Brushless DC motor vector control (1)-the similarities and differences between brushless DC motor (BLDC) and permanent magnet synchronous motor (PMSM)

1 Differences in structure and physical properties

Modern motors and control technologies divide permanent magnet brushless DC motors into square wave drive motors and sine wave drive motors based on the current drive mode. The former is called a brushless DC motor or an electronically commutated DC motor (Electronically Commutated Motor, ECM), the latter was once called a brushless AC motor BLAC, and now there are more obvious and widely recognized Title: The square wave drive is called a brushless DC motor , and the sine wave drive is called a permanent magnet synchronous motor.

On the surface, the basic structure of BLDC and PMSM are the same, and they do have the following similarities :

1. They are essentially permanent magnet motors, the rotor is composed of permanent magnets, and the stator is equipped with multi-phase AC windings;

2. The torque of the motor is generated by the interaction between the permanent magnet rotor and the alternating current of the winding stator. The essential physical principle used is that the energized conductor is forced in the magnetic field.

3. The drive current in the winding must be synchronized with the rotor position feedback

The differences between these two motors are :

1. The brushless DC motor BLDC is driven by a square wave current, while the permanent magnet synchronous motor PMSM is driven by a sine wave current. The difference is mainly caused by this point.

2. The permanent magnet synchronous motor PMSM is quieter in terms of electricity and machinery, and the torque is basically without pulsation, while the BLDC is driven by a square wave and has torque pulsation. The reason is: due to the presence of inductance, the current does not change suddenly, so the phase current is disconnected during the commutation process, and the process of current decrease and the process of newly connected new phase current increase require time, so the phase current cannot be ideal Square wave, which will produce non-commutated phase current pulsation, causing commutation torque pulsation .

3. Because of the different driving currents, the waveforms of breath magnetic field, back-EMF, and torque are all different. The following figure shows the ideal situation: magnetic flux density distribution, reverse electromotive force, phase current and electromagnetic torque waveform. The current here is an ideal square wave, which does not actually exist.

4. The current loop structure is different, and the speed feedback information is also different.

5. The design of the motor's breath magnetic flux density distribution is different from the winding design. One is for sine wave drive and the other is for square wave drive. The permanent magnet synchronous motor pursues sine flux, and its winding distribution is more and more dispersed, while the brushless DC motor Relatively simple, so the cost of the brushless DC motor is relatively cheaper.

Through the above analysis, we can find that one of the biggest shortcomings of a BLDC is torque pulsation. The reason is the inherent shortcomings of square wave drive. In many articles and practical application cases, the torque pulsation of BLDC is the same. It focuses on the problem to be solved, so this article will not elaborate on this important problem. This important problem will be analyzed in the form of a single article later. In fact, at the end of the analysis, some people have to ask, why is the permanent magnet synchronous motor so fragrant, but also use BLDC? No other, save money and trouble (manual dog head), the motor is cheap, the square wave sending method is simpler than SVPWM, and the CPU requirement is not high. The chips that we use when we study ourselves are all proper with BLDC.

2 The difference between mathematical models

The mathematical model of the permanent magnet synchronous motor generally takes the model of the dq axis coordinate system as the main analysis object. This is because the inductance of the permanent magnet synchronous motor is linear, and the back-EMF waveform is also sinusoidal . The waveform in a fundamental period is more Close to sine, the smaller the harmonic content, the higher the accuracy. The mathematical model of PMSM in the dq coordinate system is:

Voltage equation:

Stator flux equation:

Electromagnetic torque equation:

Motion balance equation:

The mathematical model of the brushless DC motor, because BLDC adopts a full-pitch concentrated winding, the induced electromotive force is a trapezoidal wave, containing many high-order harmonics, and the inductance of the BLDC is a nonlinear inductance, so the BLDC is not suitable for the dq axis coordinate system . The transformation theory is also not suitable. The main reason is that there are too many harmonics. The dq analysis is not a very effective method. Therefore, when analyzing and simulating the BLDC control system, the phase variable method is directly used. According to the rotor position, a piecewise linear expression of the induced electromotive force is used. And other physical quantities.

