AC permanent magnet motors mainly include two categories: Permanent Magnet Synchronous Motor (PMSM) and Brushless DC Motor (BDCM). The main difference between the two is that the permanent magnet excitation field is located in the stator phase winding. The electromotive force waveform induced in each phase of the PMSM is a sine wave, while the BDCM is a trapezoidal wave, as shown in Figure 1.
1. Working principle of AC synchronous motor
The permanent magnet synchronous motor uses permanent magnets to replace the excitation windings in the rotor of the wound synchronous motor, thus eliminating the need for excitation coils, slip rings and brushes. Three-phase symmetrical alternating current is passed into the stator. The permanent magnet synchronous motor model is shown in Figure 2. The rotating magnetic field is generated by the three-phase symmetrical alternating current connected to the three-phase winding of the motor stator. It is simulated by rotating magnetic poles S and N. What is the relative position of the rotating magnetic poles of the stator and the permanent magnet rotor at the beginning? The rotating magnetic poles of the stator drag the rotor synchronously due to the magnetic pulling force. The speed of the synchronous motor can be expressed as:
In the formula, fs is the power frequency; pn is the number of pole pairs of the motor; ns is the synchronous speed. It can be seen from the above formula that there is no relative movement between the motor speed and the synchronous speed.
Permanent magnet motor rotors are divided into three basic structures: convex type, embedded type and embedded type. The first two forms are also called exterior structures.
The geometric shape of the convex-mounted permanent magnet of the rotor is shown in Figure 3. Figure 3 (a) has a round sleeve-type integral magnet. In a rotor motor, such a permanent magnet ring with radially different poles can be used, but in a large-capacity motor, several separate permanent magnets must be used. If the thickness of the permanent magnet is the same and the width is less than one pole pitch, the entire magnetic field distribution is close to a trapezoid.
In Figure 4, the permanent magnets are not protrudingly mounted on the surface of the rotor, but embedded under the surface of the rotor. The width of the permanent magnets is less than one pole pitch. This structure is called embedded. For convex-mounted and embedded rotors, the permanent magnets are usually directly glued to the shaft with epoxy resin. These two structures can make the rotor small in diameter, small inertia, and small inductance, which is beneficial to improve the dynamics of the motor. performance.
Another type of rotor structure, as shown in Figure 5, has permanent magnets embedded in the rotor core, and each permanent magnet is covered by the core, which is called an embedded permanent magnet synchronous motor. This structure has high mechanical strength and small magnetic circuit air gap. Compared with external rotors, it is more suitable for weak magnetic field.
2. Mathematical model and control system of permanent magnet synchronous motor
High-performance permanent magnet synchronous motor systems usually use vector control algorithms. The d axis of the synchronous rotating magnetic field coordinate system is placed on the rotor flux linkage. The voltage equations on the d and q axes in the rotor reference coordinate system can be expressed as:
The flux equation is:
In the formula, ud and uq are d and q axis winding voltages; id and iq are d and q axis winding currents; Ld and Lq are d and q axis winding inductances; Rs is the stator phase resistance; ωr is the electrical angular velocity of the rotor; ψf Is the flux linkage of the permanent magnet fundamental excitation magnetic field linking through the stator winding; p is the differential operator.
The above equation can be represented by Figure 6.
The electromagnetic torque is
In the rotor reference coordinates, if the opposite direction of the d-axis is taken as the imaginary axis, and the q-axis is taken as the real axis, then on this complex plane, the stator current space vector is is expressed as:
The angle between is and d axis is β, which can have:
Substituting these two equations into the torque equation can be obtained:
The β angle is essentially the angle between the stator three-phase composite rotating magnetomotive force axis and the permanent magnet excitation magnetic field axis. It can be seen that the electromagnetic torque consists of two parts. The first part in the brackets is the electromagnetic torque generated by the interaction of the two magnetic fields. , The second part in the brackets is the reluctance torque, and is proportional to the difference between the two-axis inductance parameters. In the embedded or embedded PMSM motor, Ld<Lq, by adjusting the β angle, the output torque can be increased and the speed range can be expanded, achieving a wide range of constant power operation of the motor, and meeting the requirements of on-board motor drive.
The permanent magnet synchronous motor system has high application value in the electric drive system of electric vehicles due to its high efficiency, high control accuracy, high torque density and other characteristics. However, changes in motor parameters with temperature, speed and other factors caused by the complex operating conditions of electric vehicles will have a great impact on the control performance of the system. Power application in electric vehicles.
3. The working principle of brushless DC motor
The brushless DC motor replaces the stator poles of the brushed DC motor with a rotor equipped with permanent magnets, and turns the armature of the original DC motor into a stator. The brushed DC motor relies on the mechanical commutator to pass direct current to the rotor winding. The advantage of reversing the original DC motor stator and rotor and using permanent magnets is that the mechanical commutator and brushes are eliminated.
