By calculating the magnetic circuit of the permanent magnet motor, the performance of the initial design scheme is checked using relevant formulas to determine whether the error between the calculated results and the motor performance requirements is within the allowable range.
The process of magnetic circuit design
1. The following data (performance requirements) are usually given when designing a motor:
1) Rated power; 2) Rated voltage; 3) Number of phases and phase connection method;
4) Rated frequency; 5) Rated speed or synchronous speed; 6) Rated power factor, etc.
2. Motor Design Process
1) Preparation stage: Collect data, including relevant national standards, similar motor technical data, and test data. Based on the analysis of this data, prepare the technical task specification.
2) Electromagnetic design: According to the technical task specification, perform electromagnetic calculations to determine the lamination size, core length and electromagnetic performance of the designed motor.
3) Structural design: Determine the mechanical structure of the motor, the dimensions of its components, processing requirements, and the specifications of its materials.
• Magnetic circuit design involves rationally selecting the parameters and materials of the magnetic circuit based on the requirements of the magnetic field, and designing a magnetic circuit that is feasible in terms of process, meets the requirements in terms of characteristics, is economical, and can give full play to the performance of the materials.
• For a given magnetic circuit, its magnetic circuit characteristics can be uniquely obtained.
• Given the required magnetic circuit characteristics, there may be multiple magnetic circuits that meet the requirements. The purpose of the design is to find a magnetic circuit that meets the requirements.
Preliminary determination of magnetic circuit structure
Different rotor magnetic circuit structures result in different motor operating performance, control systems, manufacturing processes, and applicable applications. Based on the position of the permanent magnets on the rotor, the rotor magnetic circuit structure of permanent magnet synchronous motors is generally divided into two types: surface-mounted and embedded.
1) Surface-mounted protruding rotor magnetic circuit structure
Its structure is simple, its manufacturing cost is low, and its rotational inertia is small. It is mostly used in rectangular wave permanent magnet synchronous motors and permanent magnet synchronous motors with a narrow constant power operating range.
2) Surface-mount insert rotor magnetic circuit structure
This structure can fully utilize the reluctance torque generated by the asymmetry of the rotor's magnetic circuit, thereby increasing the motor's power density. The manufacturing process is also relatively simple. It is commonly used in certain speed-regulating permanent magnet synchronous motors.
There are also built-in and built-in hybrid types:
Determine the dimensions and materials of each part of the magnetic circuit.
Permanent magnets and their properties vary widely, and the selection of suitable permanent magnet materials directly affects the performance and economy of motors. The selection of permanent magnets should meet the following requirements:
The permanent magnet should be able to generate the required magnetic field within the specified workspace;
The magnetic field established by the permanent magnet should have a certain degree of stability, and the magnetic properties should be within the allowable range as the operating temperature and environment change.
It has good corrosion resistance;
It has good mechanical properties, including good toughness, high compressive strength, and machinability;
Reasonable price and good economic efficiency are the reasons for choosing permanent magnet materials.
• Ferrite: Suitable for applications where the size, weight, and performance requirements of the motor are not high, but the economic efficiency of the motor is.
• AlNiCo: Suitable for applications where the size, weight, and performance requirements of the motor are not high, but the operating temperature exceeds 300 degrees Celsius or good temperature stability is required and the cost of the motor is not high.
• Neodymium iron boron: Suitable for applications where the size, weight and performance of the motor are very important, the operating temperature is not high, and the temperature stability of the permanent magnet is not a major concern.
• Rare earth cobalt: Suitable for applications where there are high requirements for motor size, weight and performance, high operating temperature, good temperature stability, and where manufacturing cost is not a major consideration.
• Bonded permanent magnet materials: suitable for applications with large batch production, complex magnetic pole shapes, and low motor performance requirements.
Selection of permanent magnet size:
• When designing a motor, in order to make full use of permanent magnet materials and reduce the size of the permanent magnet and the entire motor, efforts should be made to create a magnetic field with the greatest magnetic energy in the air gap using the smallest permanent magnet volume.
For the sake of simplicity, we will begin our analysis with permanent magnet materials whose demagnetization curve is a straight line. Let the magnetic flux provided by the permanent magnet be Φ, and the magnetomotive force be F, then the magnetic energy is...
Thus, the volume (cm3) of the permanent magnet can be obtained.
The equivalent magnetic circuit calculation method is used to calculate the characteristics of the magnetic circuit and determine whether the error is within the allowable range.
• The equivalent magnetic circuit method transforms the non-uniformly distributed magnetic field within the motor space into equivalent multi-segment magnetic circuits. It approximates the magnetic flux in each segment as uniformly distributed along its cross-section and length, converting the calculation of the magnetic field into the calculation of the magnetic circuit. Various coefficients are then used for correction, ensuring that the magnetic potential difference between each segment equals the magnetic potential difference between corresponding points in the magnetic field. Through accumulating experience and obtaining various practical correction coefficients, the calculation accuracy can meet the needs of practical engineering applications.
• By calculating the magnetic circuit of the permanent magnet motor, the performance of the initial design scheme is checked using relevant formulas to determine whether the error between the calculated results and the motor performance requirements is within the allowable range. If it is within the allowable range, the magnetic circuit design is complete; otherwise, the size, material, and structure of the magnetic circuit need to be adjusted until a reasonable magnetic circuit is obtained.
