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Design and analysis of a new type of composite permanent magnet brushless DC motor for electric vehicles. Jin Jinyun, Jiang Jianzhong, Wang Xinyi (Shanghai University, Shanghai 200072). The motor uses a surface-inserted rotor pole structure. The characteristic is that the air gap flux of the motor is generated by the permanent magnet excitation and the electric field excitation of the stator current under the permanent magnet magnetic pole, so the composite characteristics of the permanent magnet brushless motor and the series excitation DC motor are particularly suitable for electric power. vehicle. The paper focuses on the unique structure and operating principle of the motor. The finite element method is used to analyze and calculate the magnetic field distribution and static characteristics. The five-phase prototype is tested to verify the theoretical analysis.
1 Introduction In the mid-1980s, with the development of power electronics technology and the emergence of new high-power electronic devices, the practical application of inverter-powered motor drive systems has grown, driving the research and development of new-type motors powered by inverters. . H., Braunschweig University of Technology, Germany
Professor Weh developed a new multi-phase reluctance motor powered by an inverter [2]. The operating principle of the motor is similar to that of a DC motor, except that the field winding and the armature winding are placed on the stator, and the rotor is axially stacked. The sheets are laminated into a salient pole structure with no coils or permanent magnets on the rotor. The stator winding is designed as seven phases. At any time, the three-phase winding in the interpole region is used as the excitation winding, and the current generates the excitation magnetic field in the lower region, and the Other four-phase winding under the salient pole is used as the armature winding. The current interacts with the excitation field to generate drive torque. When the motor is running, the role of each phase winding changes alternately with the change of the rotor position. Therefore, each phase current contains both the excitation current and the armature current. At a certain moment, the phase current acts. The rotor position is determined. Since the stator winding is powered by a seven-phase inverter and the current is controlled independently for each phase, the excitation current and armature current of the motor can be independently controlled, so that it has superior speed regulation performance with DC motor [3]. In addition, the square wave armature current and the square wave magnetic field make it have a higher power density [1]. By the mid-1990s, the American scholar TA
Lipo et al. have carefully studied the design method and performance calculation of this kind of motor. The progress has confirmed the outstanding advantages of this type of motor. However, from the perspective of the generation of electromagnetic torque, the stator current of the multi-phase reluctance motor, Only the armature current under the salient pole interacts with the excitation magnetic field to generate a positive driving torque, while the excitation current in the interpolar region interacts with the armature reaction magnetic field to generate a slight negative torque. Therefore, the excitation winding and the electric current The pivot windings are all placed on the stator, which relatively reduces the distribution space of the armature current that generates the forward drive torque, thereby limiting the increase of the motor force density, which is a major defect of the motor. In order to make up for this defect, the structure and principle of this kind of motor are improved. The permanent magnet with the proper magnetic pole direction is placed in the interelectrode space of the rotor, which is used to generate the permanent magnet excitation magnetic field and interact with the stator excitation current. A positive drive torque is generated to increase the force density of the motor. In this way, a new type of permanent magnet brushless DC motor was born. Because it evolved from a multi-phase reluctance motor and has the composite characteristics of a DC motor and a common permanent magnet motor, it is called a composite permanent magnet brushless. DC.
If roughly classified from the way the permanent magnets are placed, the motor proposed in this paper can be regarded as a new type of surface-inserted permanent magnet brushless motor, but it is essentially different from the traditional surface-mount permanent magnet brushless motor. of. First of all, the operating principle is different. The motor described in this paper evolved from a multi-phase reluctance motor. It is a multi-phase square wave motor with high power density, and the more the number of phases, the better the performance of the motor. Secondly, the rotor structure is also different. This new type of motor requires a permanent magnet width less than half a pole pitch to ensure the magnetic field adjustment characteristics of the motor and obtain a wide constant power speed regulation range. At the same time, the surface of the salient pole of the rotor needs to adopt an eccentric structure to reduce the influence of the armature reaction.
