Electronic – Obtaining DC from a tri-phase generator

It depends on your load. If you have a symmetrical load which sinks a sinusoidal current and power changes relatively slow, then what you're doing works. You can calculate the power at each sample by:

$$P = U_1 \cdot I_1 + U_2 \cdot I_2 + U_3 \cdot I_3$$

Sum all samples up and you will get the desired energy \$W\$. For example with 50 samples/s:

$$W = \frac{\Sigma P}{50} ~~[Ws]$$

But if your current is not such a nice sine and your power consumption changes fast (within 1 period) than you need much more samples than 50/s (as Andy aka said).

As the generator gives balanced 3-phase, you can use three large same resistors witch give you the neutral point and then find the frequency from neutral point and one of phases.

^{simulate this circuit – Schematic created using CircuitLab}

In the previous schematic we divided the \$100M\Omega\$ resistor into \$3M\Omega\$ and \$97M\Omega\$ to reduce the output voltage. By the generator's peak voltage (\$150V\$) the output on \$3M\Omega\$ voltage will have less than \$5V\$ peak and can be determined easily by an Op-Amp.

Another way:

Of course it is possible to find out frequency of generator from voltage of one line to another. For example from \$R\$ to \$S\$.

The waveform of \$R-S\$ voltage is like this:

This is the plot of \$f = Sin(x)-Sin(x-2 \pi / 3)\$ (Plotted by Fooplot). The period of f witch can be found by finding the \$f(x)=f(x + T)\$ is same as \$Sin(x)\$. So The frequency of generator is also can be determined by \$l-l\$ voltage. And a simple zero crossing detector can help to find the frequency.