Three phase motor formula

There are following formula's which are very useful for an electrical engineer while doing calculation on various aspects for induction motors.


High Inertia Loads

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

For calculating time required to accelerate an induction motor at a particular load formula-I will help the same

If you know want a particular motor to get accelerated at a given time then required torque can be calculated by using formula-II in above.

Both formulas are quite helpful while doing programming in Variable frequency drives and soft-starters.

Inertia reflected to motor = Load Inertia X (

Load rpm/Motor rpm

)X(Load rpm/Motor rpm)

Synchronous Speed, Frequency And Number Of Poles Of AC Motors

ns =

120 x f  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

Symbols

I	=	current in amperes
HP	=	output power in horsepower
n	=	motor speed in revolutions per minute (RPM)
ns	=	synchronous speed in revolutions per minute (RPM)
P	=	number of poles
f	=	frequency in cycles per second (CPS)
T	=	torque in pound-feet
EFF	=	efficiency as a decimal
PF	=	power factor as a decimal