By using high-energy permanent magnet as excitation mechanism, permanent magnet motor drive has the potential to be designed as a motor with high power density, high speed and high efficiency. These significant advantages make its application in electric vehicles (EV) and hybrid electric vehicles (HEV) remarkable. In the series of permanent magnet motors, brushless DC (BLDC) motor drive is the most promising choice for EV and HEV.
The main advantages of permanent magnet brushless DC motor include:
(1) High efficiency: permanent magnet brushless DC motor has the highest efficiency among all motors, because permanent magnet is used for excitation and there is no power consumption. The absence of mechanical commutators and brushes means low mechanical friction loss and therefore higher efficiency.
(2) Small size: the recent introduction of high energy density permanent magnet (rare earth permanent magnet) enables permanent magnet brushless DC motor to obtain very high flux density, which makes it possible to obtain high torque accordingly. Thus, the motor can be small in volume and light in weight.
(3) Easy to control: permanent magnet brushless DC motor is as easy to control as DC motor, because the control variables are easy to obtain and remain unchanged during the whole operation of the motor.
(4) Easy to cool: there is no circulating current in the rotor, so the rotor of permanent magnet brushless DC motor will not heat, and only heat is generated on the stator. The stator is easier to cool than the rotor because the stator is stationary and located at the edge of the motor.
(5) Low maintenance, significant long life and reliability: without brushes and mechanical commutators, there is no need for relevant regular maintenance, and the risk of failure of relevant components is eliminated. Therefore, the service life of the motor only changes with the service life of winding insulation, bearing and permanent magnet.
(6) Low noise: since the electronic commutator is used instead of the mechanical commutator, there is no noise associated with the commutator. The switching frequency of the driving inverter is high enough to make the harmonic noise in the inaudible range.
However, permanent magnet brushless DC motor also suffers from some disadvantages, such as:
(1) Cost: rare earth permanent magnets are much more expensive than other permanent magnets, which leads to the increase of motor cost.
(2) Limited constant power range: a large constant power range is very important to obtain high vehicle efficiency. It is impossible for permanent magnet brushless DC motor to obtain the maximum speed greater than twice the base speed.
(3) Safety: in the process of motor manufacturing, because the large rare earth permanent magnet can attract flying metal objects, it may be dangerous. In case of vehicle crash, if the wheels spin freely and the motor is still excited by permanent magnet, high voltage will appear at the terminal of the motor, which may endanger passengers or rescuers.
(4) Magnet demagnetization: permanent magnets can be demagnetized by large reverse magnetomotive force and high temperature. The demagnetization of each permanent material is different. Great care must be taken when cooling the motor, especially if the motor is compact.
(5) High speed performance: the motor with surface mounted permanent magnet cannot achieve high speed, which is limited by the mechanical strength of the assembly between the rotor magnet and the permanent magnet.
(6) Inverter fault in permanent magnet brushless DC motor drive: since the permanent magnet is located in the rotor, the main danger of permanent magnet brushless DC motor is in case of short circuit fault of inverter. In this way, the rotating rotor is always excited, thereby continuously inducing electromotive force in the short-circuit winding. In the short-circuit winding, the great circulating current and the corresponding large torque will block the rotor. The danger of one or more wheels of the vehicle stalling cannot be ignored. If the rear wheel is blocked and the front wheel is rotating, the vehicle will rotate out of control. If the front wheels are blocked, the driver will not be able to control the direction of the vehicle. If only one wheel is locked, it will induce the side slip torque that makes the vehicle rotate, which makes the vehicle difficult to control. In addition to the dangers of these vehicles, it should also be noted that the high current caused by the short circuit of the inverter will cause the permanent magnet to be in danger of demagnetization and damage.
The open circuit fault of permanent magnet brushless DC motor drive will not directly endanger the stability of the vehicle. However, the uncontrollable motor caused by open circuit will bring problems in vehicle control. Because the magnet is always excited and cannot be controlled, it is difficult to control the permanent magnet brushless DC motor to minimize the fault. This is a particularly important problem when the permanent magnet brushless DC motor operates in the constant power region. In the constant power region, the magnetic flux generated by the stator is opposite to the magnetic flux generated by the magnet, and the motor rotates at a higher speed. If the stator flux disappears, the magnetic flux generated by the magnet will induce a large electromotive force in the winding, which can endanger electronic components or passengers.
