Permanent magnet synchronous motor 36 question (1) -- what is the difference between salient pole motor and salient pole motor?

In fact, this issue was also discussed in the previous blog , Let's sort it out again .

The understanding of this problem can be divided into many aspects and dimensions .

The first dimension ： Numerical differences

The most direct difference between hidden pole motor and salient pole motor is , Cryptopole Motor's d Shaft inductance Ld And q Shaft inductance Lq The relationship is satisfied ：Ld = Lq, and Salient pole Motor's d Shaft inductance Ld And q Shaft inductance Lq The relationship between ： Ld ≠ Lq. Understand this level , You can do simulation , Corresponding simulation PMSM The setting of the module should be distinguished .

  The second dimension ： The difference of the maximum amplitude of the output torque

  For salient pole motors , The torque expression is as follows ：

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For non salient pole motors , The torque expression is as follows ：

You can see , The torque of salient pole motor contains two components ,Te1 = 3/2*Np*phif*iq  And   Te2 = 3/2*Np*(Ld-Lq)*id*iq

Ahead Te1 It is called electromagnetic torque ,Te2 It is called reluctance torque . The hidden pole motor only Te =  3/2*Np*phif*iq This component , There is no reluctance torque component .

Therefore, the inverter can output capacity , That is, when the capacity is certain , Salient pole motor can output more torque . The existence of reluctance torque can greatly improve the output torque capacity of the motor , Some motors even want to amplify this component , Increase the salient pole ratio of the motor , Amplify the magnitude of the reluctance torque , It is even more excessive to retain only this reluctance torque , Discard the excitation torque , Finally, it became the famous switched reluctance motor .

The second dimension ： Differences in the structure of magnetic poles

The structure of permanent magnet synchronous motor includes ： Permanent magnet 、 Rotor core 、 Rotating shafts and bearings , In the motor ,d The axis of the shaft coincides with the axis of the rotor magnetic pole of the motor ;q The shaft axis is ahead d Axis 90°, That is, it is located at the geometric center line of two adjacent magnetic poles . As shown in the figure below ：

Before distinguishing , We need to understand a concept , Air gap magnetic field , The specific explanation is on the website What is an air gap ?_ Baidu knows (baidu.com), I think it can be understood as the magnetic circuit that the air needs to walk , Part of the magnetic circuit , however The magnetoresistance of the air gap is larger （ If there is a better explanation, please send it to me ）.

In the surface mounted permanent magnet synchronous motor , The permanent magnet is close to the outside of the rotor core , Just like a layer of permanent magnet material is evenly distributed outside the iron core , Because the permeability of the permanent magnet is basically the same as that of the air gap , therefore dq The equivalent magnetic circuit of the shaft is completely equal , It can be understood as Its The real is equal to the radius of the rotor circle . When the equivalent magnetic circuits are equal , The same magnetic field comes from d Shaft conduction and q The speed of shaft conduction is exactly the same , therefore dq The axial magnetic circuit is symmetrical , It is a hidden pole motor .

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  In the built-in permanent magnet synchronous motor , The permanent magnet is buried inside the iron core , Between its outer surface and the air gap, there is a pole shoe protection of ferromagnetic material （ The meaning of extreme boots ： Why is extreme boots called extreme boots ？ - You know (zhihu.com)）, As shown in the figure below , because d Pole shoe protection exists in the magnetic circuit of the shaft ,d Equivalent air gap ratio of shaft q Larger shaft ,d Equivalent magnetic circuit ratio of shaft q The equivalent magnetic circuit of the shaft is longer , It takes longer for the magnetic field with the same strength to pass d Axis , therefore dq There is asymmetry in the axial magnetic circuit , It is a salient pole motor .

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Making this asymmetry requires special processing , This not only leads to the salient pole motor is much more expensive than the hidden pole motor , There are also obvious differences in applications .

The fourth dimension ： Differences in control strategies

As mentioned above , For salient pole motors , The torque expression is as follows ：

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For non salient pole motors , The torque expression is as follows ：

  The output torque of salient pole motor is not only determined by q Shaft current （ Torque current ） control , Also received d Influence of shaft current , The output torque of the hidden pole motor is only q Shaft current （ Torque current ） control ,d The shaft current only controls the size of the magnetic field , Therefore, the control of salient pole motor is more complex .

Control with the maximum torque current ratio MTPA Take an example to illustrate , The Lagrange equations of the torque current of salient pole motor and salient pole motor are as follows ：

 

At the maximum torque output of salient pole motor id The expression of is ：

  When the torque of the hidden pole motor is the maximum output id Expression for ：

You can see , When salient pole motor and salient pole motor realize this maximum torque current ratio control , The process is different . here MTPA And id=0 The difference of control, let's dig a hole first , Fill in later .

The fifth dimension ： There are differences in the design of observers

Take the sliding mode observer as an example , The stator voltage equation of permanent magnet synchronous motor is as follows ：

After the coordinate transformation to the two-phase static coordinate system , As shown in the figure below ：

  Salient pole motor is the model above , And hidden pole motor Ld = Lq, The two phases are directly 0. The expression is as follows

  There are also differences between the two extended back EMF expressions , The extended back EMF of salient pole motor is ：

The extended back EMF of the salient pole motor is ：

  The extended back EMF of salient pole motor includes the motor inductance 、 electric current 、 Speed factor , It is very difficult to solve the electrical angle of the motor theta, Therefore, the observation of rotor position and rotor speed is more complex . But let's look at the salient pole motor extended back EMF , Direct pair Ealpha and Ebeta Find an inverse tangent , The most critical rotor angle comes out .

The same is true of other observers , Whether full order or reduced order 、 Extended Kalman method , The complexity of observation matrix of salient pole motor is not a level compared with that of hidden pole motor , by the way Ld-Lq The interference caused by this inductive reactance component , The speed and position of the motor rotor become very complex .

In addition, it is worth mentioning that , Salient pole motor also has a very good place in observer , The salient effect caused by salient polarity can provide natural conditions for high-frequency signal injection method to extract rotor position and speed , Hidden pole motors do not have this condition . Therefore, in the current application of new energy electric vehicles , Low speed senseless strategy , Hidden pole motors have always been difficult to overcome , However, various high-frequency injection methods work well in the low-speed inductive strategy of salient pole motors .

Summary ：

1、 Judge whether the permanent magnet synchronous motor is a salient pole motor , In terms of parameters, it can be based on Ld And Lq Size relationship between , The equivalent is the hidden pole motor , Unequal is salient pole motor .

2、 Judge whether the permanent magnet synchronous motor is a salient pole motor , Structurally, it can be based on dq Judge whether the axial magnetic circuit is symmetrical ,dq If the axis equivalent magnetic circuit is equal and the magnetic circuit is symmetrical, it is a hidden pole motor ,dq The shaft equivalent magnetic circuit is unequal, and the magnetic circuit is asymmetric, which is a salient pole motor .

3、 The position of the permanent magnet in the iron core is the source of all designations , According to the position of the permanent magnet in the rotor core, it can be divided into surface type and built-in type , Among them, surface type is divided into surface paste type and plug-in type . Plug in 、 The built-in type will cause the asymmetry of the magnetic circuit , Therefore, in fact, only surface mounted permanent magnet synchronous motors are hidden polarity motors .（ This conclusion is actually quite absolute , But what I've seen so far are all tables posted like this , So if you have more precise comments, leave your opinions .）

4、 There are differences in the above five dimensions between salient pole motor and hidden pole motor in my understanding , Both control and application are worth exploring separately .