I. Overview

The main advantages of DC motor are good speed regulation and starting characteristics, and large locked-rotor torque, which are widely used in various driving devices and servo systems. However, DC motor has brushes and commutators, and the sliding mechanical contact between them seriously affects the accuracy, performance and reliability of the motor, and the sparks generated will cause radio interference. The life of the motor is shortened, and the commutator brush device makes the DC motor complicated in structure, noisy and difficult to maintain. For a long time, people have been looking for DC motors without brushes and commutator devices.

With the rapid development of electronic technology and the wide application of various high-power electronic devices, this wish has been gradually realized. The brushless DC motor introduced in this chapter uses electronic switch circuit and position sensor instead of brush and commutator, which makes this motor have the characteristics of DC motor. It also has the advantages of simple structure, reliable operation and convenient maintenance of AC motor. Its speed is no longer limited by mechanical commutation. If a high-speed bearing is used, it can also run at a speed of hundreds of thousands of revolutions per minute.

Meta-brush DC motor is widely used, which can be used as general DC motor, servo motor and torque motor, especially in advanced electronic equipment, robotics, aerospace technology, numerical control devices, medicine and chemical industry and other high-tech fields. Brushless DC motor integrates electronic circuit and motor. Applying advanced electronic technology to the field of motor will promote the faster update and development of motor technology.

Second, the basic structure and types of brushless DC motor

(A) the basic structure

Brushless DC motor is a permanent magnet synchronous motor with automatic frequency conversion. As far as its basic structure is concerned, it can be considered as a "motor system" composed of motor body, rotor position sensor and electronic switch circuit. Its basic structure is shown in Figure 5-20.

The motor body is a common salient-pole synchronous motor in structure, which consists of a main stator and a main rotor, and the spatial difference on the main stator is 120. Three-phase symmetrical armature windings Ax, BY and cz are connected in a star or triangle shape, and the main rotor is a pair of magnetic poles made of permanent magnet steel. The rotor position sensor also consists of a stator and a rotor. The stator is installed in the housing of the main motor, and the rotor and the main rotor rotate coaxially. Its function is to detect the position of the main rotor and turn it into an electrical signal to control the electronic switch circuit, so it is also called rotor position detector. The power switching elements in the electronic switching circuit are respectively connected with the windings of each phase on the main stator. The turn-on and turn-off of the triode is controlled by the signal output by the position sensor, so that the current in each phase winding on the main stator is also switched in a certain order with the change of the rotor position, thus realizing contactless commutation.

Length engine housing

Meta-brush DC motor interchanges the preset of ordinary DC motor with rotor. Its rotor is a permanent magnet, which produces air gap flux; Its stator is armature, which is composed of multiphase windings. Structurally, it is similar to a permanent magnet synchronous motor.

The stator structure of brushless DC motor is the same as that of ordinary synchronous motor or induction motor. Multiphase winding (three-phase, four-phase and five-phase. ) are embedded in the iron core. The windings can be connected in a star shape or a triangle shape, respectively connected to the power tube of the inverter for reasonable commutation. The rotors are mostly made of rare earth materials with high coercivity and high remanence density, such as samarium cobalt or NdFeB. Due to the different positions of magnetic materials in magnetic poles, they can be divided into surface magnetic poles, embedded magnetic poles and annular magnetic poles. Because the motor body is a permanent magnet motor, it is customary to call a brushless DC motor a permanent magnet brushless DC motor.

2. Rotor position sensor

Rotor position sensor is the key component of brushless DC motor. According to different principles, many different structural forms can be formed, such as electromagnetic induction, photoelectric, magnetic sensitivity and so on. Among them, electromagnetic induction is widely used because of its reliable work, simple maintenance and long service life. It determines the moment when each phase winding of the armature starts to be electrified. Its function is equivalent to the brush in ordinary DC engine. Changing the time (phase) of the signal generated by the position detector is equivalent to changing the position of the brush in space in DC motor, which has great influence on the characteristics of brushless DC motor.

A position sensor generally consists of a stator and a rotor. The rotor is used to determine the position of the magnetic pole of the motor body, and the stator is placed to detect and output the position signal of the rotor. There are many kinds of sensors, each with its own characteristics. At present, the commonly used position sensors in brushless DC motors have the following forms.

(1) electromagnetic position sensor. It is a sensing element which uses electromagnetic effect to realize position measurement. It has many forms, such as open transformer, ferromagnetic resonance circuit, proximity switch, etc., among which open _j transformer is used more.

The principle of electromagnetic induction rotor position sensor is shown in figure j 2l. Its stator is composed of a primary coil and a secondary coil wound on the same iron core, and its rotor is composed of a magnetic conductive material with a certain angle (similar to the conduction angle of a motor), which can be ferrite or silicon steel sheet. A high-frequency excitation signal is input at the primary side wl end of the coil, and the output signal of the relative position of the rotor core and the stator core is inductively coupled in the secondary side coil. The wa in the figure is processed by electronic circuit into a level signal corresponding to the position of the stator and rotor of the motor, and then it is shaped to get the commutation signal of the motor. However, the stator coils Wb and Wc without coupling rotor cores have no signal output.