- Basic principle of permanent magnet brushless DC motor drive
Permanent magnet brushless DC drive is mainly composed of Brushless DC motor, DSP based controller and power converter based on power electronics. Position detectors H1, H2 and H3 detect the position of motor rotor. The position information of the rotor is input to the controller based on DSP, and then the controller provides the gating signal to the power converter, so as to turn on and off the specific stator pole winding of the motor. In this way, the torque and speed of the motor are controlled. Brushless DC motor is a kind of synchronous motor, that is, the speed of motor rotor is affected by the speed of motor stator rotating magnetic field and the number of rotor poles (P), n = 120 · f / P. When the number of rotor poles is fixed, changing the frequency of stator rotating magnetic field can change the speed of rotor. Brushless DC motor is a synchronous motor plus electronic control (driver), which controls the frequency of the rotating magnetic field and sends the speed of the motor rotor back to the control center for repeated correction, so as to approach the characteristics of DC motor. In other words, the brushless DC motor can still control the motor rotor to maintain a certain speed when the load changes within the rated load range.
- Structure and classification of permanent magnet brushless DC motor
Permanent magnet brushless DC motor can be classified according to the position of rotor permanent magnet, that is, the way that the magnet is assembled on the rotor. The magnet can be surface mounted, embedded or inserted.
Surface mounted permanent magnet rotor. Each permanent magnet is assembled on the surface of the rotor, which is easy to construct. In particular, the inclined magnetic pole in this surface installation form is easy to magnetize, so as to reduce the cogging torque. However, at high speed, the permanent magnet may fly away from the rotor.
Permanent magnet rotor with embedded installation mode. Each permanent magnet is assembled inside the rotor. This structure is not as general as the surface mounted type, but it is preferred for high-speed operation. It should be noted that there is a change in inductance in this form of rotor, because in the magnetic circuit calculation, the permanent magnet part is equivalent to air.
Permanent magnet rotor with plug-in installation. Each permanent magnet is assembled in the rotor slot, which is more convenient to install than embedded, and the speed is higher than that of surface installation.
As far as stator windings are concerned, permanent magnet brushless DC motors are mainly divided into two categories, which can be distinguished by their respective back EMF waveforms: trapezoidal and sinusoidal waveforms.
The permanent magnet brushless DC motor with trapezoidal back EMF is designed to generate trapezoidal back EMF waveform. It has the following ideal characteristics:
(1) The magnetic flux in the air gap is rectangular.
(2) Rectangular current waveform.
(3) Centralized stator winding.
The excitation current waveform is in the form of quasi square wave, and there are two zero excitation current intervals with an electrical angle of 600 ° in each cycle. Compared with sinusoidal back EMF motor, the excitation current waveform characteristics of trapezoidal back EMF motor simplify some important systems. In particular, because only 6 commutation times are required in each cycle, the requirement for the resolution of the rotor position detector can be greatly reduced.
Waveform of trapezoidal back EMF, current and Hall sensor signal driven by three-phase permanent magnet brushless DC motor. EA, EB and EC are the back electromotive force between the line and the neutral point, which is the result of the permanent magnet flux passing through the air gap radially cutting the stator coil at a rate proportional to the rotor speed. The stator coil is arranged in a standard three-phase full pitch and centralized winding mode, so the displacement between the trapezoidal back EMF waveforms of each phase is 120 ° electrical angle. The generated current pulse is 120 ° on and 60 ° off, which means that the current of each phase flows in 2 / 3 of the 360 ° electrical angle, that is, the corresponding 120 ° electrical angle is positive current and the other 120 ° electrical angle is negative current. In order to drive the motor with the maximum and constant unit current torque, the line current pulse is required to be synchronized with the back EMF of a specific phase.
The permanent magnet brushless DC motor with sinusoidal back EMF is designed to produce sinusoidal back EMF waveform, which has the following ideal characteristics:
(1) The magnetic flux in the air gap is sinusoidal.
(2) Sinusoidal current waveform.
(3) The stator conductor is sinusoidal distribution.
The most basic characteristic of sinusoidal back EMF motor is that there is a magnet rotating. The back EMF induced in each phase of stator winding should be a sinusoidal function of rotor angle.
