forces.
The passive magnetic bearing shown in the middle consists of two sets of axially-magnetized permanent magnet rings. Analysis of radial and axial forces can be illustrated with the illustrated set of permanent magnet rings. Let two permanent magnet rings have the same thickness H and width L, the outer diameter of the outer ring is £, the average gap between the inner and outer rings is, the axial and radial displacements of the inner and outer rings are Az and A; y. The finite element analysis of the magnetic field can find the magnetic field distribution in the passive magnetic bearing. The virtual radial displacement method can be used to calculate the distribution of the radial force along the circumference and the total combined radial magnetic levitation force. In order to calculate the relationship between the radial magnetic levitation force of the permanent magnet ring and the radial offset Ay of the inner ring under different average air gaps and calculated using three-dimensional magnetic field analysis, the mean air gap refers to no radial eccentricity (Ay=0). The air gap between the inner and outer rings. The size of the magnetic ring used in the calculation model is: = / 2 = 5mm, L = 10mm, 1) The relationship between the average air gap and the radial offset of the graph can be seen from: 1 radial magnetic levitation of passive magnetic bearings The force is proportional to the radial offset of the inner ring. The larger the offset, the greater the radial levitation force. When there is no eccentricity (under a uniform air gap), the radial force is zero. 2 The radial levitation force is related to the average air gap between the inner and outer permanent magnet rings. The smaller the average air gap, the larger the radial levitation force under the same inner ring eccentricity.
3.2 Axial Force of Passive Magnetic Bearings The three-dimensional magnetic field analysis model for calculating radial forces and the calculation method for magnetic force can be used to calculate the axial force of a passive magnetic bearing. Obviously, when there is no axial displacement of the inner ring (Az = 0), the two permanent magnet rings will not generate axial force. In order to calculate the axial force of the passive magnetic bearing relative to the axial displacement of different inner rings through the magnetic field analysis, the size of the magnetic ring is the same as when calculating the radial force. The average air gap is g = 1 mm. The inner ring axial displacement is Az = Magnetic field distribution of passive magnetic bearings at 4mm.
From and can be seen: 1 The axial force of the passive magnetic bearing relative to the positive and negative axial displacement of the inner ring (+ Az and - Az) has a symmetrical relationship; 2 there is a maximum axial force, in this case The maximum axial force at the bottom is about Az=0.6mm and Az=-.6mm at the axial displacement of the inner ring. 3 The electromagnetic force is related to the strength of the magnetic field, and the electromagnetic force at the high magnetic flux density is also large. As can be seen from the illustrated magnetic field distribution, the axial force mainly produces an air gap portion at the axial interface of the inner and outer rings.
Passive magnetic bearing magnetic field spatial distribution (Az = 2mm) 3.3 Passive magnetic bearing stability analysis Passive magnetic bearings consisting of a pair of permanent magnet rings can not obtain a stable balance, and at least one degree of freedom must impose additional constraints Row.
From the above analysis of the radial force of a magnetic bearing consisting of two permanent magnet rings, it can be known that the radial magnetic bearing is radial as long as the radial perturbation force does not exceed the radial magnetic levitation force within the allowable radial offset range. stable. The axial force calculations shown indicate that a single magnetic bearing consisting of two permanent magnet rings is not axially stable, however, a combination of two symmetrical passive magnetic bearings may constitute a single axis. To a stable structure.
First, look at a set of passive magnetic bearings with symmetrical structures consisting of two magnetic bearings, assuming there is no axial displacement between the inner and outer rings of the permanent magnet, as shown in (a). At this point, the passive magnetic bearing is only theoretically stable. Assuming a disturbance from the left to the right axial force, the permanent magnet inner ring rigidly coupled to the shaft will move to the right, resulting in an axial displacement Az as shown in (b) due to the relative axial displacement of the two inner rings. As the magnetization directions of the two outer ring permanent magnets are different, it can be known that both permanent magnet inner rings are subjected to an axial force from left to right, and the resultant axial force is as shown in (b). As a result, the two inner rings are obtained. Continue to offset to the right. Therefore, this is an axially unstable passive magnetic bearing structure.
