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.

A note about slip test

It is conducted to calculate unsaturated Xd and Xq.
We feed subnormal voltage to armature, mechanically rotate  it, keep field open ( Vf) 
Here Nr is not equal to Ns.
but is almost equal to Ns.
Since mmf due to armature reaction is rotating at Ns, field poles gradually slip w.r.t. rotating armature mmf.
Thus R = 1/ u of path of mmf changes at twice slip freq. from maxm when axis of poles and axis of armature reaction  wave coincide and minm. when they are at right angles.