Calculation of 3 phase motor power - Calculation of energy consumption for all p

Hello At the very beginning I wanted to point out that I am green in matters related to electrics.
I need to calculate the electricity consumption for a three-phase motor with data:

Engine SIEMENS C-60 3 motor 15 kW
1465 rpm
cos? -0.85
power [kW] at 50 Hz 15
power [kW] at 60Hz 17.3
Voltage 380-420 / 660-690V (? / Y)

I measured the current for each phase:
I1 Avg [A] - 9.23
I2 Avg [A] - 9.45
I3 Avg [A] - 9.35

Then I calculated the power from the formula P = ?3 * U * I * cos? for each phase:
5.44kW 5.57kW 5.51kW

And here the first question arises ...
As U, I found 400V, is this correct?

Next, I would like to calculate the electricity consumption for the engine.
Is it enough to add 5.44kW + 5.57kW + 5.51kW = 16.51kW in this case?
I don't personally like this form of operation because since it consumes 16.51kW and the engine is defined as 15kw, something here probably does not work ...

I searched a little on the net and found a formula P = Um * If1cos?1 + Um * If2cos?2 + Um * If3cos?3
after data substitution it follows that P = 9.5302 kW
Of course, as Um for each phase I decided 400V and I have doubts whether it is good.

As you can see, I have a lot of confusion here ... I tried to deal with it with my own strength but unfortunately I am not sure if my thinking is correct. Please, dear PROFESSIONALS for help )

A motor with a power of e.g. 15kW has a mechanical power of 15kW on the shaft, but this means that it must take more electrically, in the said case it must take 15kW + winding loss power + wind current losses + mechanical losses (bearing and fan resistance). Zaten 16.5kW of electric power consumption is the most theoretically correct result. But for us (for a current of about 9A), the motor power is about 5.5kW, because the formula P = ?3 * U * I * cos? applies to the power of the entire motor and not one phase, and if you do not know what voltage the phases have - just Measure them with a voltmeter ... And one last note: a 15kW motor takes 15kW, but only when it is loaded. If you load a 15kW engine with 5kW power (torque corresponding to the load), the engine takes 5kW + approx. 1 kW of loss (no more), i.e. it has 6kW and this is also normal and correct.

Since P = ?3 * U * I * cos? applies to the power of the entire engine, not one phase .....
I have measurements of the intensity for each phase, each of them is not different. How to correctly apply the U value in the formula?
Should I average these 3 currents? How to do it professionally?

I am writing a diploma thesis on the optimization of technical solutions for belt conveyors. The motor in question is used in the conveyor drive on which I carry out measurements. With the help of the SkyLab device I measured the current for 3 phases within 8 hours, with an accuracy of 10s.
In my work, I want to take into account the electricity consumption of the engine, which will later be converted into conveyor operating costs.
That is why he asks about the "professional" calculation of the energy consumption of a given engine, the calculations will be used at work, so I would not like to make a mistake.

and_rog - Thank you for your reply

Maybe someone else will join the discussion?

This engine consumes 6,5kVA.

15kW rated engine consumes 32A.

Added after 2 [minutes]:

bartekgogolin wrote:

I am writing a diploma thesis

But you don't know ANYTHING how a 3 phase asynchronous motor works.
You don't know anything or power because you haven't measured it.

At this link you will find some info about 3 phases
http://elektrotechnika.po.opole.pl/cwiczenia/uklady_3f/uklady_3f.html