3 phase power measurement

[colar]

I am doing some work using a 3 phase motor. In order to get the information that I require, I need to determine the output power of the motor. I have been using a multimeter to measure the current through the three wires (red, white, black) and to measure the voltage. My question is, what is the correct formula for determining the power consumed by this motor? Also, when measuring the voltage, how do I know which voltage coresponds to which wires current?

I have been using the formula

P=3^.5 x p.f. x (EbIb+EwIw+ErIr)/3

where P= power
p.f.= power factor
Eb,Ew,Er=Measured Voltage Corresponding to wire color
Ib,Iw,Ir= Measured current corresponding to wire color

 

[jbartos]

Suggestion:
1. Normally/often, the motor power is calculated by
Pwattinput=3**0.5 x Vline-line x Iline x PowerFactor
Pwattshaft=3**0.5 x Vline-line x Iline x PowerFactor x Efficiency
2. Alternately,
Pwattinput=(Van x Ia x PFa + Vbn x Ib x PFb + Vcn x Ic x PFc)
if Van=Vbn=Vcn=V, voltages line to neutral
To obtain V for measuring purposes, you may create a neutral point over three identical resistors, fairly high ohm value depending on your voltage (on 208VAC three 120V 40W bulbs connected into Y, forming a neutral point, may do)
Ia=Ib=Ic=I, motor line currents
PFa=PFb=PFc=PF, power factor between line current and voltage line to neutral
then
Pwattinput=3 x V x I x PF
and
Pwattshaft=3 x V x I x PF x EF