The operation of sinusoidal back EMF permanent magnet brushless DC motor is similar to that of AC synchronous motor. The motor has the same rotating stator magnetomotive force wave as the synchronous motor.
The DC brushless driver includes a power supply unit and a control unit: the power supply unit provides three-phase power to the motor, and the control unit converts the input power frequency according to the demand. The power supply unit of the Prius motor driver can directly input DC or AC. if the input is AC, it must be converted to DC through the converter. Whether it is DC input or AC input, the DC voltage must be converted from converter to 3-phase voltage to drive the motor before it is transferred to the motor coil. The converter is generally composed of six power transistors (T1 ~ T6), which are divided into upper arm (T1, T3, T5) / lower arm (T2, T4, T6). The motor is connected as a switch to control the flow through the motor coil.
Three kinds of transistor conduction.
The control unit provides PWM (pulse width modulation) to determine the switching frequency of the power transistor and the timing of commutation of the converter. Brushless DC motor generally wants to use the speed control that can stabilize the speed at the set value without too much change when the load changes, so the motor is equipped with Hall sensor that can induce magnetic field as the closed loop control of speed and the basis of phase sequence control, but it is only used for speed control and cannot be used as positioning control.
- Material properties of permanent magnet
Permanent magnetic materials have wide hysteresis loop, high coercivity and high remanence. Once magnetized, they can maintain constant magnetism, which is also called hard magnetic materials. In practice, permanent magnet materials work in the demagnetization part of the second quadrant of the hysteresis loop after deep magnetic saturation and magnetization. The commonly used permanent magnet materials are divided into aluminum nickel cobalt permanent magnet alloy, iron chromium drill permanent magnet alloy, permanent magnet ferrite, rare earth permanent magnet material and composite permanent magnet material.
The first category is alloy permanent magnet materials, including rare earth permanent magnet materials (iron boron NdFeB), samarium cobalt (SmCo), aluminum nickel cobalt (alnico).
The second category is ferrite permanent magnet materials.
The most commonly used rare earth permanent magnet materials for new energy vehicle motors (NdFeB) are divided into the following three types according to different production processes.
(1) Sintered NdFeB. Sintered hinged iron boron permanent magnet is smelted after air grinding. It has high coercivity and extremely high magnetic properties. Its maximum magnetic energy product (BHmax) is more than 10 times higher than that of ferrite. Its mechanical properties are also quite good. It can cut and process different shapes and drill holes. The maximum working temperature of high-performance products can reach 200 ℃. Because its material content is easy to lead to corrosion, different coating treatments must be carried out on the surface according to different requirements (such as zinc plating, nickel, environmental protection zinc, environmental protection nickel, nickel copper nickel, environmental protection nickel copper nickel, etc.). Very hard and brittle, high demagnetization resistance, high cost / performance ratio, not suitable for high operating temperature (> 200 ℃).
(2) Bonded NdFeB. Bonded Nd-Fe-B is a composite Nd-Fe-B permanent magnet made by uniformly mixing hinged iron-b powder with adhesives such as resin, plastic or low melting point metal, and then compression, extrusion or injection molding. The product can be formed at one time without secondary processing, and can be directly made into various complex shapes. Bonded iron boron has magnetism in all directions and can be processed into neodymium iron boron compression mold and injection mold. High precision, excellent magnetic properties, good corrosion resistance and good temperature stability.
(3) Injection molded neodymium iron boron (zhusu NdFeB) has high accuracy and is easy to make thin-walled rings or thin magnets with complex anisotropic shapes. Its characteristic parameters are:
① Magnetic flux（ Φ）， The magnetic line of force (magnetic flux) of the permanent magnet comes out of the N pole and returns to the S pole of the magnet through the surrounding space to form a closed circuit. Magnetic flux is measured with a magnetic flux meter. The basic unit is called Weber (WB). This unit is too large. Usually, the small unit Maxwell (MX) is used. Their relationship is 1Wb = 10000000mx = 108mx. MX is a non legal entity, and WB should be used in official documents.
