calculating C 2C capacitors for 3phase induction motor as generator IMAG

[hydro control]

Wiring: (picture taken from the first link below)



Capacity of C2 is double that of C1, hence the abbreviation C, 2*C or even shorter C, 2C.

When an induction generator is used in this way, particular care must be taken over the connection of the capacitor C2.
If capacitor C2 is connected between phases a and c instead of c and b, then the generator will run as an unbalanced system.
Under this condition, the generator winding will overheat.

Quick formula:
C(Farad) = P(power in Watts) * efficiency of the IMAG /  ( U(Volt)pwr2 * frequency(Hz) * 11 )
The power is not the rated power of the generator, also not the rated power of the turbine. It is the actual power produced by the water, and as a consequence, the actual current through the capacitor.

e.g C = 10,000 Watts * 0.8 / ( 230V*230V * 50 * 11) = 0,000274 Farad = 274 * 10exp-6 Farad = 274 microFarad = 274 uF

For fixed values of 230V and 50Hz and 80% efficiency, the formula can be simplified to one factor:

C (in uF!) = power ( in KiloWatts!) * 27
e.g. 10kW * 27 = 270 uF



detailed formula and explanations:
http://www.who.int/management/InductionGenerators%20forSmallHydroSchemes.pdf
and
https://www.riaed.net/IMG/pdf/Pico_hydro_for_village_power_3.pdf
The pico hydro for village power formula has an error: the result is given in Farad, not in uF (microFarad).
The Line Current I_line is the single phase current through the load.

The uncertain factor here is the efficiency: the pico hydro document assumes a much lower efficiency than 0.8 (80%), which would result in lower capacitance

To get the optimum result (maximum efficiency), a bit of experimentation on site is needed.

Determine actual power output with the given capacitors:
Starting initially with a low load, the voltage will be too high.
Increase the load, until voltage reaches your target nominal voltage.
Measure the current, and/or note the total load in Watts.
(if you already have an IGC with current measurement connected, the IGC will do this automatically for you)

Increase the capacitors and measure again:
Starting initially with a low load, the voltage will be still too high, but not as much.
Increase the load, until voltage reaches your target voltage.

Is the current and the load smaller or bigger than before?

• If smaller, reduce capacitors again.
• If bigger, add more capacitors and repeat, until the total load decreases again.

Also consider that you are optimizing for a given amount of water. If the water decreases, not only the available water power decreases, but also you are using it less efficiently!
So if you have variable amounts of available water over the year, it is recommended to optimize for the situation with little water.
That means if in doubt, choose your capacitors smaller.

Make sure the voltage rating of the capacitors is well above your line voltage (ideally double, but that's a question of money)