How do you calculate output power in an AC Three phase Induction Motor?

P=IE

in three phase circuit

input power is
P=V*I*sqrt(3)

output power is
P=V*I*sqrt(3)*cos(theta)

V=voltage (effective voltage)
I=current (RMS current)
sqrt(3) = 1.73.... (square root of three)
cos(theta) = power factor

based on given voltage and current values, input power is 1215W while output power is 802W


we can also calculate same using mechanics:
P=T*w

where T is torque and w is angular velocity (rad/sec)

1422RPM is 149 rad/sec and with only 0.5Nm torque power is only 75W so your numbers don't match. are you reading current and voltage of the name plate or those are actual measurements? if the load is smaller, motor will draw less current (less than FLA listed on name plate).

Output power = 1.732(397)(1.77)(0.66)
                        = watts