Permanent magnet brushless DC motor has been widely used in many occasions since it appeared in the 1950s due to its many advantages such as no commutation sparks, reliable operation, convenient maintenance, simple structure, and no excitation loss. Conventional PM BLDC motors require an additional position sensor to provide the necessary commutation signals to the inverter bridge. Its existence brings a lot of inconvenience to the application of brushless DC motors: firstly, the position sensor will increase the size and cost of the motor; secondly, the connection of numerous position sensors will reduce the reliability of motor operation, even the most widely used one Hall sensors also have a certain degree of magnetic insensitivity; again, in some harsh working environments, such as in sealed air-conditioning compressors, due to the strong corrosiveness of the refrigerant, conventional position sensors cannot be used at all. ; In addition, the installation accuracy of the sensor will also affect the running performance of the motor and increase the technological difficulty of production. In view of the various adverse effects brought by position sensors, the position sensorless control of permanent magnet brushless DC motors has been a hot research topic at home and abroad in the past decade or two. Since the late 1970s, up to now, the position sensorless control of permanent magnet brushless DC motors has roughly gone through three stages of development. Such as back EMF method, freewheeling diode method, inductance method, etc.

The correct understanding of the so-called position sensorless control should be the control without mechanical position sensor. During the operation of the motor, the rotor position signal as the commutation and conduction sequence of the power device of the inverter bridge is still required, but this kind of signal is still required. It is no longer provided by the position sensor, but should be replaced by a new position signal detection measure, that is, to reduce the complexity of the motor by increasing the complexity of the circuit and control. Therefore, the core and key of the current research on the sensorless control of the permanent magnet brushless DC motor is to construct a rotor position signal detection circuit, which indirectly obtains a reliable rotor position signal from the two aspects of software and hardware, so as to trigger the turn-on of the corresponding power devices. , the drive motor runs.

1. Traditional back EMF detection method

In the brushless DC motor, the rotor rotates in a certain direction under the action of the synthetic magnetic field generated by the stator winding. Armature windings are placed on the stator of the motor, so once the rotor rotates, a conductor cuts the magnetic field lines in space. According to the law of electromagnetic induction, it can be known that the conductor cuts the magnetic field lines to generate induced electric heat in the conductor. Therefore, when the rotor rotates, an induced potential, that is, a moving potential, is generated in the stator winding, which is generally referred to as back EMF or back EMF.

1.1 The principle of traditional back EMF detection

The main circuit of the three-phase brushless DC motor with trapezoidal back EMF waveform, for a certain phase winding (assuming A phase), the basic circuit schematic diagram of the conduction time is shown in Figure 1.

1.2 Derivation of Back EMF

Three-phase terminal voltage equation of brushless DC motor:

Due to the two-phase conduction three-phase six-beat operation mode, only two phases are connected at any moment. If the A-phase and B-phase are connected, and A+, B-, then the A and B-phase currents are equal in magnitude and opposite in direction. Phase C current is zero.

Equation (5) is the C opposite electromotive force detection equation.

In the same way, the A and B opposite electromotive force detection equations are:

But in fact, it is difficult to directly measure the back EMF of the winding. Therefore, the usual practice is to detect the voltage signal of the motor terminal and compare it to indirectly obtain the zero-crossing point of the back EMF signal of the winding, so as to determine the position of the rotor, so this method is also called For the "terminal voltage method".

The back EMF detection circuit based on the terminal voltage is shown in Figure 2. After dividing the terminal voltages Ua, Ub, and Uc, the detection signals Ua, Ub, and Uc are obtained after filtering. The O point of the detection circuit is connected to the negative pole of the power supply, so the formula ( 5) to (7) are transformed into:

According to the above conclusion, after detecting the zero-crossing point of the back electromotive force, a delay of 30° is the commutation point of the brushless DC motor. However, the actual position detection signal is obtained after RC filtering, and its zero point will inevitably produce a phase shift, and phase compensation must be performed in practical applications.

2. Proposal of a new detection method

Aiming at the above shortcomings of the prior art, a simple circuit, low cost, constant zero phase shift filter is proposed, which does not need to construct a virtual neutral point, does not need a speed estimator and phase shift correction. Can keep the output accurate commutation signal. The commutation signal is exactly the same as the commutation signal output by the Hall sensor, no high-speed control IC is needed, and the low-cost control IC matched with the Hall sensor can be used directly.

2.1 Circuit composition

The scheme adopted in this design includes 3 voltage divider circuits, 3 constant zero phase shift filter circuits and 3 line voltage comparators, as shown in Figure 3. It is characterized in that the three voltage divider circuits are respectively connected in series by two resistors R1 and R2. One end of R1 is used as the input terminal to connect the three-phase motor line of the brushless DC motor respectively, R2 is grounded, and the connection point of R1 and R2 is used as the output terminal. They are respectively connected with the correct input terminals of the corresponding line voltage comparators; the three constant phase shift filter circuits are respectively composed of two resistors R3, R4, two capacitors C1, C2 and an integrated operational amplifier. The capacitor C1 is connected to the voltage divider circuit R2. One end of the capacitor C2 is connected to the positive input end of the operational amplifier and is connected to one end of the capacitor C1, and the other end is connected to the negative input end of the operational amplifier. One end of the resistor R4 is connected to the negative input end of the op amp, and the other end is grounded. The positive input terminals of the three line voltage comparators are respectively connected with the output terminals of the corresponding voltage dividing circuits, and the negative input terminals are respectively connected with the output terminals of the adjacent voltage dividing circuits. The output of each line voltage comparator is used as the commutation signal of the motor respectively.

2.2 Circuit Analysis

Compared with the previous technology, this design uses a constant zero phase shift filter circuit that does not change with the motor speed, so no phase shift correction is required, and the voltage sent to the positive and negative terminals of the comparator is the two-way terminal voltage without phase shift, so there is no need to construct a virtual neutral point. What the comparator detects is the zero-crossing point of the line voltage, which corresponds to the commutation point of the motor. Therefore, the output commutation signal is exactly the same as the commutation signal output by the Hall sensor. In the range of high speed ratio of brushless DC motor, no high-speed control IC is needed, and the low-cost control IC matched with the Hall sensor can be used directly. The circuit structure is simple and the cost is low. It can be widely used in home appliances and computers instead of Hall sensors. On brushless DC motors such as peripherals and electric vehicles.

After the three-phase terminal voltages Va, Vb and Vc of the motor are passed through three voltage divider circuits and a constant zero phase shift filter circuit, the smooth terminal voltages Vao, Vbo and Vco with reduced amplitude are obtained. The phase shift of each phase terminal voltage before and after filtering The angle φ is:

After the constant-zero phase-shift terminal voltages of the adjacent two phases are sent to the comparator, the comparator compares the two-phase terminal voltages, which is essentially the zero-crossing point of the detection line voltage. This zero-crossing point just corresponds to the commutation point of the motor. Therefore, the commutation signal output by the comparator is exactly the same as the commutation signal output by the Hall sensor.