With the popularization of variable frequency power supply, the starting problem of motors has become easy to solve, but for ordinary power supplies, the starting of squirrel cage rotor asynchronous motors has always been a problem. From the analysis of the starting and running performance of asynchronous motors, it can be seen that in order to increase the starting torque and reduce the current during starting, the rotor resistance is required to be larger; while when the motor is running, in order to reduce the rotor copper loss and improve the motor efficiency, the rotor resistance is required to be smaller; this is obviously a contradiction.
For wound rotor motors, since the resistor can be connected in series during starting and then removed during operation, this requirement is well met. However, the wound rotor asynchronous motor has a complex structure, high cost, and inconvenient maintenance, which limits its application to a certain extent; this prompts people to start from the rotor slot shape of the squirrel cage asynchronous motor and try to use the “skin effect” to achieve the purpose of large starting resistance and small resistance during operation. Deep slot and double squirrel cage rotor motors have this starting performance. Today, let’s talk about deep slot rotor motors.
Deep slot asynchronous motor
In order to enhance the skin effect, the slot shape of the deep slot asynchronous motor rotor is deep and narrow, and the ratio of slot depth to slot width is in the range of 10-12. When the current passes 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 closer to the slot opening, the smaller the leakage reactance.
At starting, since the rotor current frequency is high and the leakage reactance is large, the distribution of current in each small conductor will depend on the leakage reactance. The larger the leakage reactance, the smaller the leakage current. In this way, under the action of the same potential induced by the main magnetic flux of the air gap, the current density near the bottom of the slot in the bars will be very small, and the closer to the slot opening, the larger the current density.
Due to the skin effect, after most of the current is squeezed to the upper part of the conductor bar, the role of the conductor bar at the bottom of the slot is very small, which is equivalent to reducing the height and cross-section of the conductor bar, so the rotor resistance increases, and meets the requirement of large resistance at starting. When the motor is started and the motor is running normally, due to the low frequency of the rotor current, the leakage reactance of the rotor winding is much smaller than the rotor resistance, so 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 conductor bar will be evenly distributed, so the skin effect basically disappears, and the resistance of the rotor conductor bar becomes smaller again, close to the DC resistance. It can be seen that the normal operation of the rotor resistance will automatically decrease, thereby meeting the effect of reducing copper loss and improving efficiency.
What is the skin effect?
The skin effect is also called the skin effect. When the alternating current passes through the conductor, the current will concentrate on the surface of the conductor. This phenomenon is called the skin effect. When the current or voltage is conducted in the conductor with higher frequency electrons, it will gather on the surface of the total conductor instead of being evenly distributed in the cross-sectional area of the entire conductor.
The skin effect not only affects the rotor resistance, but also the rotor leakage reactance. From the path of the slot leakage flux, it can be seen that the current passing through a small conductor only generates leakage flux from the small conductor to the slot, but not from the small conductor to the bottom of the slot, because the latter has no linkage with the current. In this way, for the same current, the closer to the bottom of the slot, the more leakage flux is generated, and the closer to the slot, the less leakage flux is generated. It can be seen that when the skin effect squeezes the current in the conductor to the slot, the slot leakage flux generated by the same current is reduced, so the slot leakage reactance is reduced. Therefore, the skin effect increases the rotor resistance and reduces the rotor leakage reactance.
The strength of the skin effect depends on the frequency of the rotor current and the slot size. The higher the frequency and the deeper the slot, the more significant the skin effect will be. For the same rotor, if the frequency is different, the skin effect will be different, and the rotor parameters will also be different. Because of this, the rotor resistance and leakage reactance during normal operation and starting should be strictly distinguished and cannot be confused. For the same frequency, the skin effect of deep groove rotors is very strong, but the skin effect also has a certain degree of influence on ordinary structure squirrel cage rotors. Therefore, even for a squirrel-cage rotor with an ordinary structure, the rotor parameters at startup and operation should be calculated separately.
The rotor leakage reactance of the deep groove asynchronous motor, due to the deep groove shape of the rotor, is reduced by the skin effect, but after the reduction, the leakage reactance of the rotor is still larger than that of the ordinary squirrel cage rotor. Therefore, the power factor and maximum torque of deep groove motors are slightly lower than those of ordinary squirrel cage motors.
Post time: Jul-05-2024
