The basic principle of permanent magnet brushless DC motor
Permanent magnet brushless DC motor is composed of one or more pieces ofpermanent magnet DC motor magnetic field is established, he excited DC motor and its performance is similar to constant excitation current, armature voltage can beconveniently by changing speed. Compared with the separately excited DC motor, has the advantages of small volume, high efficiency, simple structure, less copper consumption and so on, are the main types of small power DC motor.
Permanent magnet brushless DC motor and drive by the motor body composition, is atypical electromechanical integration products.
According to the permanent magnetic materials used in different, permanent magnet brushless DC motor is divided into ferrite alnico permanent magnet DC motor,permanent magnet DC motor and the rare earth permanent magnetic DC motor.Aluminum nickel cobalt permanent magnet brushless DC motor needs to consume large amounts of precious metals, the price is higher, but the good adaptability to high temperature, for the high environment temperature or temperature on stability ofmotor demanding occasions. Ferrite permanent magnet brushless DC motor using cheap and good, and good performance, are widely used in household appliances,automobiles, toys, power tools and other fields. The rare earth permanent magneticmaterials of rare earth permanent magnet brushless DC motor pole system, volume is small and the performance is better, but the price is expensive, mainly for aerospace,computer, a downhole instrument etc.. But in recent years the emergence of a new generation of rare earth permanent magnetic DC motor - neodymium iron boron permanent magnet brushless DC motor, because our country has more than 80% of the world reserves of neodymium mineral resources be richly endowed by nature, so it has advantage in terms of price, high performance NdFeB permanent magnet materialprice went up greatly, make the neodymium iron boron excellent, cheap permanent magnetic DC motor in industrial production has been widely used, and promote the rapid development of performance and structure of permanent magnet brushless DC motor and at the same time.
1 permanent magnet brushless DC motor position sensor code makes the two-phaseelectrified winding combined into the magnetic axis position ahead of the rotormagnetic axis position, so regardless of initial rotor position where, motor starting torque is large enough to start at the moment will be on the rotor, so no additionalstarting winding.
Because the stator magnetic field axis with the rotor can be regarded as the vertical axis, the iron core is not saturated situation, produce the average electromagnetic torque and winding current is proportional to, as with current - excited DC motortorque characteristics.
The motor torque is proportional to the winding average current:
Tm=KtIav (N m)
Motor phase back EMF is proportional to the angular velocity of the motor:
(V) ELL=Ke.
So the average current of the motor winding in the:
Iav= (Vm-ELL) /2Ra (A)
Among them, Vm= Delta VDC is added to the motor line to line voltage average value,VDC is the DC bus voltage, delta is the modulation wave duty cycle, Ra for eachphase winding resistance. We can get the electromagnetic torque DC motor:
Tm= Delta - (VDC, Kt/2Ra) -Kt (Ke /2Ra)
Kt, Ke is the structure constant of the motor, Omega is the angular velocity of the motor (rad/s), so, in a certain Omega, changing the duty cycle of delta, can linearly varying electromagnetic torque motor, obtained with the separately excited DC motorarmature voltage control characteristics and mechanical properties of the same.
Permanent magnet brushless DC motor speed setting, depending on the speed of Vc instruction level, if the speed command maximum maximum speed corresponding to +5V: Vc (max) n max, then the +5V of any of the following level that corresponds to nspeed equivalent, it was the realization of variable speed setting.
When Vc is set, both the change of load, power supply voltage changes, or changes in ambient temperature, when the speed is lower than the command speed when thefeedback voltage Vfb becomes smaller, the modulation wave duty becomes largerthan the Delta will become larger, the armature current, electromagnetic torquegenerated by the motor to increase production of acceleration, the actual speed and instruction equal speed until the motor so far; conversely, if the actual motor speedthan the instruction at high speed, delta decreases, Tm decreases, the occurrence ofreducing speed, until the actual speed with the command speed equal to date. It can be said that the permanent magnet DC motor in electric network fluctuation inallowable range, overload capability allows the following, the steady-state speed with the command speed difference in about 1%, and can realize constant torque speed range of operation in the.
