With the popularization of variable frequency power supplies, the starting problem of motors has become easy to solve. However, for ordinary power supplies, the starting of squirrel cage rotor asynchronous motors has always been a problem. From the analysis of the starting and operating performance of asynchronous motors, it can be seen that in order to increase the starting torque and reduce the current during starting, a higher rotor resistance is required; When the motor is running, in order to reduce the copper loss of the rotor and improve the efficiency of the motor, it is required to have a lower rotor resistance; This is clearly a contradiction.
For wound rotor motors, the resistor can be connected in series during startup and then cut off during operation, thus meeting this requirement very well. However, the structure of wound asynchronous motors is complex, the cost is high, and maintenance is inconvenient, which limits their application to a certain extent; This prompted people to start with the rotor groove shape of squirrel cage asynchronous motors and try to use the “skin effect” to achieve the goal of starting with a large resistance and running with a small resistance. Deep slot and double squirrel cage rotor motors have this starting performance. Today, Ms. will participate in a discussion about deep slot rotor motors.
To enhance the skin effect, the slot shape of the rotor of a deep slot asynchronous motor exhibits a deep and narrow feature, with a slot depth to slot width ratio ranging from 10 to 12. When a current passes through the rotor bar, the leakage flux of the chain intersecting with the bottom of the bar is much higher than that of the chain intersecting with the slot. Therefore, if the bar is considered to be composed of several small conductors divided in parallel along the slot height, the smaller conductors closer to the slot bottom have greater leakage reactance, while the closer to the slot, the smaller the leakage reactance.
At startup, due to the high frequency of rotor current and the large leakage reactance, 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 same potential induced by the main magnetic flux in the air gap, the current density near the bottom of the slot in the conductor will be very small, while the closer it is to the slot, the greater it will be.
Due to the skin effect, most of the current is squeezed onto the upper part of the guide bar, and the effect of the guide bar at the bottom of the slot is very small, which is equivalent to reducing the height and cross-section of the guide bar. Therefore, the rotor resistance increases, meeting the requirements for high resistance during startup. When the motor is started and running normally, due to the low frequency of rotor current, the leakage impedance of the rotor winding is much smaller than the rotor resistance. Therefore, the distribution of current in the aforementioned small conductors will mainly depend on the resistance.
Due to the equal resistance of each small conductor, the current in the conductor will be evenly distributed, so the skin effect will basically disappear, and the resistance of the rotor conductor will decrease again, approaching the DC resistance. From this, it can be seen that the normal operation of the rotor resistance will automatically decrease, thereby meeting the effect of reducing copper consumption and improving efficiency.
The skin effect not only affects the rotor resistance, but also the rotor leakage impedance. 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 opening, and does not generate leakage flux from the small conductor to the bottom of the slot, because the latter is not cross-linked with the current. In this way, the closer the current of the same size is to the bottom of the slot, the more leakage magnetic flux is generated, and the closer it is to the slot, the less leakage magnetic flux is generated. From this, it can be seen that when the skin effect squeezes the current in the conductor into the slot, the slot leakage magnetic flux generated by the same current decreases, thus reducing the slot leakage impedance. So the skin effect increases the rotor resistance and reduces the rotor leakage impedance.
The strength of the skin effect depends on the frequency and slot size of the rotor current. The higher the frequency and the deeper the slot, the more significant the skin effect. The same rotor, with different frequencies, has a different effect on the skin effect, resulting in different rotor parameters. Because of this, the rotor resistance and leakage reactance during normal operation and startup should be strictly distinguished and not confused. At the same frequency, the skin effect of deep groove rotors is strong, but it also has a certain degree of influence on ordinary squirrel cage rotors. Therefore, even for ordinary squirrel cage rotors, the rotor parameters during startup and operation should be calculated separately.
The rotor leakage reactance of deep slot asynchronous motors, due to the deep rotor groove shape, although reduced by skin effect, is still larger than that of ordinary squirrel cage rotors after reduction. So the power factor and maximum torque of the deep groove motor during operation are slightly lower than those of ordinary squirrel cage motors.