Sunday, March 30, 2014

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