With the popularization of variable frequency power supply, the problem of motor starting has been easily solved, but for ordinary power supply, the starting of squirrel-cage rotor asynchronous motor is always a problem. From the analysis of the starting and running performance of the asynchronous motor, it can be seen that in order to increase the starting torque and reduce the current when starting, the rotor resistance is required to be larger; while the motor is running, in order to reduce the rotor copper consumption and improve the motor efficiency, the rotor resistance is required to be small Some; this is clearly a contradiction.

For the wound rotor motor, since the resistance can be connected in series at the start, and then cut off at the time of operation, this requirement is well met. However, the structure of the wound asynchronous motor is complicated, the cost is high, and the maintenance is inconvenient, so its application is limited to a certain extent; Resistors, while running on purpose with small resistors. Deep slot and double squirrel cage rotor motors have this starting performance. Today, Ms. participated in talking about the deep slot rotor motor.

Deep slot asynchronous motor

In order to strengthen the skin effect, the groove shape of the deep groove asynchronous motor rotor is deep and narrow, and the ratio of groove depth to groove width is in the range of 10-12. When the current passes through the rotor bar, the leakage magnetic flux intersecting with the bottom of the bar is much more than that intersecting with the notch part. Therefore, if the bar is considered to be divided by several small If the conductors are connected in parallel, the smaller conductors closer to the bottom of the slot have greater leakage reactance, and the closer to the slot, the smaller the leakage reactance.

 

When starting, because the frequency of the rotor current is high and the leakage reactance is large, the distribution of current in each small conductor will depend on the leakage reactance, and 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 in the bar near the bottom of the slot will be very small, and the closer to the slot, the greater the current density.

Due to the skin effect, after most of the current is squeezed to the upper part of the guide bar, the role of the guide bar at the bottom of the groove is very small. Meet the requirements of large resistance when starting. When the motor is started and the motor is running normally, since the rotor current frequency is very low, the leakage reactance of the rotor winding is much smaller than the rotor resistance, so the distribution of the current in the aforementioned small conductors will mainly be determined by the resistance.

 

Since the resistance of each small conductor is equal, the current in the bar will be evenly distributed, so the skin effect basically disappears, and the resistance of the rotor bar becomes smaller, close to the DC resistance. It can be seen that the rotor resistance in normal operation will automatically decrease, thereby satisfying the effect of reducing copper consumption 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 and flow. This phenomenon is called the skin effect. When current or voltage conducts in a conductor with higher frequency electrons, they 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 affects 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 the leakage flux from the small conductor to the notch, and does not generate the leakage flux from the small conductor to the bottom of the slot. Because the latter is not cross-linked with this current. In this way, for the same magnitude of current, the closer to the bottom of the slot, the more leakage flux will be generated, and the closer to the slot opening, the less leakage flux will be generated. It can be seen that when the skin effect squeezes the current in the bar to the notch, the slot leakage magnetic flux generated by the same current decreases, so the slot leakage reactance decreases. So 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 size of the slot shape. The higher the frequency, the deeper the slot shape, and the more significant the skin effect. The same rotor with different frequencies will have different effects of the skin effect, and consequently 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 the deep groove rotor is very strong, but the skin effect also has a certain degree of influence on the common structure of the squirrel cage rotor. Therefore, even for a squirrel-cage rotor with a common structure, the rotor parameters at startup and operation should be calculated separately.

The rotor leakage reactance of the deep slot asynchronous motor, because the rotor slot shape is very deep, although it is reduced by the influence of the skin effect, it is still larger than the common squirrel cage rotor leakage reactance after reduction. Therefore, the power factor and maximum torque of the deep slot motor are slightly lower than those of the ordinary squirrel cage motor. 

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Post time: Mar-31-2023