But while you need to calculate the KWH consumed by a motor,
multiplying KW of motor (nameplate KW) and hour will not
give you the energy consumed. This is because the name plate
KW in a motor is not the electrical power, but the
mechanical power output provided by the motor (Electrical
Power = Frictional losses + Eddy current losses + Copper
losses + air gap losses + Mechanical power). In order to
calculate the energy consumed by the motor you can use the
single phase AC = Voltage x Rated current x Power factor x
Hrs run. (100% load)
3 Phase AC = 1.732 x Voltage x Rated Current x Power factor
x Hrs run. (100% load)
Normally the power factor of motor is considered between 0.8
to 0.85 at full load operation.
At no-load, motor will take approx 40% of its rated full
load current (again this depends on the design of motor, but
normally assumed to be near 40%). So at 50% load, the motor
will take a current around 70 to 75% of its full load rated
current. so we can approximate as
energy consumed at 50% load (motor - 3 phaase) = 1.732 x
Voltage x 75% rated full load current x power factor x Hrs run .
at no load = 1.732 x Voltage x 40% of rated current x power
factor x Hrs run.