Analysis of motor core manufacturing

Analysis of core quality

The motor core is a whole composed of many punching sheets stacked together. The quality of punching directly affects the quality of core press-fitting, and the quality of core will have a great impact on the quality of motor products. For example, uneven slot shape will affect the quality of coin embedding, too large burrs, and the dimensional accuracy and tightness of the core will affect the magnetic conductivity and loss. Therefore, ensuring the manufacturing quality of punching sheets and cores is an important part of improving the quality of motor products.

Quality issues of punching sheets

The quality of punching sheets is related to factors such as die quality, structure, precision of punching equipment, punching process, mechanical properties of punching sheet materials, and shape and size of punching sheets.

1. Accuracy of punching sheet size.

The dimensional accuracy, coaxiality, and accuracy of slot position of punching sheets can be guaranteed from silicon steel sheets, punching dies, punching schemes, and punching machines.

From the perspective of punching dies, reasonable gaps and die manufacturing accuracy are necessary conditions to ensure the accuracy of punching sheet size. When a compound die is used, the dimensional accuracy of the working part is mainly determined by the manufacturing accuracy of the die, and has little to do with the working state of the punch.

According to the technical conditions, the difference in the width accuracy of the stator teeth is no more than 0.12mm, and the allowable difference of individual teeth is 0.20mm.

2. Burrs.

If the gap between the die is too large, the die is not installed correctly, or the die edge is blunted, the punching sheet will produce burrs. To fundamentally reduce the burrs, it is necessary to strictly control the gap between the punch and the die during mold manufacturing; when the die is installed, the gap on each side must be uniform, and the normal operation of the die must be ensured during punching. The size of the burrs should be checked frequently and the edge should be sharpened in time;

3. Burrs can cause short circuits between the core sheets, increase iron loss and temperature rise.

Strictly control the core to achieve the press-fit size. Due to the presence of burrs, the number of punching sheets will be reduced, causing an increase in excitation current and a decrease in efficiency. The burrs in the slots will pierce the winding insulation and cause the teeth to bulge outward. If the burr at the rotor shaft hole is too large, it may cause the hole size to be reduced or the ellipticity to be increased, making it difficult to press the core on the shaft. When the burr exceeds the specified limit, the mold should be repaired in time.

4. The punching sheet is incomplete and unclean: when there are ripples, rust, oil or dust, the press-fit coefficient will be reduced.

In addition, the length should be controlled during press-fitting. If the sheet is reduced too much, the core weight will be insufficient, the magnetic circuit cross-section will be reduced, and the excitation current will increase. If the insulation of the punching sheet is not well treated or managed, the insulation layer will be destroyed after press-fitting, making the core moderate and the eddy current loss will increase.

Key points of the core press-fitting process;

A. When stacking, stack the sheets in the direction of punching;

B. The burr direction is consistent, and the slots of the same number are stacked together;

C. The keyway of the rotor punching sheet acts as a marking slot;

D. When casting aluminum, the keyway and the forged shaft key cooperate to ensure the slot slope;

Technical requirements (tightness, accuracy, and firmness) that the stator core press-fitting should meet

● Three process parameters for core press-fitting: pressure, core length, and core weight

● A certain pressure is maintained between the punching sheets, generally (6.69~9.8)×105pa

● The weight must comply with the drawing, and the core length should be guaranteed. After stacking, L1±0.5mm is allowed

● The slot shape should be smooth, and the inner size of the slot is allowed to be 0.2mm smaller than the slot shape of a single punching sheet

● Stacking is reliable and fastened

Quality problems of core press-fitting

1. The stator core length is greater than the allowable value

The length of the stator core is much longer than the length of the rotor core, which is equivalent to an increase in the effective length of the air gap, which increases the air gap magnetic flux potential (increases the excitation current) and the stator current (increases the stator copper loss).

In addition, the effective length of the core increases, which increases the leakage resistance coefficient and the motor leakage resistance.

2. The stator core tooth bounce is greater than the allowable value.

This is mainly due to the excessive burrs of the stator punching sheet, and its influence is the same as above.

3. The stator core is not heavy enough.

It reduces the net length of the stator core, reduces the cross-sectional area of the stator teeth and the stator choke, and increases the magnetic flux density.

The reasons for the insufficient weight of the core are:

① The burrs of the stator punching sheet are too large;

② The thickness of the silicon steel sheet is uneven;

③ The punching sheet is rusty or stained with dirt;

④ The pressure is insufficient due to oil leakage or other reasons during press-fitting.

4. Stator core uneven

A. Outer circle uneven

For enclosed motors, the outer circle of the stator core and the inner circle of the base are not in good contact, which affects the heat conduction and increases the temperature of the motor. Because the thermal conductivity of air is very poor, only 0.04% of the core, even a small gap will greatly affect the heat conduction.

B. Inner circle uneven

If the inner circle is not ground, the stator and rotor cores may rub against each other; if the inner circle is ground, it will increase the working hours and increase the iron loss.

C. Slot wall uneven

If the slot is not filed, it is difficult to embed the wire and it is easy to damage the slot insulation. If the slot is filed, the iron loss will increase.

D. Slot uneven

If the slot is not filed, it is difficult to embed the wire. If the slot is filed, the stator card coefficient will increase, the effective length of the air gap will increase, the excitation current will increase, and the rotational iron loss (i.e. rotor surface loss and pulsation loss) will increase.

The reasons for the uneven stator core are as follows:

A. The punching sheets are not pressed in sequence;

B. The burrs of the punching sheets are too large;

C. The slot sample bar becomes smaller due to poor manufacturing or wear;

D. The inner circle of the stacking tool cannot expand the inner circle of the stator core due to wear;

E. The slots of the stator punching sheets are uneven, etc.

The stator core is uneven and needs to be filed, which will reduce the quality of the motor. In order to avoid grinding and filing the stator core, the following measures need to be taken:

A. Improve the manufacturing accuracy of the die;

B. Realize single-machine automation, so that the punching sheets are stacked in sequence and pressed in sequence;

C. Ensure the accuracy of the application of process equipment such as the mold and slot sample bar generated when the stator core is pressed

D. Strengthen the quality inspection of each process during the punching and pressing process.