What makes a motor energy efficient ( IE2 )

Core Loss in motor :- hysteresis and Eddy current loss. Mainly in stator core ( as rotor core losses are @ slip freq.).
To reduce this material of core should be low loss Si steel. M45, M43, M36, M24 grade. ( increasing order of low-loss) Stamping should be thinner than 0.4mm

Rotor Copper Loss:- Die-cast rotors reduce losses.( instead of Cu bars).

Better Heat Transfer:- Aluminium stator frame does it better.

Reduction in air-gap of 2-pole motors: helps in increasing efficiency.