general equation for estimating rotor weight 1

[electricpete]

Does anyone have a general/approximate relationship for estimating rotor weight, given horsepower and speed of an integral-horsepower horizontal motor?

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[edison123]

pete,

when we first started our repair shop, we did not have any means to weigh the rotors for balancing purposes. So, back to basics - take the cross section and length of each section of the rotor, then calculate the weight with a density of 8 gm/cc.

Nowadays, I simply use a digital crane scale. Saves time but my math is getting a bit rusty.

kumar

 

[electricpete]

Thanks. I was looking more for a general and approximate relationship that can be applied to motors which are not disassembled, without a lot of research into dimensions and scarce vendor literature... just based on horsepower and speed.

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[aolalde]

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.