quote:

Each manufacturer has different practices but for integral motors ( 1 to 500 HP) as the frame sizes and HP pole are NEMA regulated, the weight of the rotor is related to the stator outside diameter (OD1) and Rotor outside diameter (OD2) relation (R). R=OD1/OD2

For the core lamination:
RW (rotor weight) =(OD2^2*pi/4)*L*d
SW (stator core weight) = (OD1^2*pi/4)*L*d - (OD2^2*pi/4)*L*d
= (OD1^2 – OD2^2)*pi/4*L*d
L= core length and d steel lamination density.

SW/RW= (OD1^2-OD2^2) /OD2^2 = R^2-1

The windings could be considered following the same relation, most of the rotor bars are aluminum.

In general the laminations have three ratios:
for 2 poles R=2
for 4 poles R=1.666
For 6 or more poles R=1.5

For a given frame size the Frame, End-brackets, bearings and shaft are constant and will take around 25% of the total weight for ODP and 35% for TEFC.

EG. 50 HP, 4 Poles, 60 HZ. 460 V, TEFC, Frame 326T, Total Weight 795 LB.

Structure weight~ .35*795 = 278.25 LB

then;
SW + RW = 795-357.75 = 516.75
SW/RW=1.666^2-1 = 1.775

Solving the two equations,

SW= 330.53 Lb
RW=186.22 Lb

This is only an approximation; the variation of results could be very rough from motor to motor