Advantages and Disadvantages of Adopting Closed Slot for Cast Aluminum Rotor 


The cast aluminum rotor of the motor is generally manufactured by centrifugal cast aluminum process, especially the cast aluminum rotor with radial ventilation duct. The rotor punching is designed in the form of closed groove. The middle mold is not required for aluminum casting, and the risk of rotor ventilation duct blockage caused by aluminum leakage in the ventilation duct is reduced. However, when the closed slot rotor punching plate is punched, it is easy to extend outward at the punching plate notch and increase the circumference of the outer circle. After overlapping, it will cause the problem of rotor outer circle wave, resulting in the non straightness of the ventilation duct after rotor aluminum casting, which will affect the heat dissipation effect and service life of the motor.

Although the rotor with closed slot can reduce the stator copper loss and stray loss of the motor, the reduction range is positively correlated with the closed bridge arch height or magnetic bridge thickness.
The thickness of the magnetic bridge in the closed slot of the rotor is too large, the increase amplitude of the rotor leakage reactance changes greatly, and the power factor will tend to deteriorate, resulting in the increase of the load current, the increase of the copper and aluminum consumption of the stator and rotor, the offset of the improvement of the efficiency, and even the decrease of the efficiency.
The rotor adopts closed slot. Due to the increase of leakage reactance, the starting current decreases, but at the same time, the starting torque and maximum torque also decrease. Therefore, when adopting closed slot, both starting and overload capacity shall be considered, and the change of data shall not be lower than the limit allowed by the standard.
After the rotor adopts closed slot, the noise is reduced to a certain extent. In particular, it has a very obvious inhibitory effect on electromagnetic noise and vibration.
The slip rate of closed slot motor can maintain the level of semi closed slot, and the level of efficiency improvement will be higher. From some specific test data accumulated by various motor manufacturers, the effect of closed slot with bridge arch height or magnetic bridge thickness of about 0.2mm is better.
The squirrel cage rotor of ordinary cage asynchronous motor generally adopts semi closed slot. Here, closed slot is used for high-efficiency motor. There are two considerations: first, the air gap reluctance is reduced, the excitation current is relatively small, and the copper loss is also small; Second, it can effectively reduce the additional core loss caused by the pulse vibration of the motor on the core surface due to the magnetic conductivity harmonic. Obviously, the efficiency improvement effect of dual factors is quite lethal, so manufacturers have tried the rotor closed slot on a large scale.