release time：2024/05/17
The quality of the aluminum rotor directly affects the technical and economic indicators and operating performance of the asynchronous motor. During the production, design and production process, it is not only necessary to analyze the casting defects of the rotor, but also to understand the impact of the quality of the cast aluminum rotor on the efficiency, power factor, starting, operation and other performances of the motor.
Common cast aluminum rotors have larger additional losses than cast copper rotor asynchronous motors. At the same time, the aluminum casting methods used are different, and the additional losses are also different. The common aluminum casting methods include low-pressure cast aluminum, centrifugal cast aluminum, and pressure cast aluminum. Among the three methods of aluminum casting, the additional loss of the pressure-cast aluminum rotor motor is the largest. This is because the strong pressure during die-casting makes the cage bars and the core in very close contact, so that the aluminum liquid squeezes between the laminations, and the transverse current increases. Large, the additional loss of the motor will be greatly increased. In addition, due to the fast filling and pressurizing speed and high pressure during die casting, the air in the aluminum casting mold cannot be completely eliminated, causing a large amount of gas to be densely distributed in the shape of "pinholes" in the rotor cage bars, end rings, fan blades, etc., causing the casting to be damaged. The filling rate of the aluminum rotor cannot reach 100%. At the same time, the average resistance increases compared to the centrifugal cast aluminum, which increases the additional loss of the motor. Although the centrifugal cast aluminum rotor is affected by various factors and prone to defects, the additional loss of the motor is small. During low-pressure aluminum casting, the aluminum liquid comes directly from the inside of the crucible, and a relatively "slow" low-pressure pouring is used to achieve better exhaust. When the guide bar solidifies, the upper and lower end rings replenish the aluminum liquid, so the low-pressure cast aluminum rotor is of excellent quality. Through verification and comparison by some motor factories and combined with the manufacturing process, among the three aluminum casting methods, low-pressure cast aluminum rotor has the best electrical performance, followed by centrifugal cast aluminum and pressure cast aluminum is the worst.
The quality of the cast aluminum rotor directly affects the electrical performance of the motor, so some common cast aluminum problems affect the motor performance of the motor as follows:
The weight of the rotor core is not enough, which means that the net length of the rotor core is reduced, which reduces the cross-sectional area of the rotor teeth and the rotor yoke, and the magnetic flux density increases. The impact on the performance of the motor is: the excitation current increases and the power factor decreases; the motor stator current increases, the stator copper loss increases and the efficiency decreases; the temperature rises.
The reasons why the rotor core is not heavy enough are:
1.1 The thickness of the silicon steel sheet is uneven.
1.2 The rotor punching burrs are too large.
1.3 The pressure is small during press-fitting, and the aluminum is filled into the punching room.
1.4 The rotor stampings are rusty or dirty.
1.5 If the preheating temperature of the cast aluminum rotor core is too high and the time is too long, the core will be severely burned and the net length of the core will be reduced.
If the rotor pieces are staggered and the slot slash is not straight, the rotor slot will be reduced and the rotor slot leakage resistance will be increased; the conductor bar cross-sectional area will be reduced and the conductor bar resistance will be increased. It will have the following effects on the performance of the motor: the maximum torque is reduced; the starting torque is reduced; the reactance current at full load increases and the power factor is reduced; the stator and rotor currents increase, the stator copper loss increases, the rotor loss increases, and the efficiency decreases ; Temperature rises; Slip rate is large.
The reasons for rotor misalignment are:
2.1 When the rotor core is pressed, no slot-like rod is used for positioning, and the slot walls are not neat.
2.2 After the rotor is preheated, it is thrown and rolled on the ground, and the rotor punching produces angular displacement.
2.3 The pressure during press installation is small. After preheating, the burrs and oil stains on the punched sheets will be burned away, causing the rotor sheets to loosen.
2.4 The fitting gap between the oblique key on the dummy shaft and the keyway on the punched piece is too large.
The impact of the rotor slot width being larger or smaller than the allowed value on motor performance is:
(1) When the chute width is greater than the allowable value, the rotor chute leakage reactance increases, and the total leakage reactance of the motor increases; the length of the conductor bar increases, and the resistance of the conductor bar increases, which affects the performance of the motor: the maximum torque decreases; the starting torque Decrease; the reactance current at full load increases and the power factor decreases; the stator and rotor currents increase, the stator copper loss increases, the rotor loss increases, and the efficiency decreases; the temperature rises; the slip rate is large. .
(2) When the chute width is smaller than the allowable value, the rotor chute leakage reactance decreases, the total leakage reactance of the motor decreases, and the starting current is large (because the starting current is inversely proportional to the leakage reactance). In addition, the motor produces large noise and vibration.
The reason why the width of the inclined groove is larger or smaller than the allowable value is mainly because the oblique key positioning on the dummy shaft is not used when the rotor core is pressed, or the inclination size of the oblique key is out of tolerance when the dummy shaft is designed.
The impact of a broken rotor bar on the performance of the motor is: if the rotor bar is broken, the rotor resistance is very large, so the starting torque is very small; the rotor resistance increases, the rotor loss increases, and the efficiency decreases; the temperature rises; and the slip rate is large.
The reasons for broken bars are:
4.1 The rotor core is pressed too tightly. After the aluminum is cast, the rotor core expands. Excessive pulling force is applied to the aluminum bar and the aluminum bar is broken.
4.2 Before aluminum casting, there are inclusions in the rotor core slot.
4.3 After the aluminum is cast, the mold is demolded too early, the aluminum liquid is not solidified well, and the aluminum strip breaks due to the expansion force of the iron core.
