Design parameters and values of permanent magnet synchronous motor
1.I, P, Z, n values (the most relevant parameters of the motor design)
a) pole log p
The more pole pairs, the more favorable it is to increase the torque density and power density of the motor.
Without considering the rotor leakage, the more pole pairs of the motor, the weaker the armature's armature response to the rotor.
Considering the actual process capability and the mechanical strength of the stator and rotor, too many pole pairs will cause the drain coefficient to be too large, and the shrinkage-matched armature slot area will be too small, which will be beneficial to the increase of power density.
The pole pair determines the operating frequency of the motor at a certain speed, so the maximum number of poles allowed by the motor can be obtained according to the ability of the motor lock to match the switch components of the controller and the motor lock to the maximum speed.
Priority recommendation: Meet motor controllability - to meet motor manufacturability - take as many poles as possible.
b) index Z (in general, you must first decide p and then determine Z)
(Concept: the number of slots per phase of each group is Q=Z/(3*2*P). When Q is an integer, it is called integer slot winding, otherwise it is called fractional slot winding)
If the high-power motor for the vehicle selects the concentrated winding, then the motor has Q=0.5, then the number of slots is Z=3*P, and there are also a few small-power motors with 8-pole 9-slot or 10-pole 12-slot.
The larger the Q value is, the smaller the back EMF spectrum of the motor is, and the smaller the fluctuation of the cogging torque and torque of the motor is, but the improvement effect of the spectral wave can be neglected according to the experience of Q>3.
Since the power of the drive motor is large and the number of single-phase series turns is small, it is often necessary to select the appropriate number of slots Z to ensure reasonable motor turns.
The commonly used Q value of the motor is recommended as: Q=0.5; Q=1.5; Q=2; Q=2.5; Q=3 (the flat copper wire winding process is often applied when the vehicle drive motor Q takes a larger value.
C) Number of turns N
As the number of turns increases, the back EMF coefficient of the motor increases, and the torque increases at the same current.
The increase in the number of turns means that the cross-sectional area of the conductor is reduced, which may cause the problem of excessive thermal load on the armature.
Changing the volume of the motor changes the magnetic flux area of the motor. Changing the magnetic circuit structure can change the air gap magnetic close-coupled arc coefficient, creating conditions for adjusting the motor turns.
2. Main size determination
1) Armature outer diameter value
Under normal circumstances, according to the size requirements of the whole vehicle, the outer diameter of the stator core is obtained by removing the thickness of the outer casing. The thickness of the motor casing varies with the outer dimensions of the motor and the casing process. Water-cooled casing, the thickness of the casing is recommended to range from 18 to 30mm
2) Armature inner diameter value
Definition: After determining the outer diameter of the stator, the inner diameter of the armature can be determined. The key is to design the ratio of the inner and outer diameter of the motor.
Influence: The larger the Kd value is, the smaller the influence of the motor armature magnetic potential is, but the rotor magnetic flux is increased, the rotor magnetic potential is enhanced, and the power capability of the motor is easy to be improved, but the copper loss of the motor needs to be increased, and vice versa. Power capability, but can create conditions for improving motor efficiency. Kd value also affects the size and shape of the armature slot. The smaller the Kd value, the deeper the slot, the smaller the armature slot, and the slot leakage resistance increases.
3) Motor stator and rotor air gap selection
The smaller the air gap, the better the performance of the motor is improved, but the electric noise also likes the problem of too small. The assembly accuracy of the parts with too small clearance requirements is too high, and the centrifugal force deformation at the high speed of the rotor cannot be adapted. The size of the rotor can be determined mainly by the relevant process level and the deformation of the rotor under high speed conditions.
3. Magnetic density value
a) relationship between output and magnetic density
Electromagnetic force: F=BIL
Electromagnetic torque: Te=BINLfeR=BJV
The torque density of the motor depends on the load magnetic density in the air gap of the motor and the current density of the inner conductor of the stator.
b) There are two ways for the motor to obtain a higher magnetic density:
High magnetic force (increased magnetic field strength)
High current density (increased magnetic permeability of material materials, which is difficult in current technology)
C) Value of empty and load magnetic density
No-load: Under the premise of satisfying the magnitude of the back EMF, it is recommended to take a lower no-load stator magnetic density and a reasonable rotor magnetic density.
Peak load:
Properly adjust the AC and DC current distribution ratio of the motor to alleviate the saturation of the magnetic circuit without sacrificing the torque of the motor.
The stator and rotor are mostly saturated, but the magnetic circuit should be optimized to reduce the saturation of the magnetic circuit caused by the armature leakage reactance.
4. Back EMF value
a) Influence of back EMF on motor and controller
Under the condition that the working current of the motor is constant, the output torque of the motor is proportional to the back potential of the motor. Increasing the motor back EMF can reduce the operating current of the motor at the same output torque
When the motor is not weakly magnetically operated, the working speed of the motor is inversely proportional to the back electromotive force under the condition that the motor voltage is constant. For controlling the synchronous motor, the magnitude of the back electromotive force basically determines the inflection point position of the peak torque of the motor.
The highest back EMF threatens the safety of the main components of the controller (capacitors and IGBTs). Excessive back EMF can cause device damage.
b) the value of the back EMF
In general, the existing film capacitors in the market can withstand a back EMF of less than 500V; for a 300V power supply system, if the device is customized, the value is generally less than 700v, in reality more than 650v, the film capacitor will be broken down regardless of whether it is working or not. Therefore, many factories require values below 450v.
