Quality control and inspection of motor manufacturing

Motor is an electromagnetic mechanism to realize electromechanical energy transmission and conversion. Compared with general machines, the products have the same: similar mechanical structure, different: special conductive and magnetic conduction and insulation structure; Technological similarities: casting, forging, machining, stamping and assembly. Differences: unique iron core punching, winding manufacturing, paint dipping and plastic sealing parts.
The motor process has the following characteristics:

  • (1) There are many types of work and a wide range of processes;
  • (2) There are many non-standard equipment and non-standard tooling;
  • (3) There are many kinds of manufacturing materials;
  • (4) High machining accuracy;
  • (5) Large amount of manual labor.

Manufacturing of iron core of motor

The core manufacturing process is divided into two parts.

Key points of stamping process for silicon steel sheet

  • A. The dimensional accuracy of punching sheet includes inner circle, outer circle, shaft hole groove shape and groove bottom;
  • B. Burr of punching piece, stator 0.05, re punching 0.1, rotor 0.1;
  • C. Coaxiality of inner and outer circles: ovality greater than 0.06;
  • D. The groove shape position is correct and the groove position is marked;
  • E. The surface is flat.

Key points of press mounting process of iron core

  • A. Lamination according to the punching direction during lamination;
  • B. The burr direction is consistent and the same groove is stacked together;
  • C. The keyway of rotor punching plate acts as a marking groove;
  • D. When casting aluminum, the keyway is matched with the forged shaft key to ensure the groove inclination.

The press mounting of stator core shall meet the technical requirements (tightness, accuracy and firmness)
Three process parameters of iron core press fitting: pressure, iron core length and iron core weight.

  • A certain pressure is maintained between punching plates, generally (6.69 ~ 9.8) × 105pa;
  • The weight shall comply with the drawing, the length of iron core shall be ensured, and L1 ± 0.5mm is allowed after lamination;
  • The groove shape shall be smooth, and the size in the groove is allowed, which is 0.2mm smaller than the groove shape of single sheet punching;
  • The lamination is firm and reliable.

Quality analysis of iron core

The motor core is a whole made up of many punched pieces. The quality of punched pieces directly affects the quality of core press fitting, and the quality of iron core will have a great impact on the quality of motor products. For example, the irregular groove shape will affect the quality of embedded money, the burr is too large, and the dimensional accuracy and tightness of iron core will affect the magnetic conductivity and loss. Therefore, Ensuring the manufacturing quality of stamping and iron core is an important link to improve the quality of motor products.

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Quality problems of stamping

The quality of punching sheet is related to the quality of punching die, structure, accuracy of punching equipment, punching process, mechanical properties of punching sheet materials, shape and size of punching sheet, etc.
1. Accuracy of punch size. The dimensional accuracy, coaxiality and groove position accuracy of the stamping sheet can be guaranteed from the aspects of silicon steel sheet, stamping die, stamping scheme and punch press. From the aspect of die, reasonable clearance and die manufacturing accuracy are the necessary conditions to ensure the accuracy of die size. When the compound die is used, the dimensional accuracy of the working part mainly depends on the manufacturing accuracy of the die, but has nothing to do with the working state of the punch. According to the technical conditions, the difference of stator tooth width accuracy is no more than 0.12mm, and the allowable difference of individual teeth is 0.20mm.
2. Burr. If the die clearance is too large, the die is installed incorrectly or the die edge is blunt, it will produce burrs on the die. To fundamentally reduce the burrs, the gap between the punch and the female die must be strictly controlled during die manufacturing; During the installation of the die, ensure the uniform clearance of each side, ensure the normal operation of the die during punching, regularly check the size of the burr and grind the edge in time;
3. Burr will cause inter chip short circuit of iron core and increase iron consumption and temperature rise. Strictly control the pressing size of the iron core. Due to the existence of burrs, the number of punching plates will be reduced, resulting in the increase of excitation current and the reduction of efficiency. The burrs in the slot will stab the winding insulation and cause the external expansion of the teeth. The burr at the rotor shaft hole is too large, which may cause the reduction of hole size or ovality, making it difficult to press and install the iron core on the shaft. When the burr exceeds the specified limit, the mold shall be repaired in time.