The electromagnetic position sensor has the advantages of large output signal, reliable operation, long service life, low requirements for use environment, strong adaptability and simple and compact structure. However, this kind of sensor has low signal-to-noise ratio, large volume, and the output waveform is AC, which needs rectification and filtering before it can be used, which greatly limits its application in ordinary occasions.

(2) Magnetic sensitive position sensor. The magnetic sensor works by using the magnetic effect of current, and the position detector consists of a permanent magnet detection rotor coaxial with the motor and having the same number of poles as the motor rotor, and a plurality of magnetic sensors evenly distributed in space. At present, the commonly used magnetic sensor is Hall element or Hall integrated circuit, which will generate Hall electricity under the action of magnetic field.

The potential, after shaping and amplification, can output the required level signal, which constitutes the original position signal. Figure 5-22 shows the Hall IC and its switching output characteristics.

So as to obtain three groups of square wave original position signals with a phase difference of 120 electrical angle and a width of 180 electrical angle. Three Hall elements are needed, which are distributed at different mechanical angles in space, and the users are motor pole pairs. Figure 5-23 shows the complete structure of Hall position detector of four-pole motor. Three Hall elements h 1, H2 and H3 are distributed in space at a mechanical angle of 60. When the permanent magnet detection rotor passes through the Hall element in turn. According to different polarities, three square wave position signals with a phase difference of 120 electrical angle and a width of 180 electrical angle are generated, and these three signals just reflect the spatial position information of the motor rotor poles installed coaxially. After shaping circuit and logic circuit, the trigger letters and logic circuit numbers of six power electronic switches are output. Hall position detector is one of the most widely used permanent magnet brushless DC motors.

(3) The structure of photoelectric position sensor and photoelectric position sensing element. This is a detection method that uses a notched turntable coaxially installed with the motor rotor to control the on-off of photoelectric elements and generate a series of pulse signals reflecting the spatial position of the rotor. Because the three-phase permanent magnet brushless DC motor generally commutates once every l/6 cycle, it only needs to adopt a simple detection method similar to electromagnetic or Hall position detection, instead of the complicated method of photoelectric encoder. The structure of a simple photoelectric element is shown in Figure 5-24, which consists of an infrared light emitting diode and a phototransistor. When the light in the groove of the element is blocked by the disk, the phototransistor is not conductive: when the light in the groove

When it is released from the disc gap, the phototransistor is turned on, thereby outputting a switch-type position signal. The radian of the disc gap and the spatial arrangement of photoelectric elements are the same as those of the open transformer position detector.

In addition to the above three kinds of position sensors, there are other position sensing elements, such as sine and cosine resolver, photoelectric encoder, etc., but they are rarely used because of high cost, large volume and complicated circuit. Due to the problems of mechanical installation, maintenance and operation reliability of position detectors, meta-position detectors have appeared in recent years.

The successful application of meta-position sensor detection technology has solved the problem of difficult installation of position sensor, reduced its volume and improved its reliability, which has attracted wide attention at home and abroad. At present, the commonly used methods are: back electromotive force detection, free-wheeling diode working mode detection, stator third harmonic detection and instantaneous voltage equation method.

It must be noted that the position signals obtained by various methods can not be directly used to control the on-off of the power tube, and often need to be processed by logic before they can act on the logic control unit.

3. Electronic commutation circuit

The electronic commutation circuit of brushless DC motor is used to control the sequence and time of energizing the stator winding of the motor. It is mainly composed of power switch tube and logic control circuit. The power switch unit is the core part. Its function is to distribute the power of power supply to the winding of stator E of brushless DC motor according to a certain logical relationship, so that the motor can generate continuous torque. The control part converts the signals obtained by position detection into corresponding pulse signals, and drives the power switch tube as required. At present, the main switch of brushless DC motor generally adopts IGBT or M0sFET, and some main circuits already have integrated power module (PIc) and intelligent power module (IPM). Their application can greatly improve the reliability of the whole system.

(=) Types of brushless DC motors

In recent years, brushless DC motors have replaced brushes and commutators with transistor switching circuits and position sensors. According to the different transistor switching circuits, brushless DC motors can be divided into bridge type and non-bridge type. According to the different forms of position sensors used, they can be divided into photoelectric, electromagnetic, magnetic sensor and proximity switch.

Third, the basic working principle of brushless DC motor

In practical application, most permanent magnet brushless DC motors are in the form of three-phase bridge power main circuit, but for the convenience of explanation, its working principle is analyzed from the three-phase half-bridge main circuit first.

1. Three-phase half-bridge main circuit

Fig. 5-25 shows three photoelectric position sensing elements H 1 H2 H3 of a three-phase half-bridge permanent magnet brushless DC motor (P= 1), which are evenly distributed with a spatial difference of 120, and a notched light shield with a width of 180 is installed coaxially with the motor rotor. Adjust the relative position between the disc notch and the rotor pole, so that the notch edge can reflect the spatial position of the rotor pole.

