The quality of cast aluminum rotors affects the efficiency, power factor, and starting and running performance of motors. To comprehensively improve rotor quality, it is necessary not only to analyze the defects of cast aluminum rotors but also to study the various factors affecting their quality, thereby proposing improvement measures. This article provides a brief introduction to the methods of casting aluminum rotors and analyzes the problem of broken bars.
The casting of aluminum rotors is a closely guarded project. The quality control of rotors relies on process parameters. In a sense, the focus of the performance and quality competition of cast aluminum rotors is on the casting of aluminum rotors.
Aluminum casting method and rotor quality
The change from copper bar casting to aluminum casting in squirrel-cage rotors increases stray losses in motors by 2 to 6 times. Different aluminum casting methods result in different stray losses. Pressure-cast aluminum rotors exhibit the highest stray losses. While centrifugal and vibratory casting are susceptible to defects due to various factors, their rotor stray losses are lower. Further research reveals that die-cast rotors have approximately 8% lower aluminum cage density and 13% higher average resistivity compared to centrifugally cast rotors. This worsens the key technical and economic indicators of die-cast asynchronous motors, increasing core loss, slip, and temperature rise, while reducing minimum torque and efficiency.
The density of aluminum in a die-cast rotor decreases while its resistance increases because gas mixed into the molten aluminum during die casting forms pinhole-like inclusions throughout the rotor cage bars, end rings, and fan blades. Furthermore, the intense pressure causes the cage bars and core to be in very close contact (even between the core laminations, aluminum conductors can be introduced), resulting in a significant increase in stray losses in the motor due to the lateral current.
Low-pressure cast aluminum molten aluminum comes from inside the crucible and is poured at a relatively "slow" low pressure, resulting in better venting and better quality cast aluminum rotors. Low-pressure cast aluminum rotors are of the best quality, followed by centrifugal cast aluminum, with pressure cast aluminum being the worst.
Defects in cast aluminum rotors are diverse, and their causes are complex. Domestic motor manufacturers mostly use centrifugal cast aluminum; the following analysis will primarily focus on the quality problems of centrifugal cast aluminum rotors. Quality issues arising from other casting methods will be briefly introduced.
Why do the bars of a cast aluminum rotor break?
(1) Misalignment or blockage of the core slot: Misalignment reduces the cross-section of the slot, leading to blockage; missing punching of individual lamination slots and inclusions introduced during preheating can also cause blockage of the slots, resulting in the molten aluminum being cut off during casting.
(2) The rotor cage bars are broken. When the rotor core is stacked too tightly, the cage bars may break when the core expands outward after the dummy shaft is removed. If the molten aluminum has not completely solidified during demolding, the cage bars may also break due to premature impact on the mold. During vibratory casting of aluminum, the cage bars may also break due to excessive vibration time.
(3) Interruption during pouring. Because aluminum is easily oxidized, the molten aluminum poured after the interruption cannot be fused with the molten aluminum in front, causing the cage bars to break.
(4) The air holes in the trough cause the cage bars to break.
(5) The core temperature is too low, which affects the fluidity of the molten aluminum. This has a particularly large impact on gravity casting of aluminum.
Measures to prevent rotor bar breakage
(1) Strengthen the inspection before pouring to eliminate misalignment and blockage of the slots.
(2) When stacking the rotor core, control the stacking pressure to ensure it does not exceed 4 MN/m². After casting the aluminum, wait for the molten aluminum to completely solidify before demolding.
(3) The pouring should be completed in one go, without interruption.
(4) Appropriately increase the core preheating temperature.
