Technical Requirements for Transformer Core Pressing

The task of core pressing is to arrange, compress, and fix a certain number of laminations into a single, uniformly sized, and tightly packed unit. After core pressing, the core should meet the following technical requirements:

1. Core weight meets the drawing requirements, and the deviation should generally not exceed +3% to -1%.

Insufficient core weight will increase magnetic flux density, leading to increased motor iron losses, increased excitation current, and reduced power factor and efficiency.

2. Uniform pressure and appropriate tightness

The core should not loosen or deform under the combined effects of mechanical vibration, electromagnetic forces, and thermal stress. For externally pressed cores, it must also be ensured that they do not loosen or deform during transportation. If the core is too loose, the number of laminations in a certain length will be insufficient, not only resulting in insufficient magnetic cross-section but also causing vibration noise and damaging the insulation; if it is pressed too tightly, the inter-lamination insulation resistance will decrease, or even the insulation will be damaged, leading to a sharp increase in iron losses. Uneven pressure can easily damage the insulation in areas that are too tight, while areas that are too loose often cause the core to loosen.

3. Accurate geometric dimensions

The total length of the core, the slot dimensions, and the dimensions and positions of the radial ventilation slots should all meet the specified requirements. After core stacking, some unevenness is inevitable, and the slot dimensions are always slightly smaller than the lamination dimensions. The smaller this error, the better. This is because the magnetic circuit calculation of the motor is based on the geometric dimensions of the laminations, while the coil dimensions must be calculated according to the clear dimensions of the core slots. Generally, the slot dimensions after pressing are allowed to be 0.2 mm smaller than the lamination dimensions. For large motors stacked with sector-shaped laminations, this can generally be 0.4-0.5 mm smaller.

The outer diameter tolerance of small asynchronous motor cores is related to the process of ensuring the coaxiality of the externally pressed stator core. For medium-sized asynchronous motor cores, the inner diameter of the stator, the outer diameter of the rotor, and the outer diameter tolerance of the DC motor armature core generally use h7. For press-fitting stator cores, the main challenge is ensuring the coaxiality of the frame's locating surface and the inner diameter of the core during the press-fitting process. Although grinding or turning the inner diameter of the core after press-fitting, using the locating surface for positioning, can improve coaxiality, it will increase iron losses and affect motor performance.

4. Shape Requirements

The coaxiality of the core should be within the specified range. The laminations should not show any waviness after core press-fitting. The core edges, especially the teeth, should not be warped. The spring-back of the core teeth (the difference between the length of the tooth tip and the slot bottom) should generally not exceed the values in the table.

For cores twisted at a certain angle, the twisting should be done according to specifications.

1. For cores composed of sector-shaped laminations, they must be stacked in a crisscross pattern according to specifications, and there should be no overlapping between the laminations. For segmented cores (such as those in hydroelectric generators with a diameter exceeding 3.25m), the joint gap must meet the specifications to prevent uneven magnetic flux distribution, which can cause noise and additional stress.

2. The axial centerline position of the core should meet the specified requirements to ensure symmetry of the stator and rotor magnetic centers.

3. Inter-lamination insulation and core losses must be stable.

Based on the requirements for core quality in terms of electromagnetic performance, the ultimate manifestation is the magnitude of core losses. A good core should have sufficient inter-lamination insulation resistance and small and stable iron losses.

Rotating Core Structure Types and Press-fitting Processes

(I) Rotating Core Structure and Press-fitting Process

For medium and small motor rotor cores, the expansion joint generally uses full-circle laminations; sector-shaped laminations are only used when the rotor diameter exceeds 990 mm. The core of cast aluminum rotors is secured by the cast squirrel cage winding.

(II) Armature Core Structure and Press-fitting Process

Most armature cores of medium and small DC motors are directly press-fitted onto the shaft. Large DC motors are generally press-fitted onto a support bracket, and then the bracket is fixed to the shaft using a key or shrink-fitting method.