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4. Incomplete and unclean punching: when there is ripple, rust, oil stain or dust, the press fitting coefficient will be reduced. In addition, the length should be controlled during press fitting. Too much reduction will not enough weight of the core, reduce the magnetic circuit section and increase the excitation current. Poor processing or management of punching insulation, and the insulation layer is damaged after press fitting, so that the iron core is moderate and the eddy current loss increases.

Quality problems of iron core press fitting

1. The length of stator core is greater than the allowable value. The stator core length is too much longer than the rotor core length, which is equivalent to the increase of the effective length of the air gap, which increases the air gap magnetic flux potential (excitation current) and the stator current (stator copper consumption). In addition, the increase of the effective length of the core increases the leakage reactance coefficient and motor leakage reactance.
2. The spring of stator core teeth is greater than the allowable value. This is mainly due to the excessive burr of stator punching plate, and its influence is the same as above.
3. The weight of stator core is not enough. It reduces the net length of stator core, reduces the cross-sectional area of stator teeth and stator choke, and increases the magnetic flux density. The reasons for insufficient weight of iron core are:

  • ① The burr of stator punching plate is too large;
  • ② Uneven thickness of silicon steel sheet;
  • ③ The punched sheet is rusty or stained with dirt;
  • ④ During press fitting, the pressure is insufficient due to oil leakage of hydraulic press or other reasons.

The reasons for uneven stator core are as follows:

  • A. The punching plate is not pressed in the forward direction in sequence;
  • B. The punching burr is too large;
  • C. The groove sample bar becomes smaller due to poor manufacturing or wear;
  • D. The inner circle of the laminating tool cannot expand the inner circle of the stator core due to wear;
  • E. The stator punching groove is not neat, etc.

4. Uneven stator core

  • A. Uneven outer circle: for closed motor, the outer circle of stator core is not in good contact with the inner circle of base, affecting heat conduction and increasing motor temperature. Because the thermal conductivity of air is very poor, only 0.04% of that of iron core, even if there is a small gap, the thermal conductivity is greatly affected.
  • B. Uneven inner circle: if the inner circle is not ground, the stator and rotor cores may rub; If the inner circle is ground, not only the working hours will be increased, but also the iron consumption will be increased.
  • C. Uneven groove wall: if the groove is not filed, it is difficult to embed wires, and it is easy to damage the groove insulation. If the groove is filed, the iron loss increases.
  • D. Uneven notch: if the notch is not filed, it is difficult to embed the wire. If the notch is filed, the clamping coefficient of the stator increases, the effective length of the air gap increases, the excitation current increases, and the rotating iron loss (i.e. rotor surface loss and pulsation loss) increases.

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Slot filing is required due to uneven stator core, so as to reduce the quality of the motor. In order to prevent stator core from grinding and filing, the following measures shall be taken:

  • Improve die manufacturing accuracy;
  • Realize single machine automation, so that the punching pieces are stacked and pressed in sequence;
  • Ensure the application accuracy of tooling, slot sample bar and other process equipment generated during stator core press fitting;
  • Strengthen the quality inspection of each process in the process of punching and press fitting.

