A hydro plant needs a generator that turns at the speed its turbine can supply and a way to keep the electrical load constant. Use a salvaged car alternator for 12 V battery charging, a permanent magnet alternator for variable-frequency AC, or an induction motor spun above its rated speed with a capacitor bank, and add a dump load, because a hydro turbine delivers nearly constant power and the generator speeds up dangerously within seconds if the load disconnects.
Part of Micro-Hydro Turbine
The generator turns the turbine’s spinning shaft into electricity, and the electrical system around it decides what the electricity can be used for and what happens when the load changes.
Three Machines Can Be The Generator
The carrier lists three options.
| Generator | What it produces | What it needs | Watch for |
|---|---|---|---|
| Car alternator | 12-14 V DC | belt drive from the turbine | made for battery charging, not for loads without a battery |
| Permanent magnet alternator (PMA) | AC at variable frequency | rare-earth magnets and copper wire to build | needs rectifying to charge a battery; frequency follows speed |
| Induction motor run as generator | AC | spinning faster than its rated speed, plus a capacitor bank for excitation | the capacitor bank must be right for the machine |
The carrier names the PMA as the best for micro-hydro, with the build described under Advanced Wind Turbine. The Hydro Generator page says the generator must spin at 500 to 1,500 RPM, which sets the drive ratio from the turbine (Generator Coupling (Belt vs Direct)).
Rate the generator at or above the shaft power. If the turbine gives 750 watts at the shaft, a 500 watt generator will overheat at full flow, and a 750 W or larger one will not.
A Hydro Turbine Does Not Know What The Load Is
The Load Control page puts it plainly: a turbine at constant head and flow delivers nearly constant power. When the load drops, the generator speeds up, and voltage and frequency rise; if all load disconnects, the generator can be wrecked within seconds to a few tens of seconds. That is the defining difference from wind or solar:
Different sources, different problems
A hydro plant has steady power and a changing load, so the electrical system must keep the load steady by sending any unused power to a dump load such as a water-heater element. A wind turbine has a changing power, so its electrical system is built to store the surplus in batteries and burn the rest. See Electrical System on the wind side.
Regulation: Three Ways To Hold The Speed
The carrier gives three approaches, covered in Voltage Regulation and Load Control:
- Electronic load controller. It diverts surplus power to a dump load. It is the most common micro-hydro solution.
- Mechanical governor. It adjusts the nozzle or gate to match flow to load; more complex, but no energy is wasted.
- Battery buffer. The batteries absorb variation, and an inverter supplies AC (Battery Charging vs Direct AC).
Size a dump load at 100 to 120% of the turbine output, as the Load Control page says, so that it can absorb everything when all other loads are off.
Current Grows As Voltage Falls
For 350 W of output: at 12 V the current is 350 ÷ 12 = 29.2 A, at 24 V it is 14.6 A, and at 48 V it is 7.3 A. Thicker wire is needed for the same power at a lower voltage (Conductor Selection).
Common Mistakes
- Running the generator with no load connected — it overspeeds; the dump load must always be connected.
- A generator that wants 1,500 RPM on a 10 RPM wheel — see the gearing arithmetic in Overshot Waterwheel.
- An undersized generator — it overheats at full flow.
Safety
Overspeed and live output
If the load disconnects, a hydro generator accelerates and its voltage rises with speed. Keep the dump load wired in permanently, close the intake gate before disconnecting any load, and treat the output as live whenever the water is running. Name the stop: the intake gate.
What’s Next
- Learn next: Generator Coupling (Belt vs Direct) for connecting turbine and generator
- Combine with: Charge Controller if the generator charges batteries