The two inner permanent magnet rings in the illustrated structure are respectively moved in the opposite direction by a known offset Az which is larger than the corresponding maximum axial force shown in (a). Structure. From (a), for each magnetic bearing, the axial displacement of the inner ring is the same as the magnetization direction of the outer ring permanent magnet. The axial force generated is along the magnetization direction of the permanent magnet, and due to the installation in The magnetization direction of the two magnetic bearings on the same axis is opposite, so the two inner rings actually have opposite forces. At the same time, because the axial displacements of the two inner rings are equal, the resulting axial forces are equal and cancel each other out, so that the magnetic bearings can obtain axial static stability.
(a) The initial position of the picture (Cb) The position after disturbance by the axial force When the magnetic bearing is subjected to axial external force disturbance, if it is subjected to a disturbance force from left to right, the inner ring of the bearing will generate a leftward direction. The right displacement Az, as shown in (b). With respect to the outer ring itself, the displacements of the two inner rings are Az+Az and Az-Az, respectively, ie the axial offset of the inner and outer rings of the left magnetic bearing is larger than the axial offset of the inner and outer rings of the right magnetic bearing. It can be known that the axial force generated by the left magnetic bearing is smaller than the axial force generated by the right magnetic bearing. The axial force of the two magnetic bearings is combined into the right-to-left Fz shown in (b). The result is the magnetic bearing. The inner ring moves to the left to restore the position before the disturbance. Therefore, the magnetic bearing structure has axial dynamic stability.
By the same token, the passive magnetic bearing structure shown also has radial and axial stability.
瞧 Another axially stable passive magnetic bearing structure Fig.8Anotheraxiallystablestructure 4 Design of brushless DC motor 4.1 Radial force of brushless DC motor For the design of general brushless DC motor, the main task is to meet the load characteristics of the electromagnetic rotation of the motor Moment requirements. However, for brushless DC motors using magnetic bearings, the influence of the radial and axial forces due to the interaction between the stator and rotor on the magnetic bearings needs to be considered.
The two-dimensional magnetic field analysis model can be used to calculate the radial force of a brushless DC motor due to rotor eccentricity. The magnetic field distribution in the motor is shown when there is a displacement of the 4-pole external permanent magnet rotor from right to left in the horizontal direction. The magnetic lines of the left air gap are denser than the magnetic lines of the right air gap, indicating the magnetic flux density in the left air gap. More than the right air gap.
The radial force is proportional to the square of the magnetic flux density, so the rotor will experience a unilateral magnetic pull from right to left.
The curve of the combined radial force calculated by the magnetic field analysis and the radial offset of the rotor and the average air gap is shown as 0, and the average air gap is still defined as the average air gap without radial deviation of the rotor. It can be seen that: (1) The radial force due to eccentricity of a permanent magnet rotor is proportional to the eccentricity of the rotor; (2) Under the same eccentricity, the larger the average air gap, the smaller the radial force generated; 3 The eccentricity of the rotor The radial force is a kind of unilateral magnetic tension, which is opposite to the direction of the radial magnetic levitation force generated by the passive magnetic bearing, so it is unfavorable to the magnetic levitation of the rotor.
0 Relationship between radial force and eccentricity of the rotor and mean gas 4.2 The axial force of the brushless DC motor can be seen: 1 Only when the axial displacement of the rotor is small, calculate the axis of the brushless DC motor with axial displacement of the stator and rotor. Qiu Qiu, etc. (Zhu a/). Permanent-magnet-biased Radial-Axial Magnetic Suspension Bearing Operating Principle and Parameter Design (The Analysis of Excitation and Control Method of Suspension Force Winding of Bearingless Motor with Magnetic Suspension for Wang Baoguo, Wang Fengxiang (Wang Baoguo, Wang Fengxiang) (Excitation and control analysis Wang Fengxiang (1938-), Male, Professor, doctoral supervisor, research direction: special motor and its control, power electronics and power transmission; Wang Jiqiang (1977-), male, doctoral student, research direction for special motor and its control: Kong Zhiguo (1981-), male, master Graduate student, research direction is power electronics and electric drive.
(Editor Ding Yuyu)
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