Quality analysis of cast aluminum rotor

The quality of cast aluminum rotor directly affects the technical and economic indexes and operation performance of asynchronous motor. When studying the quality of cast aluminum rotor, we should not only analyze the casting defects of rotor, but also understand the impact of cast aluminum rotor quality on motor efficiency, power factor, startup and operation performance.
1. Relationship between aluminum casting method and rotor quality
The additional loss of cast aluminum rotor is much greater than that of copper bar rotor asynchronous motor, and the cast aluminum methods are different. The additional loss is also different, and the additional loss of pressure cast aluminum rotor motor is the largest. This is because the strong pressure during die casting makes the cage bar and the iron core contact very closely, and even the molten aluminum is squeezed between the laminations, the transverse current increases, and the additional loss of the motor increases greatly. In addition, during die casting, due to the fast pressurization speed and high pressure, the air in the mold cavity can not be completely eliminated, and a large amount of gas is densely distributed in the rotor cage bar, end ring, wind blade, etc. as a result, the proportion of aluminum in the cast aluminum rotor is reduced (about 8% less than that of centrifugal cast aluminum) and the average resistance is increased (about 13%), which greatly reduces the main technical and economic indexes of the motor, Centrifugal cast aluminum rotor is easy to produce defects due to various factors, but the additional loss of the motor is small. During low-pressure aluminum casting, the molten aluminum directly comes from the inside of the crucible, and the “slow” low-pressure casting is adopted, with better exhaust; When the guide bar solidifies, the aluminum water is supplemented by the upper and lower rings. Therefore, the quality of low-pressure cast aluminum rotor is excellent.
2. Influence of rotor quality on motor performance
The quality of cast aluminum rotor has a great impact on the performance of the motor. The causes of these defects and their impact on the performance of the motor are discussed in detail below.
(1) The reasons for insufficient weight of rotor core are as follows:

  • The burr of rotor punching plate is too large;
  • Uneven thickness of silicon steel sheet;
  • The rotor punching plate is rusty or unclean;
  • The pressure is small during press fitting (the press fitting pressure of rotor core is generally 2.5 ~. MPa).

The preheating temperature of cast aluminum rotor core is too high and the time is too long, the core is seriously burned, and the net length of the core is reduced. If the weight of rotor core is not enough, it is equivalent to reducing the net length of rotor core, reducing the cross-sectional area of rotor teeth and rotor choke, then the magnetic flux density increases. The influence on the motor performance is that the excitation current increases, the power factor decreases, the motor stator current increases, the rotor copper loss increases, the efficiency decreases and the temperature rise increases.
(2) The reasons for rotor misalignment are as follows:
The rotor core is not positioned with groove bar during press fitting, and the groove wall is not neat.

  • The fit clearance between the oblique key on the dummy shaft and the keyway on the punching plate is too large;
  • The pressure during press fitting is small. After preheating, the burr and oil stain of the punching plate are burned to loosen the rotor plate;
  • After the rotor is preheated, it is thrown and rolled on the ground, resulting in angular displacement of rotor punching.

The above defects will reduce the rotor slot, increase the leakage reactance of the rotor slot, reduce the section of the guide bar and increase the resistance of the guide bar, and have the following effects on the motor performance: the maximum torque is reduced, the starting torque is reduced, the reactance current at full load is increased and the power factor is reduced; The stator and rotor current increases, and the stator copper consumption increases; The rotor loss increases, the efficiency decreases, the temperature increases and the slip rate is large.
(3) The rotor chute width is greater than or less than the allowable value
The reason why the width of the inclined slot is greater than or less than the allowable value is mainly that the inclined key on the dummy shaft is not used for positioning during the press fitting of the rotor core, or the inclined size of the inclined key is out of tolerance during the design of the dummy shaft.
The impact on motor performance is:
(1) When the chute width is greater than the allowable value, the leakage reactance of rotor chute increases and the total leakage reactance of motor increases; When the length of the guide bar increases, the resistance of the guide bar increases, which has the same impact on the motor performance (2).
(2) When the chute width is less than the allowable value, the leakage reactance of rotor chute decreases, the total leakage reactance of motor decreases and the starting current increases (because the starting current is inversely proportional to the leakage reactance). In addition, the noise and vibration of the motor are large.
(3) Broken rotor bars are caused by:

  • The rotor core is pressed too tightly, the rotor core expands after aluminum casting, and excessive tension is added to the aluminum strip to break the aluminum strip.
  • Early demoulding after aluminum casting, the molten aluminum did not solidify well, and the aluminum strip broke due to the expansion force of the iron core.
  • There are inclusions in the rotor core groove before aluminum casting.