② Magnetic flux density (b), magnetic flux passing vertically on unit area (s)（ Φ） It’s called flux density (b), B= Φ/ S. The basic unit of B is “Tesla”, the symbol “t”, the smaller unit is “Gauss”, and the symbol “g”, which is a non legal unit and has been abolished in the official literature. 1 Tesla = 1 Weber / 1 square meter (1t = 1Wb / m2), 1 Gauss = 1 maxwell / 1 square centimeter (1g = 1mx / cm2), 1 Tesla = 10000 Gauss (1t = 10000g). The magnetic flux density (b) is measured with a Tesla meter (Gauss meter).
③ Remanence (BR or MR), remanence is short for “residual magnetic induction” (BR) or “residual magnetization” (MR). These two terms are different in the strict scientific sense, but they are the same in the field of practical permanent magnet technology. The permanent magnet is clamped between the two pole heads of the electromagnet, and the demagnetization curve is measured by the hysteresis loop tester to obtain the residual magnetism of the permanent magnet, whose unit is the same as the magnetic flux density. The demagnetization curve can also be measured by vibrating sample magnetometer, and the demagnetization curve measured by drawing method in superconducting solenoid is the most standard, but these two instruments are expensive. The remanence of Al Ni Co is 0.8t ~ 1.4T (8000g ~ 14000g), that of Ba al ferrite is 0.2T ~ 0.44t (2000g ~ 4400g), that of SM co 2:17 is 1t ~ 1.14t (10000g ~ 114000g), that of SM 1:5 is 0.8t ~ 1.05t (8500g ~ 10500g), and that of Qin iron boron is 1.1t ~ 1.52t (11000g ~ 15200g).
④ Magnetic field strength (H): the current generates a magnetic field around it. The magnetic field strength (H) represents the strength of the magnetic field. Its units are two, ampere / meter (A / M) and oerster (OE), and the latter is a non legal unit. For permanent magnet materials, ampere / meter (A / M) is too small, and kiloampere / meter (Ka / M) is commonly used. The size of OE is the same as that of Gauss (g), and the magnetic field intensity is also measured by Gauss meter.
The conversion factor between Ka / M and OE is 103 / 4 π, i.e. 1oe = (103 / 4 π) a / m ≈ 80A / m.
⑤ The difference between coercivity (HC), HCB and HCI is that the permanent magnet shows magnetism after effective magnetization, and the magnetic flux comes out of pole N and returns to pole s. Under the action of reverse magnetic field (demagnetizing field), the permanent magnet tenaciously maintains the magnetism until the magnetism returns to zero under a certain reverse magnetic field. The value of this magnetic field is the coercivity value of the permanent magnet.
When measuring the demagnetization curve of the permanent magnet, a detection coil is sleeved with the sample, the magnetic flux of the permanent magnet passes through the coil, and the magnetic flux of the reverse magnetic field also passes through the coil. The direction of the electromotive force generated by the two in the coil is opposite. In this way, when the reverse magnetic field increases, the decrease of the magnetic flux of the permanent magnet is measured by the detection coil; At the same time, the magnetic flux of the reverse magnetic field offsets the magnetic flux of the permanent magnet, which is also measured by the detection coil. The coercivity measured in this way is expressed by HCB. The detection coil is improved so that the electromotive force generated by the reverse magnetic field in the coil is zero. In this way, when measuring the demagnetization curve of the permanent magnet, the value of the reverse magnetic field when the magnetic flux of the permanent magnet drops to zero is the internal observation coercivity of the permanent magnet, expressed by HCI.
⑥ Maximum magnetic energy product (BH) max, on the B-H curve (demagnetization curve), each point corresponds to a set of values (b I, H I) and its product B IHI. At br point, H value is 0, so BH product is 0; At HCB point, b value is 0, so BH product is also 0. Between these two points, the BH product of one point must reach the maximum, which is recorded as (BH) max and called the maximum magnetic energy product. The maximum magnetic energy product of permanent magnet material represents the magnetic energy density stored in it, and its units are kilojoules / cubic meter (kJ / m3) and megahertz (mgoe), which are non legal units. The conversion factor between the two units is 100 / 4 π = 7.96 (also approximately 8).
- Performance analysis and control of permanent magnet brushless DC motor
Speed torque performance is the most important for traction and other applications. Like any other motor, torque is generated by the interaction of magnetic field and current. The magnetic field in the permanent magnet brushless DC motor is generated by the permanent magnet, and the current depends on the power supply voltage, control and back EMF, which is determined by the magnetic field and motor speed. In order to obtain the desired torque and speed under a given load, the current needs to be controlled.
