Squirrel-Cage Induction Motors: Why Choose Deep-Slot Rotors?

With the popularization of variable-frequency power supplies, the starting problem of motors has been easily solved. However, for ordinary power supplies, the starting of squirrel-cage rotor induction motors has always been a challenge. From the analysis of the starting and operating performance of asynchronous motors, it can be seen that during starting, a larger rotor resistance is required to increase the starting torque and reduce the current; while during motor operation, a smaller rotor resistance is required to reduce rotor copper loss and improve motor efficiency. This is obviously a contradiction.

For wound-rotor induction motors, since resistance can be connected in series during starting and then cut off during operation, this requirement is well met. However, wound-rotor induction motors have complex structures, high costs, and inconvenient maintenance, which limit their applications. This has prompted people to start with the rotor slot shape of squirrel-cage induction motors and try to use the "skin effect" to achieve large resistance during starting and small resistance during operation. Deep-slot and double-squirrel-cage rotor motors have such starting performance. Today, Ms. will discuss deep-slot rotor motors with everyone.

Deep-Slot Induction Motors

To enhance the skin effect, the rotor slot shape of deep-slot induction motors is deep and narrow, with a ratio of slot depth to slot width in the range of 10-12. When current flows through the rotor bars, the leakage flux linked with the bottom of the bars is much more than that linked with the slot opening part. Therefore, if the bar is regarded as a parallel connection of several small conductors divided along the slot height, the small conductors closer to the slot bottom have a larger leakage reactance, and the closer to the slot opening, the smaller the leakage reactance.

During starting, due to the high frequency of the rotor current and the large leakage reactance, the current distribution in each small conductor depends on the leakage reactance-the larger the leakage reactance, the smaller the leakage current. Thus, under the action of the same electromotive force induced by the air-gap main flux, the current density near the bottom of the bar will be very small, and it will be larger closer to the slot opening.

Due to the skin effect, most of the current is squeezed to the upper part of the bar, and the part of the bar at the slot bottom plays a very small role. This effect is equivalent to reducing the height and cross-sectional area of the bar, so the rotor resistance increases, meeting the requirement of large resistance during starting. When the motor finishes starting and operates normally, since the frequency of the rotor current is very low, the leakage reactance of the rotor winding is much smaller than the rotor resistance. Therefore, the current distribution in each small conductor mentioned earlier is mainly 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 small again, close to the DC resistance. It can be seen that the rotor resistance during normal operation will automatically become small, thereby achieving the effect of reducing copper loss and improving efficiency.

What is the Skin Effect?

The skin effect refers to the phenomenon that when alternating current passes through a conductor, the current will concentrate on the surface of the conductor. When current or voltage is transmitted in a conductor by electrons with high frequency, it will gather on the surface of the total conductor instead of being evenly distributed over the entire cross-sectional area of the conductor.

The skin effect affects not only the rotor resistance but also the rotor leakage reactance. It can be seen from the path of the slot leakage flux that the current passing through a certain small conductor only generates leakage flux from the small conductor to the slot opening, not from the small conductor to the slot bottom, because the latter is not linked with the current. Thus, for the same magnitude of current, the closer to the slot bottom, the more leakage flux is generated, and the closer to the slot opening, the less leakage flux is generated. It can be seen that when the skin effect squeezes the current in the bar to the slot opening, 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 decreases the rotor leakage reactance.

The intensity 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 shape, the more significant the skin effect. For the same rotor, different frequencies result in different effects of the skin effect, and thus different rotor parameters. For this reason, the rotor resistance and leakage reactance during normal operation and starting should be strictly distinguished and not confused. For the same frequency, the skin effect of the deep-slot rotor is very strong, but for the squirrel-cage rotor with a common structure, the skin effect also has a certain impact. Therefore, even for the squirrel-cage rotor with a common structure, the rotor parameters during starting and operation should be calculated separately.

Although the rotor leakage reactance of the deep-slot induction motor is reduced due to the skin effect because of its very deep rotor slot shape, it is still larger than that of the ordinary squirrel-cage motor after reduction. Therefore, the power factor and maximum torque of the deep-slot motor during operation are slightly lower than those of the ordinary squirrel-cage induction motor.