Electrical Engineering – Basic Motor Formulas

Basic Motor Formulas

Mechanical Formulas
Torque in lb.ft. = HP x 5250

rpm HP = Torque x rpm

5250 rpm = 120 x Frequency

No. of Poles

Temperature Conversion

Deg C = (Deg F – 32) x 5/9
Deg F = (Deg C x 9/5) + 32

High Inertia Loads
t = WK2 x rpm

308 x T av. WK2 = inertia in lb.ft.2
t = accelerating time in sec.
T = Av. accelerating torque lb.ft..
T = WK2 x rpm

308 x t
inertia reflected to motor = Load Inertia Load rpm

Motor rpm 2

Synchronous Speed, Frequency And Number Of Poles Of AC Motors
ns = 120 x f
Where Ns = Synchronous Speed
f = Frequency
P f = P x ns

120 P = 120 x f

ns

Relation Between Horsepower, Torque, And Speed
HP = T x n

5250 T = 5250 HP

n n = 5250 HP

T

Motor Slip
% Slip = ns – n

ns x 100

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