Join turbine and generator directly only when they want the same speed and the shafts can be held in line; otherwise use a V-belt, where generator RPM equals turbine RPM times the turbine sheave diameter divided by the generator sheave diameter. A 309 RPM crossflow turning a 1,500 RPM alternator needs a 4.85:1 ratio, such as a 300 mm sheave driving a 62 mm sheave, and the drive must be rated at 2 to 3 times the running torque of 31.8 N·m.
Part of Micro-Hydro Turbine
The coupling or belt is the mechanical link that carries the turbine’s torque to the generator while absorbing misalignment and vibration.
Speed Match Decides Direct Or Belt
A turbine and an alternator are each built for a speed. The carrier notes that a Pelton wheel at 500 to 1,500 RPM often drives a generator directly, while a crossflow at 200 to 500 RPM and a waterwheel at 5 to 15 RPM need a step-up. The Generator Coupling page describes the hardware: flanged rigid, pin-and-bush, chain and V-belt. This page does the arithmetic of choosing between them.
Worked Example: The Carrier’s Crossflow (5 m, 30 L/s)
From Banki (Medium Head): shaft power 1,030 W at 309 RPM.
Torque. ω = 309 × 2π ÷ 60 = 32.4 rad/s, so torque = 1,030 ÷ 32.4 = 31.8 N·m. The Generator Coupling page’s formula gives the same: 1,030 W is 1.38 hp; 1.38 × 5,252 ÷ 309 = 23.5 ft·lb = 31.8 N·m.
Rated torque. The page multiplies by 2 to 3 for shock and starting, so rate the coupling or belt for 64 to 96 N·m.
Ratio. 1,500 ÷ 309 = 4.85. Choose a 300 mm sheave on the turbine and 300 ÷ 4.85 = 62 mm on the alternator. Check: 309 × 300 ÷ 62 = 1,495 RPM.
Torque at the generator. 1,030 ÷ (1,500 × 2π ÷ 60) = 1,030 ÷ 157.1 = 6.6 N·m, which is 31.8 ÷ 4.85, so the belt trades torque for speed.
Belt speed. π × 0.300 × 309 ÷ 60 = 4.85 m/s, the same as the runner’s rim speed.
Wrap on the small sheave. With centers 600 mm apart, the belt leaves each sheave at an angle whose sine is (300 − 62) ÷ (2 × 600) = 0.198, so 11.4°. The wrap on the small sheave is 180° − 2 × 11.4° = 157°. A larger ratio or closer centers shrinks that, and a belt wrapped on less of the small sheave slips more easily.
For a Pelton (see Pelton Wheel (High Head)) at 568 RPM and 12.6 N·m, a generator wound for about 568 RPM can be coupled directly; one that wants 1,500 RPM needs 1,500 ÷ 568 = 2.64:1.
Direct Or Belt: What Each Costs
| Factor | Direct coupling | V-belt |
|---|---|---|
| Speed ratio | 1:1 only | any, by changing sheaves |
| Alignment | tight: the Generator Coupling page gives 0.002 in (0.05 mm) parallel and 0.1° for a rigid flange | tolerant |
| Misalignment tolerated | ±0.5° and ±1 mm for pin-and-bush | belt absorbs it |
| Overload | transmits it | slips, protecting both machines |
| Side load on bearings | none | belt tension pulls on both shafts |
| Losses | negligible | a few percent (recalled, not from the corpus) |
A belt adds a failure path. If the belt breaks, the generator is no longer attached, so a dump load cannot help: the turbine is unloaded and speeds up. The turbine, wheel and any belt on it must survive its free-run speed, or have its own overspeed trip on the turbine shaft (Load Control).
Method: Fit And Check The Drive
Step 1: Compute shaft power, speed and torque as above.
Step 2: Choose the type. Direct if the speeds match within the generator’s tolerance, belt otherwise.
Step 3: Key every hub. The Generator Coupling page checks a 12 × 12 mm key in a 50 mm hub as 600 mm² × 0.6 × 250 MPa = 90,000 N, far above this drive’s needs.
Step 4: Align. Straight-edge across the sheave faces, or dial indicators for a rigid coupling.
Step 5: Tension the belt until it deflects only slightly under a thumb, and re-check after the first hour as new belts stretch.
Step 6: Guard it, then run it light before connecting any load.
Common Mistakes
- Over-tight belts — they pull the bearings out of true.
- Sizing from running torque alone — starting and shock loads are 2 to 3 times higher.
- A big ratio on a short center distance — the belt wraps too little of the small sheave and slips.
Safety
A belt is a nip point you cannot stop with your hands
A belt and sheave pull in a sleeve or hand in an instant, and the turbine keeps driving them as long as water flows. Guard every belt and shaft. The stop is the intake gate or nozzle valve, and whoever works near the drive must be able to reach it. After the gate is shut, wait for the turbine to coast down; time the coast on the first run and post it by the gate.
What’s Next
- Learn next: Battery Charging vs Direct AC for what the generator then feeds
- Combine with: Bearing Maintenance, since belt tension sets the bearing load