Manufacturing of winding

Winding is the heart of motor. Its life and operation reliability mainly depend on the manufacturing quality of winding, electromagnetic effect in operation, mechanical vibration and environmental factors;
The selection of insulating material and structure, insulation defects and insulation treatment quality in the manufacturing process directly affect the winding quality. Therefore, attention should be paid to winding manufacturing, winding falling binding and insulation treatment.
Winding manufacturing, winding materials: copper conductor and aluminum conductor;
Most of the electromagnetic wires commonly used in motor windings are insulated wires. Therefore, the conductor insulation is required to have sufficient mechanical strength, electrical strength, good solvent resistance and high heat resistance. The thinner the insulation is required, the better.

Insulating material

Insulating material is a material with high resistivity, and the current flowing through it can be considered as negligible. Generally, the resistivity is greater than 107 Ω * M.
1. Electrical performance:

  • ① Dielectric strength (also called insulation strength or breakdown strength)
  • ② Insulation resistivity kV/MMM Ω ratio of applied voltage of insulating material/leakage current of insulating material;
  • ③ Dielectric constant, energy of static charge storage capacity;
  • ④ Dielectric loss, energy loss generated in alternating magnetic field;
  • ⑤ Corona resistance, arc resistance and leakage resistance.

2. Thermal properties: the thermal properties of insulating materials include heat resistance rating, heat shock resistance, heat expansion coefficient, thermal conductivity and curing temperature.
3. Mechanical properties: such as peeling resistance, scratch resistance and bending resistance of enamelled wire paint. For groove insulation and heat-resistant insulation, it is required to have certain compression resistance, tensile strength, bending resistance, shear resistance, bonding humidity, impact toughness and hardness.
4. Physical and chemical properties: refer to water absorption, acid and alkali resistance, solvent resistance, mildew resistance, etc.

Quality inspection of coil

The quality inspection of stator winding after wire embedding includes appearance inspection, DC resistance measurement and withstand voltage test.
1. Appearance inspection

  • 1) The dimensions and specifications of materials used for inspection shall comply with the provisions of drawings and technical standards.
  • 2) The winding pitch shall comply with the provisions of the drawing, the connection between windings shall be correct, the straight part shall be straight and neat, there shall be no serious crossing at the end, and the insulation shape at the end shall comply with the provisions.
  • 3) The slot wedge shall have sufficient tightness. If necessary, it shall be checked with a spring scale. Its end shall not be broken. The slot wedge shall not be higher than the inner circle of the iron core, and the length extending from both ends of the iron core shall be equal.
  • 4) Check the shape and size of the winding end with a sample plate, which shall meet the requirements of the drawing, and the end binding shall be firm.
  • 5) Both ends of the slot insulation shall be repaired reliably. For motors with less than 36 slots, it shall not exceed three places and shall not be broken to the iron core.

2. Allowable DC resistance ± 4%
3. Withstand voltage test
The purpose of voltage withstand test is to check whether the insulation strength of winding to ground and between windings is qualified. The withstand voltage test is conducted twice, one after wire embedding and one during the delivery test of the motor. The test voltage is AC, the frequency is 50Hz and the actual sinusoidal waveform. During delivery test, the effective value of test voltage is 1260v (P2 < 1kW) or 1760v (P2 ≥ 1kW); When the test is conducted after wire embedding, the effective value of the test voltage is 1760v (P2 < 1kW) or 2260v (P2 ≥ 1kW). The stator winding shall be able to withstand the above voltage for 1min without breakdown.