The permanent magnet excitation source of Brushless DC motor in permanent magnet, so unlike the asynchronous machine that needs to learn the excitation current from the power grid; because there is no magnetic flux in the rotor, the rotor has no copper loss and iron free consumption, so the efficiency ratio of about the same capacity asynchronous motor high 10% in general, permanent magnetic DC motor force pointer (ETA cos theta) than the same volume of three-phase asynchronous motor with high 12%-20%.
2 because of the permanent magnet brushless DC motor is running in automatic control type, so it won't be like synchronous motor variable frequency speed regulation under heavy load starting as the startup winding on the rotor plus, also won't produce vibration and losing step in step load.
Permanent magnet permanent magnet brushless DC motor with small capacity, more use of rare earth neodymium iron boron high magnetic energy product (Nd-Fe-B)materials. Therefore, the rare earth permanent magnet brushless motor, reduce the volume of a frame size than the same volume of three-phase asynchronous motor.
Research on variable frequency speed regulation of asynchronous motor in recent thirty years, in the final analysis is looking for ways to control the torque of asynchronous motor, and the permanent magnet DC motor armature current orvoltage quantity is directly control the motor torque. In the past, due to the rare earth permanent magnet prices relatively higher factors, limiting the application field of rare earth permanent magnet DC motor, but as the technology innovation, its price hasdecreased rapidly, for example, the introduction of our company have BS price series of permanent magnet DC motor and asynchronous motor and has not much difference between the price and the common frequency converter. Rare earth permanent magnet permanent magnet DC motor is bound to its characteristics of wide speed, small volume, high efficiency and small error in steady state speed speed regulating field advantage.
3 motor stator windings made symmetrical three-phase star connection, with three phase asynchronous motor is very similar. The rotor of the motor is stuck on apermanent magnet have been magnetized, in order to detect the polarity of the rotormotor, a position sensor is arranged in the motor. Drive by power electronic devices and integrated circuit components, its function is: accept the motor starting, stopping,braking signal, to control the motor start, stop and brake; accept the position sensor signal and a positive reversal signal, used to control the inverter bridge each powertube off, continuous torque; accept the speed command and speed the feedback signal, to control and adjust the speed to provide protection and display and so on;
The main circuit is a typical AC-DC-AC voltage type inverter circuit, such as amplitude and frequency to provide 5-26KHZ modulation wave symmetric alternating rectangular wave.
N-S permanent magnet alternating exchange, produced 120 DEG phase difference of U, V, W square wave makes the position sensor, combined with positive / reversalsignal generated six status code signal effectively: 101, 100, 110, 010, 011, 001,produce T1-T4 conduction, T1-T6 conduction, T3-T6 conduction, T3-T2 conductionT5-T2 T5-T4, the conduction and conduction through the logic componentprocessing, that is to say the DC bus voltage are added on A+B-, A+C-, B+C-, B+A-,C+A-, C+B-, so that the rotor each turned a pair of N-S pole, T1-T6 power tubeaccording to the fixed combination into six States conducted in turn. Every state, onlytwo phase winding is energized, then change a state, the magnetic field axis stator winding to generate rotating in space 60 electrical degrees of rotor stator magnetic field rotation, follow the equivalent of 60 electrical degrees rotor space position, in the new position, the position sensor U, V, W generates a new group code according to the agreement, the new coding and change of power tube combination, magneticshaft so that the stator winding to generate the go 60 electrical degrees, so the cycle,the permanent magnet DC motor will produce continuous torque and drag load for continuous rotation. Because of the permanent magnet DC motor commutation isproduced by itself, and not by the forced commutation of the inverter, so it is also called self controlled synchronous motor.
