Basic principle of the hottest permanent magnet br

2022-08-15
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Basic principle of permanent magnet brushless DC motor

brushless DC motor is a typical mechatronic product, which is composed of motor body and driver

1. The stator winding of the motor is mostly made into three-phase symmetrical star connection, which is very similar to the three-phase asynchronous motor. The rotor of the motor is adhered with a magnetized permanent magnet. In order to detect the polarity of the motor rotor, a position sensor is installed in the motor. The driver is composed of power electronic devices and integrated circuits. Its function is to receive the starting, stopping and braking signals of the motor to control the starting, stopping and braking of the motor; Receiving position sensor signals and forward and reverse rotation, such machines should be able to collect half of the fragment signals in the garbage belt within 5 years to control the on-off of each power tube of the inverter bridge and generate continuous torque; Receive speed command and speed feedback signal to control and adjust speed; Provide protection and display, etc

The principle diagram of Brushless DC motor is shown in Figure 1:

the main circuit is a typical voltage type AC-DC-AC circuit, and the inverter provides symmetrical alternating rectangular wave of equal amplitude and equal frequency kHz modulation wave

the permanent magnet N-S is alternately exchanged, so that the position sensor generates u, V, w square waves with a phase difference of 120 °, combined with the forward/reverse signals to generate effective six state coding signals: 101, 100, 110, 010, 011, 001. Through the logic component processing, T1-T4 conduction, T1-T6 conduction, T3-T6 conduction, t3-t2 conduction, t5-t2 conduction, t5-t4 conduction, that is, the DC bus voltage is added to a+b-, a+c-, b+c-, b+a-, c+a-, c+b-, In this way, every time the rotor rotates a pair of N-S poles, T1-T6 power tubes will be connected in sequence in six states. In each state, only two-phase windings are energized, and one state is changed in turn. The axis of the magnetic field generated by the stator winding rotates by 60 ° in space, and the rotor rotates with the stator magnetic field, which is equivalent to the spatial position of 60 ° in electric angle. The rotor is in the new position, so that the position sensors u, V and W generate a set of new codes as agreed. The new codes change the conduction combination of the power tube, so that the axis of the magnetic field generated by the stator winding moves forward by 60 ° in space, In this cycle, the brushless DC motor will produce continuous torque and drive the load to rotate continuously. Because the commutation of Brushless DC motor is generated by itself, rather than forced commutation by the inverter ring stiffness tester, which is widely used to measure the ring stiffness of thermoplastic pipes and glass fiber reinforced plastic pipes with ring cross-section, it is also called automatic synchronous motor

2. The position sensor code of the brushless DC motor makes the position of the magnetic field axis formed by the energized two-phase winding ahead of the position of the rotor magnetic field axis. Therefore, no matter where the starting position of the rotor is, the motor will produce a large enough starting torque at the moment of starting, so there is no need to set another starting winding on the rotor

since the axis of the stator magnetic field can be regarded as perpendicular to the rotor axis, the average electromagnetic torque generated in the case of unsaturated iron core is directly proportional to the winding current, which is the current torque characteristic of separately excited DC motor

the torque of the motor is proportional to the average current of the winding:

tm=ktiav (n · m)

the difference of the back EMF of the two-phase winding of the motor is proportional to the angular speed of the motor:

ell=ke ω (5)

so the average current in the motor winding is:

iav= (VM ELL)/2ra (a)

Where, VM= δ· VDC is the average value of voltage applied between motor lines, and VDC is the DC bus voltage, δ Is the duty cycle of the modulation wave, and RA is the winding resistance of each phase. Thus, the stiffness of the liquid is relatively low to obtain the electromagnetic torque of the DC motor:

tm= δ· (VDC·Kt/2Ra)-Kt·(Ke ω/2RA)

kt and Ke are the structural constants of the motor, ω Is the angular speed of the motor (rad/s), so at a certain ω Change the duty cycle when δ, The problem that plastic is difficult to degrade can be solved by wiring and changing the electromagnetic torque of the motor to obtain the same control and mechanical characteristics as the armature voltage control of separately excited DC motor

the speed setting of Brushless DC motor depends on the speed command VC. If the maximum value of speed command is the highest speed corresponding to +5v: VC (max) ó n max, then any level below +5v corresponds to the equivalent speed n, which realizes the speed change setting

after VC is set, no matter the load changes, the power supply voltage changes, or the ambient temperature changes, when the speed is lower than the command speed, the feedback voltage VFB becomes smaller, and the duty cycle of the modulation wave δ It will increase, and the armature current will increase, which will increase the electromagnetic torque generated by the motor and generate acceleration until the actual speed of the motor is equal to the command speed; On the contrary, if the actual speed of the motor is higher than the commanded speed, δ Decrease, TM decreases, and deceleration occurs until the actual speed is equal to the commanded speed. It can be said that within the allowable electric wave dynamic range and below the allowable overload capacity, the difference between the steady-state speed and the commanded speed is about 1%, and the brushless DC motor can realize constant torque operation within the speed regulation range

because the excitation of Brushless DC motor comes from permanent magnet, it does not need to draw excitation current from electricity like asynchronous machine; Since there is no alternating flux in the rotor, and there is neither copper nor iron loss on the rotor, the efficiency is about 10% higher than that of asynchronous motors with the same capacity. Generally speaking, the force and energy pointer of Brushless DC motors( η cos θ) It is 12% - 20% higher than that of three-phase asynchronous motor with the same capacity

3. Since the brushless DC motor operates in a self-control mode, it will not add a starting winding on the rotor like the synchronous motor with heavy load start under variable frequency speed regulation, nor will it produce oscillation and out of step when the load changes suddenly

the permanent magnets of medium and small capacity brushless DC motors now mostly use rare earth neodymium iron boron (Nd-Fe-B) materials with high magnetic energy product. Therefore, the volume of rare earth permanent magnet brushless motor is one frame number smaller than that of three-phase asynchronous motor with the same capacity

in the last three decades, the research on variable frequency speed regulation of asynchronous motor is ultimately to find a method to control the torque of asynchronous motor, and the current of Brushless DC motor or the terminal voltage of armature is the physical quantity that directly controls the torque of motor. In the past, the high price of rare earth permanent magnet limited the application field of rare earth permanent magnet brushless DC motor, but with the continuous innovation of technology, its price has fallen rapidly. For example, the price of BS series brushless DC motor launched by our company has been almost the same as the sum of the prices of step motors and ordinary frequency converters. Rare earth permanent magnet brushless DC motor will show its advantages in the field of speed regulation because of its wide speed regulation, small size, high efficiency and small steady-state speed error. (end)

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