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Quality inspection of winding insulation treatment

1. Electrical performance of winding
The electrical breakdown strength of insulating paint is dozens of times that of air. After insulation treatment, the air in the winding is replaced by insulating paint, which improves the initial free voltage and other electrical properties of the winding;
2. Moisture resistance of winding
After the winding is impregnated, the insulating paint fills the capillary and gap of the insulating material, and forms a layer of dense and smooth paint film on the surface, which makes it difficult for water to immerse in the winding, so as to significantly improve the moisture resistance of the winding.
3. Thermal conductivity and heat resistance of winding
The thermal conductivity of insulation is much better than that of air. After impregnation, the thermal conductivity of the winding can be significantly improved. At the same time, the aging speed of insulating materials slows down and the heat resistance is improved.
4. Mechanical properties of winding
After the winding is impregnated, the conductor and insulating material are bonded into a solid whole, which improves the mechanical properties of the winding and can effectively prevent insulation loosening and wear caused by vibration, electromagnetic force, thermal expansion and cold contraction.
5. Chemical stability of winding
The paint film formed after insulation treatment can prevent the insulation material from being damaged by direct contact with harmful chemical media. After special insulation treatment, the winding can also have the ability of anti mildew, anti corona and anti oil stain, so as to improve the chemical stability of the winding.

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Process characteristics of motor assembly

The characteristics of motor assembly are mainly determined by the use requirements and structural characteristics, mainly including:

  • (1) All parts shall be interchangeable. That is to say, during the structural design, each part should have clear requirements for size, geometric tolerance and surface roughness, which is the basis to ensure the product quality of micro and special motors. When the parts of some precision micro and special motors are completely interchangeable and can not meet the requirements, they need to be assembled in groups.
  • (2) Ensure shaft assembly quality. Shaft assembly has a great impact on motor life, noise, static friction and temperature rise. The shaft accuracy and installation requirements of each sharp motor are different, which shall be clearly specified and guaranteed in technology.
  • (3) Ensure the coaxiality of stator and rotor and the perpendicularity of end cover bearing installation. If necessary, the inspection of assembly coaxiality and verticality can be increased during assembly.
  • (4) Ensure the static and dynamic balance requirements of the rotor. Because the static imbalance and dynamic imbalance make the motor produce additional torque during operation. The light ones have vibration and noise, and the heavy ones may have chamber sweeping, resonance, etc. Special equipment is required for careful calibration.
  • (5) Special attention shall be paid to the deformation and damage of light, small and thin-walled parts. There are many light and small parts and thin-walled parts of motor, with poor stiffness and easy deformation. During processing and assembly, special tools must be used for transmission, transfer and storage. It shall not be subjected to undue external force to cause deformation and damage.

In addition, the assembly process route shall adapt to the production batch. For mass-produced motors, assembly can be carried out in a flow process. The assembly process is divided into very detailed processes to ensure quality step by step. For multi variety and small batch products, color group process assembly should be adopted, which is often divided into stator and rotor. Unified special process procedures can be formulated for the general assembly process, including the specific requirements of each product. In this way, it is convenient to ensure the quality. If necessary, an intermediate inspection process can be added.
Relevant national ministries: formulate some general standards according to the commonness of various motors and a large class of motors. According to the special requirements of a series or a variety, the standards are formulated. Each enterprise shall formulate detailed rules for the implementation of standards according to its own situation and formulate enterprise special product standards.
In standards at all levels, especially in national standards, there are mandatory standards, recommended standards and guiding standards.
2. Standard number composition:

  • Part I: composed of letters (Chinese, Chinese pronunciation). Indicates: standard level, international industrial standard and enterprise standard; Nature: compulsion, recommendation and guidance;
  • Part II: for example, gb755 is the National Standard No. 755, and Arabic numerals are used to represent the serial number in this level of standard.
  • Part III: Yes – separate from part II, use Arabic numerals to indicate the year of implementation.

Source: China Permanent Magnet Manufacturer www.ymagnet.com

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