2 Operating principle The operating principle of the composite permanent magnet brushless DC motor. As shown in the figure, there are 30 slots on the stator of the motor, the number of slots per phase per pole is 1, and the two-slot conductors under each pole form a full-circle coil. Therefore, for a six-pole motor, there are three coils per phase. These three coils are connected in series to form a phase winding. For example, the three coils of the A-phase winding are placed in the 1st and 6th slots, the 11th and 16th slots, and the 21st and 26th slots, respectively. The rotor surface of the motor is embedded with six permanent magnets, the polarity of which is shown in the figure. The width of the permanent magnet is about two slots or 2/5 poles. Obviously, this motor requires less permanent magnet material than conventional surface-mounted permanent magnet brushless motors, thus reducing its cost. In addition, the dovetail-inserted permanent magnet pole structure makes the motor suitable for high speed operation.
In order for the limited excitation current to produce a sufficiently large air gap magnetic flux in the salient pole region of the rotor to achieve a high power density of the motor, the air gap in this region must be designed to be as small as possible. However, the too small air gap enhances the armature reaction of the stator armature current, which easily causes the magnetic circuit to saturate and the average magnetic density of the air gap in the region decreases, which affects the output of the motor. Therefore, the surface of the salient pole of the rotor of the motor adopts an eccentric structure, so that the air gap under the salient pole is uneven, that is, the air gap at the center of the salient pole is small, and the air gap on both sides is large, thereby weakening the armature reaction. Improve the performance of the motor.
The waveform can be seen. At any time, the four-phase winding is in the energized state, the phase winding is in the power-off state or the commutation state, and the phase windings are alternately commutated. During each cycle, each phase winding is turned on 144 ° first, then turned off (a) clockwise (b) counterclockwise broken 36°, then reversed 144 °, then turned off 36 °. The phase difference between adjacent two phases is 36°. When the rotor of the motor is in the position shown in Figure 1, the A-phase winding is in the off state, and the current direction of the remaining four phases is marked in the corresponding slot. In order to more clearly illustrate the operating principle of the motor, the opposite pole of the motor is briefly drawn in Figure 3a. As shown in the figure, only phase B and phase are under the permanent magnet pole, and the remaining phases are below the rotor salient pole. The permanent magnet pole only provides a permanent magnet excitation magnetic field for phase B and phase current (see magnetic line 1), and the interaction between them generates driving torque on the rotor. Its principle of action and a new type of composite permanent magnet brushless for electric vehicles The DC motor is the same as the ordinary permanent magnet brushless motor, so it is called permanent magnet torque. At the same time, phase B and phase currents generate an electric field in the salient pole region (see lines of force 2 and 3), which interact with the phase D and phase E currents in the region to produce electromagnetic torque. The multiphase reluctance motor is the same, so it is called reluctance torque.
To ensure the direction of the two torque components, the direction of each phase current must be determined by the rotor position signal and the direction of rotation. For the case shown in Figures 1 and 3a, the two torque components acting on the rotor are all clockwise. If the phase windings are turned on and reversed in the waveform shown in Figure 2a, the motor will continue to run in a clockwise direction. If phase B and phase current are reversed, see Figure 3b, the two torque components acting on the rotor will be counterclockwise. Since the direction of rotation is changed, in this case, A is energized and D The phase is in the off state. In order for the motor to run continuously in a counterclockwise direction, each phase winding must be turned on and reversed according to the waveform shown in Figure 2b.
(a) Clockwise rotation (b) Counterclockwise rotation, the air gap magnetic field of this motor is generated by permanent magnet excitation on the rotor and electrical excitation on the stator. The former makes the motor have an extra permanent magnet torque component compared with the multi-phase reluctance motor, thereby increasing the power density of the motor, and the latter makes the motor have a reluctance torque component, since the excitation magnetic field can pass the current of each phase Independent control to adjust. Therefore, it makes the motor have the characteristics of the DC motor and plays an important role in the speed control of the motor. For example, by reducing the corresponding stator excitation current, the magnetic field is weakened, thereby achieving weak magnetic control and expanding the constant power adjustment of the motor. Speed ​​range.
