A motor factory has manufactured nearly 100 YKK high-voltage motors for customers, with frame sizes ranging from H450 to 710. The heat load was designed to be very low, and the design temperature rise was lower than 68K. However, within less than half a year of operation, more than 40 motors had high temperature problems. From the test data, the temperature rise of motors with fewer slots is acceptable, while motors with too high temperature rise all use more slots . Today, Ms. Can will talk to you about the relationship between slot matching and motor performance.

The additional losses of asynchronous motors are mainly caused by air gap harmonic flux. These harmonic fluxes generate high-frequency iron loss in the stator and rotor cores and high-frequency current loss in the squirrel cage rotor. Among them, the effect of stator and rotor tooth harmonic flux is the most significant.

When the number of stator and rotor slots is very close , the width of the rotor tooth tip will be very close to the wavelength of the stator tooth harmonic, so the pulsation in the rotor teeth caused by the stator tooth harmonic flux is small, and the pulsation loss is also small. Similarly, the pulsation loss in the stator teeth caused by the rotor tooth harmonic flux is also small. 

Therefore, when selecting the slot combination of the asynchronous motor, in order to reduce additional losses, the number of stator and rotor slots should be as close as possible, but not equal, because this will produce synchronous additional torque and make the motor unable to start.

When using a multi-slot rotor, the harmonic loss in the rotor increases compared to a rotor with fewer slots. However, since the harmonic amplitude of the rotor teeth is reduced, the pulsation loss it generates in the stator teeth is also reduced. Therefore, the total additional loss is similar to that of a rotor with fewer slots.

However, for the cast aluminum rotor with skewed slots, the additional loss on the rotor side is much larger than that on the stator side because the loss between the bars caused by the transverse current is large and increases rapidly with the increase in the number of slots.

Therefore, in the skew slot cast aluminum rotor motor, the few slots - near slots combination is generally adopted , that is, the number of stator and rotor slots is close, and the number of rotor slots is slightly smaller than the number of stator slots.

When the stator is an open slot or semi-open slot and the rotor is a straight slot cast aluminum rotor, if the number of rotor slots is greater than the number of stator slots, the no-load additional loss will increase. Therefore, it is best to adopt a few slots and near slots combination.

There may be a serious defect in selecting a motor with multiple slots , which is the spacing between the rotor teeth. The ventilation slots occupy a large area, and the distance between the tubes is too small, which affects ventilation and heat dissipation . This is especially true for multi-pole motors with radial ventilation.

Therefore, when using multiple slots , be sure to carefully check the rotor tooth spacing. A more direct and clear method is to remove the air-to-air cooler on the top of the motor, run the motor at no load, and use an anemometer to test the wind speed at the stator ventilation duct from the top of the motor. If the wind speed is found to be almost zero, it is possible that the rotor slot tooth spacing is too small, and the number of rotor slots must be appropriately reduced.

●  The use of magnetic slot wedges can reduce the temperature rise of the motor by about 10K.

●  The rotor adopts closed slots , which can also reduce the additional surface loss.

● Properly increasing the air gap is also a very convenient and effective way for manufacturers to reduce temperature rise. This method can not only reduce the additional loss on the motor surface, improve the motor efficiency, but also improve the ventilation of the motor. Its disadvantage is that it will increase the no-load current and reduce the power factor of the motor.

●For YKK series motors, the air-to-air cooler can be replaced .