How does the quality of cast aluminum rotor affect asynchronous motors

The quality of cast aluminum rotors directly affects the technical and economic indicators and operating performance of asynchronous motors. When studying the quality of cast aluminum rotors, it is necessary not only to analyze the casting defects of the rotors, but also to understand the impact of cast aluminum rotor quality on the efficiency, power factor, and starting and operating performance of the motor.

1. The relationship between aluminum casting method and rotor quality

The additional loss of cast aluminum rotors is much greater than that of copper bar rotor asynchronous motors, and the aluminum casting methods used are different. The additional loss is also different, among which the additional loss of pressure cast aluminum rotor motors is the largest. This is because the strong pressure during die casting makes the cage bars and the iron core contact very closely, and even the aluminum liquid squeezes into the laminations, the lateral current increases, and the additional loss of the motor increases greatly. In addition, due to the high speed and high pressure during die casting, the air in the mold cavity cannot be completely removed, and a large amount of gas is densely distributed in the rotor cage bars, end rings, fan blades, etc. in the form of "pinholes", resulting in a decrease in the specific gravity of aluminum in the cast aluminum rotor (about 8% less than centrifugal cast aluminum) and an increase in average resistance (about 13%), which greatly reduces the main technical and economic indicators of the motor. Although the centrifugal cast aluminum rotor is affected by various factors and is prone to defects, the additional loss of the motor is small. In low-pressure aluminum casting, the aluminum water comes directly from the inside of the crucible, and a relatively "slow" low-pressure pouring is used, which has better exhaust; when the guide bar solidifies, the aluminum water is supplemented by the upper and lower end rings. Therefore, the low-pressure cast aluminum rotor has excellent quality. The electrical performance of the low-pressure cast aluminum rotor is the best, followed by the centrifugal cast aluminum, and the pressure cast aluminum is the worst.

2. The influence of rotor quality on motor performance

The quality of the cast aluminum rotor has a great influence on the performance of the motor. The causes of these defects and their influence on the performance of the motor are discussed in detail below.

(1) Insufficient weight of the rotor core

Insufficient weight of the rotor core is equivalent to reducing the net length of the rotor core, which reduces the cross-sectional area of the rotor teeth and the rotor choke, and increases the magnetic flux density. The impact on the motor performance is: the excitation current increases, the power factor decreases, the motor stator current increases, the rotor copper loss increases, the efficiency decreases, and the temperature rise increases.

The main reasons for the weight of the rotor core are:

a. The burrs of the rotor punching are too large;

b. The thickness of the silicon steel sheet is uneven;

c. The rotor punching is rusty or not clean;

d. The pressure during press-fitting is low (the press-fitting pressure of the rotor core is generally 2.5~.MPa).

f. The preheating temperature of the cast aluminum rotor core is too high and the time is too long, the core is severely burned, and the net length of the core is reduced.

(2) Rotor misalignment, slot oblique lines are not straight, and rotor misalignment occurs

The above defects will reduce the rotor slot opening, increase the rotor slot leakage, reduce the conductor bar cross section, increase the conductor bar resistance, and have the following effects on the motor performance: maximum torque is reduced, starting torque is reduced, reactance current at full load is increased, power factor is reduced; stator and rotor currents increase, stator copper loss increases; rotor loss increases, efficiency decreases, temperature rises, and slip rate is large.

The reasons for rotor misalignment are:

a. The slot sample rod is not used for positioning during the rotor core press-fitting, and the slot wall is not neat.

b. The matching clearance between the oblique key on the dummy shaft and the keyway on the punching plate is too large;

c. The pressure during press-fitting is small, and the burrs and oil stains on the punching plate are burned off after preheating, causing the rotor plate to loosen;

d. The rotor is thrown and rolled on the ground after preheating, causing angular displacement of the rotor punching plate.

(3) The width of the rotor skew slot is greater than or less than the allowable value

The reason why the width of the skew slot is greater than or less than the allowable value is mainly that the skew key on the dummy shaft is not used for positioning when the rotor core is pressed, or the skew key size exceeds the tolerance when the dummy shaft is designed.

The impact on the motor performance is:

(1) When the skew slot width is greater than the allowable value, the rotor skew slot leakage reactance increases, and the total leakage reactance of the motor increases; the length of the conductor bar increases, and the conductor bar resistance increases.

(2) When the skew slot width is less than the allowable value, the rotor skew slot leakage reactance decreases, the total leakage reactance of the motor decreases, and the starting current increases (because the starting current is inversely proportional to the leakage reactance). In addition, the noise and vibration of the motor are large.

(3) The reasons for the broken rotor bars are:

a. The rotor core is pressed too tightly, and the rotor core expands after aluminum casting. There is too much tension on the aluminum bar, which breaks the aluminum bar.

b. The mold is demolded too early after aluminum casting, and the aluminum liquid has not solidified well, and the aluminum bar breaks due to the expansion force of the core.

c. Before aluminum casting, there are inclusions in the rotor core slots.

NIDE provides customers with various types of motor rotor aluminum casting machines.

This rotor automatic centrifugal aluminum casting machine adopts four-station casting to improve production efficiency.

The whole process is automated, reducing labor costs, only one operator required. Automatic get aluminium melt liquid, automatic loading and unloading the rotor, oven heating, manipulator reclaiming, four-station aluminum casting station, manipulator discharging, cooling assembly line, shaping station, discharging platform, automatic recycling and reuse of aluminum scraps.