- Performance analysis
The performance analysis of permanent magnet brushless DC motor is based on the following assumptions.
(1) The motor is in unsaturated state.
(2) The resistance of all stator windings is equal, and their self inductance and mutual inductance are constant.
(3) The power semiconductor device in the inverter is an ideal device.
(4) Iron consumption is negligible.
The speed torque performance of permanent magnet brushless DC motor powered by constant voltage source produces a large torque at low speed, especially at start-up. The reason is the low back electromotive force, resulting in a very large current. This very large current may damage the stator winding. When using variable voltage source, the winding current can be limited to its maximum value by actively controlling the power supply voltage. Therefore, the maximum constant torque can be generated,
- Control of permanent magnet brushless DC motor drive
In the application of vehicle traction, the output torque is required to be controlled according to the driver’s expectation and through the accelerator and brake pedal. Therefore, torque control is its basic requirement.
Block diagram of torque control strategy driven by permanent magnet brushless DC motor. The desired current I * is given by the controlled torque T * through the torque controller. The current controller and commutation sequence generator receive the position information of the desired current I * from the position detector, or may get the current feedback through the current detector, and then generate the gating signal. These gating signals are sent to the three-phase inverter (power converter) to generate the desired phase current of the permanent magnet brushless DC motor.
In traction applications, speed control may be required, such as cruise control operation. Many high-performance applications include current feedback for torque control. At least the current feedback of DC bus is required to protect the driving circuit and motor from overcurrent. The control module, namely “speed controller”, can be any type of traditional controller, such as PI controller, or more advanced controller, such as artificial intelligence controller. Hysteresis current (current chopper) control or voltage source (PWM) current control is adopted to compare the current value detected by the detector with the reference current value, so that the “current controller and commutation sequence generator” provide a specific sequence of gating signals to the “three-phase inverter” to maintain constant peak current control. Through the application of position information, the commutation sequence generator makes nibianqi9 implement “electronic commutation”, which is equivalent to the mechanical commutator of traditional DC motor. Generally, the commutation angle related to brushless DC motor is set so that the peak value of moment angle characteristic curve is ± 30 ° electrical angle. When the motor position moves beyond the 30 ° electrical angle of the peak position, the commutation detector will switch the stator excitation phase, resulting in the sudden movement of the motor to the – 30 ° electrical angle relative to the peak value of the next torque angle characteristic curve.
- Extended speed technology
As mentioned above, the inherent constant power range of permanent magnet brushless DC motor is small due to its limited flux weakening ability. This is due to the existence of permanent magnet magnetic field, which can only be weakened by the stator magnetic field component opposite to the rotor magnetic field. Its speed ratio x is usually less than 2.
Recently, additional excitation windings have been developed to expand the speed range of permanent magnet brushless DC motors. The key of this technology is to control the excitation current. The air gap magnetic field provided by the permanent magnet can be weakened during high-speed constant power operation. Due to the existence of permanent magnet and excitation winding, this kind of motor is called permanent magnet hybrid motor, and the speed ratio is about 4. However, the permanent magnet hybrid motor has the disadvantage of relatively complex structure. Its speed is still not enough to meet the performance requirements of vehicles, especially in off-road vehicles. Therefore, a multi gear transmission device is required.
- Detector free technology
The operation of permanent magnet brushless DC motor drive mainly depends on the position detector to obtain the rotor position information, so as to properly perform the on or off of each phase. Position detectors are usually either three-dimensional Hall effect sensors or optical encoders. These position detectors are high cost and vulnerable components. Therefore, the existence of position detector not only increases the cost of motor drive, but also seriously reduces the reliability of the system, and limits its application in some environments, such as military. If the position detector fails, the non position detector technology can effectively continue the operation of the system. This is critical in some applications, such as military vehicles. Several detector free technologies have been developed. Most of these technologies are based on the detection of voltage, current and back EMF, which are mainly divided into four categories.
(1) Using the detected current and voltage, the basic equation of motor and a class of algebraic calculation.
(2) A class of observers is used.
(3) A class using the back EMF method.
(4) Different from the first three categories, it adopts new technology.