No load current of a 3phase induction motor varies
according to its KW and rpm. Normally as a thumb rule
fraction HP motors (below 2.2KW) will be 50% of full load
current. Above 2.2KW may be 20% to 30% OF full load
There is no standerd formula for calculating no load
current, It depend on the desine and material used in
motor. There is a major role of material(winding wire&
stamoing)to anhancing the efficiancy of motor.
Torque of a rotor depends on its diameter and weight along
with winding as well as material and methodology of
construction and also on applied bearings and lubrication. A
bulky, heavier rotor will draw more no load current than
small, light weight rotor. Formula : Torque (T) = qr (Where
q=Tangential force acting at PCD of winding due to effect of
current flowing through it, and r= distance of winding from
centre of the shaft). This torque only tries to rotate the
dead weight(W) of the rotor on no load. It can be said
T=qr=(W + rolling friction in bearings + wind resistance).
These all factors can be calculated easily and then finally
to arrive the result i.e. no load current, however bearing
frictional force and wind force can be ignored.