The influence of cast aluminum rotor on motor performance
The influence of contact resistance of cast aluminum rotor on motor performance
The aluminum conductor and iron core of the cast aluminum rotor are tightly attached together, and the low contact resistance generates a considerable lateral current, especially when the rotor is skewed, which has a significant impact on the stray losses and operating performance of the motor. Ms., today we will have a brief discussion on the contact resistance of cast aluminum rotors.
Measurement of Contact Resistance
The contact between the cage bars of the cast aluminum rotor and the rotor slot wall is very tight. When the rotor surface is precision machined, the cutting pressure of the tool further compresses the aluminum conductor and iron core in the slot, and the contact resistance between them is very small.
Measurement of contact resistance between rotor core and squirrel cage: Introduce direct current between the end ring of the squirrel cage and the shaft, and measure the average voltage drop between the squirrel cage and the core. At this point, the contact resistance can be calculated using the following formula:
Rc=c l Q2Uav/I………………(1)
In equation (1):
Rc - Contact resistance (Ω· mm2);
I - Direct current through the rotor (A);
Uav - average voltage drop between rotor cage and iron core (V);
Q2- Number of rotor slots;
L - length of rotor iron core (mm);
C - The circumference of the rotor cage section, i.e. the circumference of the rotor slot (mm).
The contact resistance between the end ring of the squirrel cage and the iron core is not considered in equation (1) because its contact resistance value is much larger than that between the cage bar and the iron core.
The Influence of Contact Resistance on Motor Performance
The analysis results of various cast aluminum rotor motors in actual production show that the average stray loss is 2-3%, with a maximum of 6.5% and a minimum of about 0.7%. This variation is mainly due to the different sizes of contact resistance. The excessively small contact resistance value also significantly reduces the minimum torque of the motor.
Taking a 6P-7.5kW motor with 36 stator slots and 44 rotor slots from a certain motor factory as an example, this study qualitatively illustrates the influence of rotor contact resistance on motor performance. From the relationship curve between the iron core loss of the motor rotor and the contact resistance between the cage bars and the iron core, it can be seen that when the contact resistance increases from 0.04 ohms · mm2 to around 30 ohms · mm2, the iron loss decreases by about 30%. The reduction in losses is due to the decrease in losses caused by the current (i.e. transverse current) passing through the rotor core between the cage bars. From the relationship curve between the contact resistance of the cage bars and the stray losses and minimum torque under load, it can be seen that increasing the contact resistance value to 30 ohms · mm2 will reduce the stray losses of the load by about 58%. Similar results have also been obtained from experiments on many other types of electric motors.
From this, it can be seen that in order to effectively reduce the stray losses of the motor, the contact resistance between the cage bars and the iron core needs to be increased to 30 ohms · mm2; To improve the torque curve by increasing the minimum torque, the contact resistance value only needs to be greater than 0.3~0.6 ohms · mm2.
Experiments have shown that increasing the contact resistance between the rotor cage bars and the iron core reduces the losses of the rotor iron core and load stray losses, improves the efficiency of the motor, and reduces the temperature rise of the stator winding.
The contact resistance values of different aluminum casting methods are also different. When using centrifugal cast aluminum, the contact resistance between the rotor cage and the iron core is approximately 0.15~7.0 ohms · mm2. The contact resistance value for pressure cast aluminum rotors is approximately 0.01~0.09 ohms · mm2.
From the experimental data, it can be seen that the contact resistance value of the rotor using centrifugal cast aluminum can basically ensure the required torque characteristics. Compared with the indicators obtained using pressure cast aluminum, the efficiency of the motor is improved and the temperature rise is reduced. This change in motor specifications is particularly evident when the production plant changed from centrifugal cast aluminum to pressure cast aluminum. Because pressure cast aluminum is an efficient casting process, in order to not reduce the power performance indicators of the motor, it is necessary to seek practical and feasible methods to increase the contact resistance between the rotor cage bars and the iron core.
