6. What among the following are considered for the selection of number of rotor slots?

For the selection of the number of rotor slots in electric machines, especially in induction motors, various factors are considered to ensure optimal performance, efficiency, and to minimize undesired effects. The principal considerations include:

1. Magnetic Compatibility (Harmonics): The number of rotor slots must be chosen carefully to minimize the harmonics induced by the slot combination between the rotor and the stator. This is crucial for reducing magnetic noise and minimizing losses due to harmonic fluxes.

2. Torque Ripple: The interaction between the stator and rotor slot numbers can affect the smoothness of the torque produced by the motor. Selecting an appropriate number of rotor slots can help in reducing torque ripple, leading to smoother operation.

3. Starting Performance: The starting characteristics of an induction motor, such as starting torque and starting currents, can be influenced by the number of rotor slots. An optimal slot number can improve starting performance while minimizing inrush currents.

4. Efficiency and Losses: By optimizing the number of rotor slots, the efficiency of the motor can be maximized, and losses minimized. This includes reducing eddy current losses and hysteresis losses in the rotor.

5. Mechanical Considerations: The mechanical strength of the rotor, cooling aspects, and manufacturing feasibility also play a critical role in deciding the number of rotor slots. Enough slots need to be provided to ensure structural integrity and to facilitate efficient heat dissipation.

6. **Cogging and Crawling