how to reduce rotor losses in inductuction motors?

Answer 1

Rotor losses are comprised of two groups: load-sensitive (i.e. copper loss) and load-insensitive (i.e. windage, friction).

To reduce load-sensitive losses:
- Use the lowest resistance conductor (i.e. pure copper) for the rotor winding that still meets the other required performance characteristics (starting torque, breakdown torque, inrush current).
- Keep the temperature rise of the conductor down by using a generous cross-section of material.
- Use a non-rectangular (and non-circular) slot/bar shape to utilize the material most effectively during starting conditions.
- Make the shorting ring (also called an end ring) of pure copper, regardless of actual bar material - at least as long as the bar is a copper alloy.
- Used brazed joints between bar and ring, or contiguous casting to limit joint resistances that can create a thermal buildup.
- Use low-loss electrical steels for the rotor core. Use them as laminations (lamination direction is perpendicular to shaft). Coat them with an insulating coating to further reduce eddy effect and localized heating.
- Use radial ducting (or better yet, axial ducting under the winding) to enhance air flow and thus cooling capacity of rotor.
- Keep axial length (and thus resistive path) as short as possible, yet still achieve required torque performance.

To reduce load-insensitive losses:
- Make rotor surface as continuous as possible (a smooth surface, not slotted).
- Keep additional fan surface to a minimum while ensuring sufficient volume of cooling air is moved over the rotor and windings.

Note that sometimes there will have to be a tradeoff between possible means of reduction - and that the specified performance criteria will usually outweigh the 'low loss' rotor approach in larger machines.

Answer 2

1. Use electric steel 8050 to improve the magnetism of the motor and decrease iron losses.
2. Use cast copper instead of Al for the casing.
3. Reduce copper losses by temperature control.
4. Improve the power factor.