4.4 During single punching, some slots in the rotor punching sheet are missed.
4.5 There are pores in the aluminum strip, or the slag cleaning is not good, and there are inclusions in the aluminum liquid.
4.6 Pause during pouring. Because liquid aluminum is easily oxidized, the liquid aluminum poured in successively cannot be combined together due to oxidation, resulting in "cold isolation".
The rotor is thin, the rotor resistance increases, the efficiency decreases, the temperature increases, and the slip rate is large.
The reasons for thin strips are:
5.1 The centrifuge speed is too high and the centrifugal force is too large, so that the guide bar at the bottom of the tank is not filled (empty).
5.2 The rotor slot is too small, making it difficult for aluminum liquid to flow, and the core preheating temperature is unreasonable.
5.3 The rotor is misaligned and the groove slopes are not in a straight line, which hinders the flow of aluminum liquid.
5.4 The core preheating temperature is low, and the fluidity becomes worse after the aluminum liquid is poured into it.
The impact of pores on motor performance will increase rotor resistance, reduce efficiency, increase temperature, and increase slip.
The causes of pores are:
6.1 If the aluminum liquid is not cleaned well, the aluminum liquid contains serious gas, the pouring speed is too fast or the exhaust groove is too small, the gas in the model has no time to be discharged, which is more obvious in pressure casting aluminum.
6.2 The preheating temperature of the iron core is too low, and the oil stains are not burned out before casting aluminum. The oil stains volatilize and form pores during work.
6.3 When casting aluminum at low pressure, if the liquid riser tube leaks seriously, the compressed air flowing into the crucible will enter the liquid riser tube and run into the rotor together with the aluminum liquid to form pores.
If the pouring is not full, the rotor resistance will increase, the efficiency will decrease, the temperature will increase, and the slip rate will increase.
The main reasons for dissatisfaction include:
7.1 The temperature of the aluminum liquid is too low and the fluidity is poor.
7.2 The preheating temperature of the core and mold is too low. The aluminum liquid cools down rapidly after being poured in, and the fluidity becomes poor.
7.3 The centrifuge speed is too low, the centrifugal force is too small, and the aluminum liquid cannot be filled.
7.4 The amount of molten aluminum poured is not enough.
7.5 The cross-sectional area of the inner gate of the cast aluminum mold is too small, and the aluminum liquid solidifies prematurely, blocking the aluminum liquid channel.
Shrinkage holes will increase the rotor resistance and reduce efficiency; the temperature will rise; the slip rate will be large. The main reasons for shrinkage holes are:
8.1 The temperature combination of the aluminum liquid, mold and iron core is inappropriate, and the purpose of sequential solidification and reasonable feeding cannot be achieved. If the preheating temperature of the upper mold is too low, the preheating temperature of the core will be uneven at the upper and lower ends, causing the aluminum liquid at the gate to solidify first. When the upper end ring aluminum liquid solidifies, the aluminum liquid cannot be replenished, causing shrinkage holes in the upper end ring. Because shrinkage cavities always occur where the aluminum liquid finally solidifies.
8.2 The mold structure is unreasonable. For example, if the cross-sectional area of the inner gate is too small or the diverter is too high, the channel of the molten aluminum will grow at the inner gate, and the aluminum liquid will solidify first at the inner gate, resulting in poor feeding and shrinkage holes in the upper ring. For another example, if the mold is poorly sealed or improperly installed, causing leakage of aluminum liquid, the amount of aluminum liquid at the gate will be too small, unable to achieve the feeding effect, and it is easy to cause shrinkage cavities.
Cracks in the cast aluminum rotor will have an impact on the mechanical strength of the rotor, which is not conducive to the high-speed operation of the rotor. At the same time, it will increase the resistance of the rotor and reduce the efficiency; the temperature will rise; the slip rate will be large, and it is mainly due to the cooling process of the rotor. The casting stress exceeds the material ultimate strength of the aluminum guide bar at that time (referring to the moment when cracks occur). Most cracks in cast aluminum rotors are radial. Cracks are divided into hot cracks and cold cracks: hot cracks are generated at high temperatures during the crystallization process; cold cracks are generated during the further cooling process of solidified aluminum.
The main reasons for cracks are:
9.1 The impurity content in industrial pure aluminum is unreasonable. Common impurities in industrial pure aluminum are iron and silicon. A large number of experimental analyzes have confirmed that the silicon and iron impurity content ratio has a great impact on cracks, that is, cracks are prone to occur when the silicon to iron ratio is between 1.5 and 10.
9.2 When the temperature of the aluminum liquid is too high (over 800°C), the aluminum grains become coarser and the elongation decreases. It cannot withstand the shrinkage force generated during the condensation process and forms cracks.
9.3 The size design of the rotor end ring is unreasonable (the ratio of thickness to width is less than 0.4).
9.4 The fillets at the connections between the fan blades, balance columns and end rings are too small, causing cracks due to concentration of casting stress.
When the purity of the aluminum is not enough, the conductivity decreases, which increases the rotor resistance, reduces the efficiency, increases the temperature, and increases the slip rate. The main reasons for low conductivity are: poor quality of aluminum or too much recycled scrap aluminum; but if too high-purity aluminum ingots are used, the rotor resistance decreases and the starting torque of the motor is low (because the starting torque is approximately the same as when starting proportional to the rotor resistance). Therefore, aluminum ingots must be selected and used according to design requirements.
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