3 Electromagnetic field analysis Due to the use of non-uniform air gap and multi-source excitation, the finite element method must be used to analyze and calculate the electromagnetic field in the motor to optimize the design of the motor. The steps in the motor design are as follows: 1) Initially select the structure and dimensions of the motor from the given data.
2) Select the appropriate solution area and discretize the solution area.
3) Calculate and analyze the magnetic field using the finite element method.
4) Calculate the parameters and performance of the motor.
5) Based on the performance calculation results, correct the geometry of the motor and return to step 2) until the best design is obtained.
3. 1 The finite element model can be considered to be substantially the same due to the magnetic field distribution on each cross section of the motor, if the end leakage is ignored. The two-dimensional finite element method can be used to analyze the electromagnetic field in the motor. From the symmetry of the motor structure and the periodicity of the magnetic field distribution, the opposite pole is selected as the solution area. In this region, the vector magnetic potential equation of a two-dimensional field can be expressed as: where A is the z-axis component of the vector magnetic position, J is the current density in the z-direction (axial direction), and v is the magnetoresistance, respectively x , the magnetization in the y direction. The magnetic density B can be expressed as: the components in the x and y directions are: the boundary condition of the solution region is: where T is the upper and lower circumferential boundary, and N is the left and right ray boundary.
In addition, when the magnetic density value is obtained, the electromagnetic force can be calculated by the Maxwell tension method. The force density at each point on the tension line in the air gap is: the subscripts t and n in the equation represent the tangential and normal directions of the points on the tension line, respectively.
3. 2 Magnetic field distribution Using the above model, the magnetic field of the proposed motor is analyzed and calculated. Figure 4 shows the distribution of magnetic field lines, where Figure 4a shows the permanent magnet field, ie the no-load magnetic field. The electric excitation field at rated load is shown in Figure 4b. By changing the magnitude of the excitation phase current under the permanent magnet pole, the strength of this magnetic field can be adjusted. The overall magnetic field distribution at rated load is shown in Figure 4.
(a) Permanent magnet excitation magnetic field (b) Electric excitation magnetic field () Magnetic field at rated load 3. 3 Rotational potential In addition to the magnetic density distribution, the flux linkage and rotational potential of the winding can also be obtained from the magnetic field calculation results. For a two-dimensional field, the magnetic flux passing through the coil can be calculated by the following equation: where A is the vector magnetic position value on the two coil sides, which can be obtained directly from the finite element calculation result as the core length. Using the finite element calculation results of equation (9) and no-load magnetic field, the magnetic flux value of the permanent magnet excitation magnetic field passing through the coil can be calculated, and the magnetic flux shown in Fig. 5 can be obtained by performing multiple calculations on different rotor positions. The corner curve, in which the rotor position angle θ = 0 corresponds to the permanent magnet pole at the middle of the coil, and the flat top shape of the curve is caused by the permanent magnet width only occupying about 2 /5 pole pitch.
As mentioned above, each phase winding has only one full-circle coil under the counter electrode. Therefore, the flux linkage of each phase winding is: where: N is the number of series turns of each phase winding. When the motor rotates at an electrical angular velocity k, the permanent magnet field induces a back EMF in the phase winding, commonly referred to as the rotating potential. According to Faraday's law of electromagnetic induction, the rotational potential is: where: θ is the angle between the magnetic field and the phase winding (electrical angle), that is, the rotor position angle shown in FIG. Therefore, it can be easily obtained from Fig. 5 by simple differentiation. Thus, the rotational potential waveform at any speed can be calculated from Fig. 5 and equation (11). At the rated speed (1000r / in), the calculated rotational potential waveform is shown in Figure 6. Only the positive half-cycle waveform is given. From the symmetry of the potential waveform, the negative half-cycle waveform is directly known.