The commutation device composed of the position sensor of the DC brushless motor, the control circuit and the power switch device makes the DC brushless motor in the operation process of the magnetic field generated by the stator winding and the permanent magnetic field generated by the rotating rotor magnet. The electrical angle of 90° is always maintained in the space. The following describes the three-phase star winding half-controlled bridge circuit. In Figure 7, the installation positions of the three photoelectric position sensors VP1, VP2, and VP3, differ by 120°. V1, V2 and V3 are three power tubes. When VP1 is at high level, V1 is turned on and current flows into A-A’. Under the action of the mutual magnetic field of the stator and rotor, the magnetic poles of the rotor rotate in a clockwise direction. When the rotor passes 120°, VP2 is at high level, and VP1 It is low level, so that V1 is cut off, V2 is turned on, and the current is disconnected from winding A-A’ and flows into B-B’. After the rotor continues to rotate 120° clockwise under the action of the magnetic field, VP3 is high. , VP2 is low level, V2 is off, V3 is on, and current flows from C-C’. It can be seen from this that it is the position sensor that realizes the commutation of the winding current of each phase. In the three-phase half-controlled bridge circuit, each phase winding of the brushless current motor continuously works at an electrical angle of 120°.
4. Mathematical model and control system of brushless DC motor
The air gap magnetic induction intensity is distributed along the air gap trapezoidal wave, and the back electromotive force generated by the three-phase stator cutting magnetic field is a trapezoidal wave, and the phase difference is 120° electrical angle.
The voltage equation of the stator three-phase winding can be expressed as:
In the formula, ua, ub, and uc are the stator three-phase voltages; ia, ib, and ic are the stator three-phase currents; ea, eb, and ec are the stator three-phase winding back EMF; Rs is the stator three-phase resistance; L is Stator phase inductance; M is the stator mutual inductance.
The electromagnetic torque of the motor system is:
When the induced electromotive force and current of each phase winding are in the same phase, the electromagnetic torque can be expressed as:
In the formula, KT is the torque constant; Φ is the air gap magnetic flux per pole of the motor; Im is the maximum value of a phase current, and its torque formula is the same as that of a DC motor.
Because the magnetic field of the brushless DC motor is non-sinusoidal, the vector control algorithm similar to the model transformation of the induction motor can no longer be used. Instead, the PWM algorithm that adjusts the duty cycle is the same as that of the DC motor.
Figure 8 is a schematic diagram of the main circuit of a brushless DC motor.
The brushless DC motor control system usually adopts a two-by-two energization method. Only two power tubes of the upper and lower bridge arms are turned on at a moment, and the reversal is performed every 60° electrical angle, and each power tube is turned on by 120° electrical angle. The turn-on sequence of each power tube is +AB, +AC, +BC, +BA, +CA, +CB, which are marked as states S1, S2, S3, S4, S5, S6. Similar in each state Like the chopper of a DC motor, PWM acts on the power tube of the upper bridge arm, and the corresponding lower bridge arm tube is in the normally-on state.
The control block diagram is shown in Figure 9. The three-phase winding currents ia, ib, and ic are added after absolute value processing, and then divided by 2 to obtain the feedback current IF. This feedback current represents the armature current of the equivalent current motor. . The given current IREF is compared with the feedback current, and the error value is adjusted by PI, and the pulse width of PWM is adjusted by the output of the PI regulator. The adjustment process is: when IF<IREF,the regulator widens the pulse of PWM; when IF>IREF, the regulator narrows the pulse. As the time response of the regulator is very fast, IF always follows the current given value IREF. The output signal of PWM modulates the output signal of the rotor position sensor to obtain six PWM drive signals.
The external characteristic curve of the brushless DC motor is similar to that of the permanent magnet DC motor. The advantages of the brushless DC motor drive system: because there are no brushes and commutators, it can run at high speed, so that the size of the motor can be reduced, the weight is reduced, the reliability is improved, and the control of the brushless DC motor is relatively simple , These are very meaningful for electric vehicle driving.
5. Characteristics of AC permanent magnet motor drive system
Compared with the permanent magnet synchronous motor, the brushless DC motor has the advantages of simple controller and large output torque; the disadvantage is that the torque ripple is larger. The advantage of the latter is: the torque ripple is small; but the controller is more complicated, for the motor of the same power, its torque is slightly smaller than that of the brushless DC motor. Permanent magnet synchronous motors use vector control algorithms to achieve a wide range of constant power field weakening speed regulation, while there is no mature technology for brushless DC motor field weakening speed regulation.
AC permanent magnet motors use rare earth permanent magnets for excitation, which have the characteristics of high efficiency and high power density, and have advantages in medium and small power systems. At present, the most used NdFeB rare earth permanent magnet motor should not have too high a temperature rise during operation, otherwise it will cause demagnetization. Therefore, the AC permanent magnet motor is also a very important technology in terms of heat dissipation. Generally speaking, compared with the excitation motor, the AC permanent magnet motor drive system has high efficiency, small size, and light weight. It has also been used in electric vehicles to a certain extent, but this type of drive system currently has the disadvantage of high cost. , It is also significantly worse than induction motors in terms of reliability and service life. In addition, for PMSM and BDCM with higher power, it is still difficult to achieve small size and light weight.