Voltage equation: where R is the phase stator resistance, i is the phase current, P is the differential operator, Lx is the phase stator inductance, Lxy is the mutual inductance between phases, and e is the opposite electromotive force.

Electromagnetic torque equation: where w is the angular velocity and the unit is rad/s. It can be seen that the electromagnetic torque of BLDC is similar to that of ordinary DC motors. The output torque is proportional to the magnetic flux and current amplitude. Therefore, the output torque can be controlled by controlling the current amplitude output by the inverter .

Motion balance equation: where B is the friction coefficient, w is the angular velocity, TL is the load speed, dw/dt is the acceleration, and J is the moment of inertia.

The equivalent circuit is shown in the following figure: It can be seen that BLDC is analyzed based on each phase circuit, which is a resistive load and an inductive load, and the three phases are connected together in a star connection. Therefore, there are also key voltage equations:

3 Drive mode (the difference between modulation modes)

I think the third point that everyone is most concerned about is the difference between BLDC and PMSM in the way of generating waves. In fact, most controllers are PI regulators. After I have this control command, how do I drive the inverter? There are also big differences between BLDC and PMSM. Harm and modulation are the most difficult. The best way to deal with fear is to face it. It's all done. The power circuits of the two are the same, which also means that the main circuit that drives the BLDC can also drive the PMSM.

First of all, the driving wave mode of PMSM is mainly based on FOC/SVPWM. The theoretical basis of SVPWM is the principle of average value equivalence, that is, the basic voltage vector is combined in a switching cycle to make the average value and the given voltage vector equal. At a certain moment, the voltage vector rotates into a certain area, which can be obtained by different combinations of two adjacent non-zero vectors and zero vectors in this area. The action time of the two vectors is applied multiple times within a sampling period, so as to control the action time of each voltage vector, so that the voltage space vector closes to a circular trajectory, and the actual magnetic flux generated by the different switching states of the inverter Approach the ideal magnetic flux circle, and determine the switching state of the inverter by the comparison result of the two, thus forming the PWM waveform. There are detailed explanations in many places. Recommending a big guy is an explanation, and a big guy is a big guy. For details, thank them for their work.

https://zhuanlan.zhihu.com/p/47766452

And BLDC has a clear gap with this, there are mainly the following 6 PWM wave sending methods,

The first type: on_pwm, the first 60 cycles of 120 cycles are constant on, and the latter 60° chopped PWM modulation. ABC has the same principle.

The second type: pwm_on, the last 60 cycles of the 120 cycles are constant on, and the first 60° chopped PWM modulation.

The third type: H_pwm_L_pwn, is full 120 full modulation.

The fourth type: H_pwm_L_on, the upper tube is modulated, and the lower tube is Hengtong.

The fifth type, H_on_L_pwm, is modulated by the upper tube and Hengtong on the lower tube.

The sixth type: H_L_pwm, the upper and lower tubes alternately modulate.

Among the six modulation methods, (c) and (f> are called "double chop" mode, that is, two switching tubes are chopped; the other four are called "single chop" mode, that is, one switching tube is constant on, The other is chopping. In the single chopping method, only the upper arm (d) or only the lower arm (e) is easier to achieve, but it will cause uneven heating of the upper and lower tubes. And (a) (b) The switching losses of these two single-cut methods are the same as (d)(e), but are equally distributed in the two tubes. In the double-cut method (c), the switching loss is twice that of the single-cut method, which reduces the efficiency of the controller , And the heat is increased by one-fold. Therefore, this modulation method is rarely used . In (f), two tubes are used for modulation in turn, the switching loss of the upper and lower tubes is the same, and the equivalent switching frequency is doubled. Especially when the switching frequency of some devices is low, this method can be used to increase the equivalent switching frequency, which is an ideal modulation method.

summary:

The brushless DC motor and the permanent magnet synchronous motor are actually a kind of motor, the structure is the same, but because of the different driving current mode, they also have various different characteristics. Under the premise of reducing costs, performance is correspondingly reduced, and it needs to be optimized through specific control strategies.