4 Static characteristic analysis It is known from the operating principle of the motor that the air gap flux is composed of two parts: the permanent magnet excitation flux and the electric excitation flux. The former is basically constant, and the latter varies with the stator excitation current. Therefore, the static torque angle characteristics of the motor are different for different stator currents. Since this characteristic reflects the torque ripple of the motor, it is shown in Figure 7. 0 ° in the figure corresponds to the rotor position shown in Figure 1, and the angle at which the rotor turns counterclockwise is negative, and vice versa. The results in the figure are obtained while maintaining the energization state of the five-phase winding. However, according to the conduction waveform shown in Fig. 2, the energization state of the five-phase winding can only remain unchanged within the 36° interval. A new type of composite permanent magnet brushless DC motor for electric vehicles is derived from the perspective of torque generation. It can be seen that the currents of the phases are reversed in sequence, so that the relative position between the stator current and the magnetic pole of the rotor is repeated in a period of 36°. Therefore, the torque variation curve in the region from 18 ° to 18 ° in Fig. 7 can be used. To evaluate the static torque ripple of the motor. In order to quantitatively analyze the torque ripple condition, a torque ripple rate r is introduced, which is defined as follows: where T is the maximum torque, the minimum torque and the average torque, respectively. From Figure 7 and Equation (12), the torque ripple rate r at different phase currents can be calculated. As mentioned earlier, the values ​​for different phase energies are also different. Compared with other permanent magnet brushless DC motors, the torque ripple rate of the motor is not large and is within an acceptable range.
Curve. When the phase current is equal to the rated current of 21.67A, the permanent magnet torque accounts for 64. 8 of the total torque, while the reluctance torque accounts for 35. 2. By adjusting the width of the permanent magnet during the design phase, or during the motor running phase By adjusting the magnitude of the phase current, you can change the ratio of the two torque components. Fig. 9 is obtained by calibrating the quantity in Fig. 8, and each quantity is taken as the base value of each of the rated points. In this way, the rate of change of each torque with current can be more clearly compared. If the magnetic circuit is not saturated, the reluctance torque component will be squared with the phase current, and the permanent magnet torque component will be linear with the phase current. Therefore, the reluctance torque rises much faster than the permanent magnet torque.
It can be seen from Fig. 9 that when the motor is overloaded, this important characteristic of the reluctance torque increases the rate of increase of the total torque with the current, so that the motor has a higher overload starting torque. This feature is desirable for electric vehicles because electric vehicles require as much short-term overload torque as possible to start and climb, to reduce start-up time and improve the ability of the vehicle to climb, while on a flat road. When driving, a smaller drive torque [6] is required. It can be seen that this type of motor is particularly suitable for use as a drive motor for electric vehicles.
5 Test results Table 1. The prototype of 8 kW, 1000r / in has been designed and manufactured, the technical data of the motor. When the motor is running at rated power, rated voltage, rated speed, rated current, phase, stator outer diameter, inner diameter, iron core, long slot, winding, single-layer winding, coil, pitch, coil, number of turns, phase resistance, (75 °C), rotor outer diameter, inner diameter Permanent magnet maximum thickness Permanent magnet width Permanent magnet residual magnet permanent magnet NdFeB 4 Calculating current and voltage with equivalent circuit According to the previous analysis, the positive and negative sequence voltages in the stator coil are calculated by the following equation: The following equation is calculated: since the equivalent circuit of the corresponding equation (9) is as shown in Fig. 5, and Fig. 5 and Fig. 1 are compared, the two are the same. By the formula (9), the positive sequence component, the negative sequence current and the total current of the stator coil current at the rated voltage can be calculated under the equivalent parameters of the known single-phase operation of the asynchronous motor. The positive and negative sequence voltages in the stator coil can be calculated by equation (7).
