Installation manual

3.5. Understanding open circuit voltage (Voc)

When there is no current, because the turbine spinning is disconnected, we can measure the

"open circuit voltage" (Voc). The Voc depends directly on the turbine speed in rpm. Double

the speed and you double the Voc. Different turbines will have different Voc/rpm ratio,

depending on the site and the required voltage. But any turbine will have a much higher

voltage when it is running faster off load than it will when working optimally.

Imagine you put your foot on the accelerator/gas pedal in a car when the engine is out of

gear. The rpm will go up quickly and the engine will race. Let in the clutch and the rpm

slows down as the engine comes under load. If you open the valves for the turbine when it

is electrically disconnected then it will overspeed to almost double the best operating speed

for energy production. The Voc will now be much higher than the design operating voltage.

If it is designed to work at 56 volts (to directly charge a 48-volt battery) then the Voc may rise

above 150 volts. This could endanger you if you touch it, and it most likely would kill any

inverter that is designed to connect to a 48-volt battery. So it's very important never to

disconnect the battery when the turbine is running and connected to an off-grid inverter.

In the case of solar PV the Voc is also somewhat higher than the battery voltage, but the

difference is not so huge, and the dangers are less. Most controllers designed for solar PV

will disconnect the PV to control current to the battery. They can withstand the Voc from a

PV array but may be destroyed by the high Voc of the hydro turbine.

Never connect a controller in series between the PowerSpout and the battery unless:

 It's an MPPT controller

 you have checked rigorously that the Voc will not damage it.

If the battery voltage is 48 V then you will need to either use a protection crowbar (for

example a PLT100C turbine) or you will need to choose a controller with a high enough Voc

rating.

If you intend to run a PLT turbine on a MPPT controller/inverter you must:

 Tell us prior to ordering

 Report an accurate measure of the head of your system

 Check runaway Voc prior to hook up