This notch position makes photoelectric element H 1 receive light and output high level, and triggers power switch VTl to make DC current flow into A-phase winding Ax, forming armature magnetic potential located on the axis of A-phase winding. At this time, adjust the relative position between the disc notch and the rotor magnetic pole, so that the plane magnetic potential Ff of the rotor permanent phase winding is located on the B-X plane of the B-phase winding, as shown in Figure 5-26 (a), and the interaction between them generates a driving torque, which drives the rotor to rotate clockwise. When the rotor pole turns to the position shown in fig. 5 (b), if the A-phase winding is still energized, the space angle of armature magnetic potential Ff will decrease to 30 and continue to decrease, eventually leading to the disappearance of driving torque. However, due to the synchronous rotation of the coaxial turntable, the photoelectric element H2 just receives the light, and H 1 blocks the light, so that the power switch VT2 is turned on, the current is disconnected from the A-phase winding and flows to the B-phase winding, and the current is reversed, so that the armature magnetic potential becomes Fb, which makes the permanent magnet magnetic potential Ff 150 phase again in the rotation direction. The interaction between them produces a driving torque, which drives the rotor to continue to rotate clockwise. When the rotor pole rotates to the position shown in Figure 5-26 (c), similarly, the armature current commutates from phase B to phase C, which ensures the continuous generation of electromagnetic torque and the continuous rotation of the motor until it returns to the initial position in Figure 5-26 (d) or Figure 5-26(a).

It can be seen that due to the coaxial rotor position detection disk, the stator winding is fed in turn under the control of the position detector, and the phase current is a rectangular wave with a width of 120, as shown in Figure 5-27. Such a three-phase current makes the armature magnetic field generated by the stator winding and the rotating rotor permanent magnetic field always keep an approximately vertical relationship in space, which creates conditions for generating the maximum torque. At the same time, it can be seen that the armature magnetic field of stator winding is uneven during commutation.

The high-speed rotating magnetic field is a stepping magnetic field. There are three magnetic states in the range of one rotation of the rotor, and each state lasts 1/3 cycles (120. Electrical angle), as shown in Figure 5-26, FA, FB and Fc. As you can imagine, the resulting electromagnetic torque has great pulsation, especially at low speed, which will make the speed fluctuate. In order to solve this problem, only by increasing the number of magnetic states in a rotor cycle, a three-phase bridge main circuit structure should be adopted.

2. Three-phase bridge main circuit

The main circuit of the three-phase bridge is shown in Figure 5-28. The power electronic switch has a standard three-phase bridge structure, and upper bridge arm elements vt 1, VT3 and VT5 provide forward current to each phase winding to generate forward electromagnetic torque. The lower bridge arm elements VT4, VT6 and VT2 provide reverse current to each phase winding, which will generate reverse electromagnetic torque under the action of the permanent magnetic field of the rotor with the same polarity. There are two ways to connect power components (120. Conductive type) and three three electrification (180. Conduction type), its output torque is different.

(1) Paired power-on mode. The so-called pairwise conduction mode means that two power tubes are conducting at all times, and they are commutated once every 1/6 period (60 electric angle), one power tube at a time, and commutated left and right between different bridge arms. Each power tube is conducted at an electrical angle of 120. The turn-on sequence of power tubes is: vt 1, vT2；; vT2、vT3vT3、vT4vT4

VT5VT5、VT6VTL VT6 ...........................................................................................................................................................................

When two switching elements are connected, the square wave current of 120 will flow in the positive and negative directions in each phase winding. The current waveform in the three-phase winding is shown in Figure 5-29.

At any moment, one upper arm element is turned on, so that one phase winding obtains positive torque with positive current, and the other lower arm element is turned on, so that the other phase winding obtains negative torque with negative current. At this time, the resultant torque should be the vector sum of the positive and negative torques generated by energizing the relevant phase winding, as shown in Figure 5-30. It can be seen that the resultant torque is √3 times of the torque generated when one phase is electrified, and the direction of the resultant torque is rotated by 60 electrical angle every time it is commutated. In an output cycle, the torque changes in six directions, so the torque ripple is much smoother than that in the three-phase half-bridge main circuit.

(2) Three-three boot mode. The so-called "three-three conduction" mode means that three power tubes are turned on at a time and commutated once every l/6 cycle (60 electrical angle). Each commutation is carried out between the upper pipe and the lower pipe of the same bridge arm. Each power tube is conducted at an electrical angle of 180. The turn-on sequence of power tubes is: vt 1, VT2, VT3；; VT2、VT3、VT4VT3、VT4、VT5VT4、VT5、VT6VT5、VT6、vt 1； VT6, VT 1, vt ...: It can be seen that the diagram of the rotor synthetic driving torque is the same as that of one cycle in two modes, with six states, but the difference is that the amplitude of the synthetic torque at this time is 1.5 times that of the single-phase winding torque, which is the result of the simultaneous action of three-phase currents. The synthetic diagram of torque vector when the motor is running is shown in Figure 5-3 1.

Although the three-phase permanent magnet brushless DC motor is the most widely used one, people have developed multi-phase motors, such as four-phase, five-phase or even ten-phase and twelve-phase motors, from the perspective of reducing torque ripple and expanding single-machine capacity. In order to improve the utilization ratio of motor windings, multi-phase current operation mode should be adopted.