5 Using the equivalent circuit for motor performance analysis From the analysis in Section 4, it can be seen that the positive and negative sequence voltages U and U of the asynchronous motor single-phase operation can be calculated, and the variables and equivalent circuits corresponding to the positive sequence components are regarded as positive. The three-phase asynchronous motor is turned on, the applied voltage is U 1 , and the variable corresponding to the negative sequence component and the equivalent circuit are regarded as an inverted three-phase asynchronous motor. When the applied voltage is U 1 , the positive and negative torque is calculated as follows: Medium: p is the pole pair of the motor, k is the power angular frequency, is the square of the positive sequence impedance mode, and Z is the square of the negative sequence impedance mode. Substituting equation (9) into equation (7) to obtain U and U, then substituting equation (10), and canceling the positive and negative moments, the expression of the resultant moment is calculated: 6 Conclusion Although only the asynchronous motor is analyzed in this paper. The action of the fundamental magnetic potential during phase operation, but this superposition principle analysis method can also be used to analyze the action of the higher harmonic magnetic potential of asynchronous motors, especially single-phase asynchronous motors, to the mechanical characteristics of the motor / More efficient research and analysis of efficiency characteristics, because the frequency of the potential formed by the higher harmonic magnetic potential in the stator coil is the same as the frequency of the applied power supply. It is relatively simple to analyze by the superposition principle, and it can also be derived from high order. The equivalent circuit of harmonic components, this analysis is especially important and effective for single-phase asynchronous motors, because the higher harmonic components of the magnetic potential of single-phase asynchronous motors are larger, and the impact on the performance of the motor is very large. Do a detailed analysis.
Li Longnian. Principle and design of single-phase motor [ ]. Beijing: Tsinghua University Press, when the motor state, its no-load potential was measured. Figure 10 shows the no-load potential waveform measured at rated speed, which is in good agreement with the theoretical waveform given in Figure 6, confirming the validity of the theoretical results obtained with the finite element method.
6 Conclusions This paper introduces a new type of multi-phase hybrid permanent magnet brushless DC motor, which uses a multi-phase stator winding and a unique rotor pole structure (permanent magnet pole width of about 2 /5 pole pitch) It has the advantages of permanent magnet brushless DC motor and series-excited DC motor. This new type of motor has high power density, high efficiency, high starting torque, electromagnetic analysis of single-phase operation of asynchronous motor and superposition principle analysis method. Parameters and display the results in real time. The system runs the corresponding program according to the input parameters to determine the running direction (positive and reverse), operation mode (integral, half step) and control mode (open, closed loop) of the motor. The control circuit is composed of a programmable interface chip 8255, and the A and B ports are used as input ports, and the position feedback information outputted by the sensor to the counter is used as a position counting port as an output port, and the required control commands are output, including synchronization commands and mode selection. command.
In the system, the speed loop adopts incremental digital control, and the parameter is quantized into the clock unit of the 8253 timer T. The algorithm is as follows: where: u(k)—the lead angle delay value (k)—the speed reference (k)—the speed Measured value - error scale factor - error integral coefficient - saturated output value of the error differential coefficient.
Considering that the sampling period is too long at low speed, it is easy to cause output saturation. At low speed, the integral separation method is used: 4 Experimental research and conclusions The low-frequency operation of the stepping motor often produces slight oscillation due to excess energy (Fig. 5). A pulse signal appears in the reverse channel of the signal, and an extra pulse appears in the forward channel. It can be seen from Fig. 6 that if the pulse of the reverse channel cancels the extra pulse of the forward channel, the number of feedback pulses in the forward channel is still correct. It can be seen from Fig. 7 and Fig. 8 that the self-synchronized operation system does not have the capacity of loading. After the speed is closed, the lead angle is dynamically adjusted. At this time, the speed can be stabilized at a given value, and the steady-state performance of the closed-loop control is obviously excellent. Run at self-synchronization.
The rotor position detection is realized by the magnetoresistance change of the stepping motor, and then the closed-loop control system of the two-phase hybrid stepping motor is realized, which reduces the hardware cost and improves the flexibility, reliability and versatility of the control. The feasibility of this method is also proved by experiments.
Wang Zongpei, Yao Hong. Research on calculation and design method of stepping motor [ ]. Nanjing: Southeast University Press, 1994.
(Upward page 17) Wide speed range and strong mechanical robustness, especially suitable for electric vehicles.