Why Do Motors Have Deep Slotted Rotors?

In the previous topic we talked about the motor starting, efficiency and other performance parameters of the relationship between the fish and the bear's paw is a very desirable effect, but how to try to achieve, is indeed a topic of motor research.

We have talked about, the starting process we expect a large starting torque and small starting current, this time the need for motor rotor resistance, but the motor running process, in order to meet the motor's efficiency targets, and hope that the rotor resistance is smaller. For this requirement, the winding rotor motor can be started through the process of series resistance, the operation process to cut off the resistance of the way to solve, but for the cast aluminum rotor motor, how to achieve, which is our topic today.

Cast aluminum rotor slot shape relative to the wire-wound rotor slot shape to be a lot more freedom, not subject to the limitations of the winding shape, and try to follow the theoretical performance requirements to design, deep slot rotor is a good example.

The definition of deep groove rotor is carried out according to the depth-to-width ratio of the groove shape of the rotor punch, deep groove rotor groove depth-to-width ratio is more than 10, generally between 10 and 12, when interested, we can observe and count the groove shape of the rotor of the motor.

Deep groove rotor motor is mainly the use of the skin effect, that is, when the conductor in the alternating current or alternating electromagnetic field, the current distribution inside the conductor is not uniform, the current is concentrated in the conductor's "skin part, that is to say, the current is concentrated in the conductor of the outer layer of the thin, the closer to the surface of the conductor, the higher the current density, the conductor inside the current is actually small, the result is that the conductor's resistance increases. For the motor rotor, the skin effect of the final effect is like the current is squeezed to the rotor slot position, and therefore also customarily known as the squeeze effect.

When the motor starts, the current in the rotor guide bars will be evenly distributed. The change of resistance before and after starting is mainly due to the change of rotor current frequency, the deep groove rotor makes full use of the skin effect of the conductor, which effectively improves the starting performance of the motor without affecting the operating efficiency of the motor.

When the skin effect squeezes the current in the guide bar 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 decreases 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, the deeper the slot, and the more significant the skin effect. For rotors with the same slot size, the skin effect will be different if the frequency is different. When the motor is running normally and starting, the equivalent resistance of the rotor has a large difference. Under the same frequency condition, the skin effect of deep groove rotors is very strong, but for ordinary structure squirrel cage rotors, the skin effect also has a certain degree of influence. 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 deep-slot asynchronous motor, due to the deep rotor groove shape, although reduced by the influence of skin effect, but ultimately still larger than the ordinary squirrel cage rotor leakage reactance. Therefore, the power factor and maximum torque of the deep slot motor are slightly lower than those of the ordinary squirrel cage motor.

For motor products, should be combined with the specific conditions of use, the performance advantages associated with appropriate trade-offs. But with the progress of frequency conversion technology, squirrel cage motor starting may not be too big a problem, how to combine the traditional motor control and new control technology, is the irreversible trend of future motor development.