Installation manual

3.4. How can we be sure of getting the right voltage out of the turbine?

Your turbine will have been designed to produce maximum power at the chosen operating

voltage. Turbine voltage depends on speed. The design process involves predicting the

best rotational speed for the turbine, which depends in turn on the pressure of the water,

which depends on the head you measured.

Accurately measure the head at your site, and use the recommended pipe size, so that

the actual pressure of the water ends up being close to the value that the turbine is designed

for. Every site is different, so the design process is critical and the measurement of head is

key.

3.4.1. Voltage and turbine speed

The voltage produced by your turbine will vary depending on how fast it is running.

Renewable energy sources such as turbines and PV panels are actually quite volatile in their

voltage. You need a load (for example a battery) connected to keep that voltage stable. If a

turbine or a PV panel is hooked up directly to a battery, it has no choice: its voltage is the

same as the battery voltage. Two things that are connected to each other in parallel must

have the same voltage. If the turbine tries to push its output up to a higher voltage then

there will be a charging current into the battery that holds the turbine voltage down through

its internal impedance. The extra current will raise the battery voltage a little, but the battery

is very stable compared with the turbine or PV. So the battery itself actually governs the

runner speed, by loading (restraining) the turbine when it runs fast enough to produce the

desired voltage.

Your turbine will be designed to work best at the head you have reported for the site and the

voltage of your battery (or controller). We recommend that you opt for an MPPT controller in

the electrical design of the system (later) because this gives more flexibility. The controller

will automatically adjust the voltage (within its operating range) to find the best speed. This

maximises the power. Hence the title Maximum Power Point Tracker.

3.4.2. Manual optimisation of the turbine

If the voltage is fixed (turbine feeding the battery directly without MPPT) then we can still

manually adjust the speed of the alternator by tweaking a third variable: magnetic field

strength.

The voltage actually depends on several factors: the rpm, the design of the windings in the

stator, and the strength of the magnetism moving past it. By putting spacers behind the

magnet rotor we can move it slightly away from the stator and then the runner will move

faster to create the same voltage. So, if we find that the turbine is a little too slow compared

to it's best speed we add spacers and optimise the output. (see 3.58.4.3 for more on this)

This may be tricky to do and may not achieve the best operating speed and conversion

efficiency for the turbine. It may not be easy to optimise the turbine for all flow conditions,

using packing washers.

One big advantage of MPPT controllers is that they will alter the turbine voltage until they

find the maximum power (corresponding to the maximum speed). So we get the best out of

the turbine under all conditions and we don't have to take it apart and play with spacers to

achieve this.