Why do cast aluminum rotors have thin or broken bars?

Thin bars or broken bars are commonly used fault terms in cast aluminum rotor motors. Both thin bars and broken bars refer to the rotor bars. Theoretically, once the rotor's punching slot shape, iron length, and slot slope are determined, the rotor bars are outlined in a very regular shape. However, in the actual manufacturing process, various reasons often cause the final rotor bars to be twisted and deformed, and even shrinkage holes appear inside the bars. In severe cases, the bars may break.

Since the rotor core is made of rotor punchings, the circumferential positioning is performed by the slotted rods matching the rotor punchings during the lamination process. After completion, the slotted rods are taken out and cast aluminum with the mold. If the slotted rods and the slots are too loose, the punchings will have different degrees of circumferential displacement during the lamination process, which will eventually lead to wavy surfaces on the rotor bars, sawtooth phenomena on the rotor core slots, and even broken bars. In addition, the aluminum casting process is also the solidification process of liquid aluminum entering the rotor slots. If the liquid aluminum is mixed with gas during the injection process and cannot be discharged well, pores will be formed in a certain part of the bars. If the pores are too large, it will also cause rotor bar breakage.

Knowledge expansion - deep groove and double cage asynchronous motors

From the analysis of the start of the cage Asynchronous Motor, it can be seen that when starting directly, the starting current is too large; when starting with reduced voltage, although the starting current is reduced, the starting torque is also reduced. According to the artificial mechanical characteristics of the series resistance of the asynchronous motor rotor, it can be seen that increasing the rotor resistance within a certain range can increase the starting torque, and increasing the rotor resistance will also reduce the starting current. Therefore, a larger rotor resistance can improve the starting performance.

However, when the motor is running normally, it is hoped that the rotor resistance is smaller, which can reduce the rotor copper loss and improve the efficiency of the motor. How can the cage asynchronous motor have a larger rotor resistance when starting, and the rotor resistance automatically decreases during normal operation? Deep slot and double cage asynchronous motors can achieve this goal.
Deep slot asynchronous motor
The rotor slot of the deep slot asynchronous motor is deep and narrow, and the ratio of slot depth to slot width is usually 10 to 12 or more. When current flows through the rotor bars, the leakage flux interlinked with the bottom of the bars is much greater than the leakage flux interlinked with the slot opening. Therefore, if the bars are regarded as a number of small conductors divided along the slot height connected in parallel, the small conductors closer to the bottom of the slot have a larger leakage reactance, and the small conductors closer to the slot opening have a smaller leakage reactance.

When the motor starts, due to the high frequency of the rotor current, the leakage reactance of the rotor bars is large, so the distribution of current in each small conductor will be mainly determined by the leakage reactance. The larger the leakage reactance, the smaller the current. In this way, under the same electromotive force induced by the main magnetic flux of the air gap, the current density near the bottom of the slot in the conductor will be very small, and the closer to the slot, the larger it will be. This phenomenon is called the skin effect of the current. It is equivalent to the current being squeezed to the slot, so it is also called the squeeze effect. The effect of the skin effect is equivalent to reducing the height and cross-section of the conductor bar, increasing the rotor resistance, and thus meeting the starting requirements.

When the start is completed and the motor is running normally, the rotor current frequency is very low, generally 1 to 3 Hz, and the leakage reactance of the rotor bars is much smaller than the rotor resistance. Therefore, the distribution of current in the aforementioned small conductors will be mainly determined by the resistance. Since the resistance of each small conductor is equal, the current in the bars will be evenly distributed, and the skin effect basically disappears, so the rotor bar resistance returns to its own DC resistance. It can be seen that during normal operation, the rotor resistance of the deep slot asynchronous motor can automatically decrease, thereby meeting the requirements of reducing rotor copper loss and improving motor efficiency.

Double-cage asynchronous motor

There are two cages on the rotor of the double-cage asynchronous motor, namely the upper cage and the lower cage. The upper cage bars have a smaller cross-sectional area and are made of materials with a higher resistivity such as brass or aluminum bronze, and have a larger resistance; the lower cage bars have a larger cross-sectional area and are made of copper with a lower resistivity, and have a smaller resistance. Double-cage motors also often use cast aluminum rotors; it is obvious that the leakage flux of the lower cage is much more than that of the upper cage, so the leakage reactance of the lower cage is also much larger than that of the upper cage.