Above are conceptual line drawings that show the basic difference between direct-drive and belt-drive for a generator. The component that generates electricity is called an ‘alternator’ because it generates alternating current.
With direct-drive, the engine and alternator are bolted directly together (usually no gearbox between). The engine crankshaft has to be perfectly aligned with the shaft of the alternator rotor and both rotate at the same speed in lockstep. The engine and alternator are usually very close together so that the shaft is not exposed. Because of the precise alignment required, this type of arrangement is difficult to do on an ad-hoc DIY basis unless the parts are specifically designed to be fitted together.
With belt-drive, there is a robust flexible belt (possibly several parallel belts), which are driven by the engine pulley/sheave and transfer the energy to a pulley on the alternator ‘head’. This is the same type of arrangement as most vehicle engines have to drive the water pump, alternator, air-con etc. There is a tiny loss of energy in the belt drive, but it is not significant if it is adjusted properly. Misalignment of pulleys for belts will increase wear. They can be aligned with a laser guide, but a belt drive is far more forgiving in terms of alignment of the components. Belt tension likewise has an optimum value, but will operate satisfactorily across quite a wide range.
| Shaft drive | Belt drive |
|---|---|
| No loss of energy in the shaft | Tiny loss of energy in the belt |
| Engine has to rotate at 3000 RPM for 50 Hz with a 2-pole alternator (traditional non-inverter) | Required engine speed can be set by modifying pulley diameters. This enables the use of slow-running engines such as Lister, Petter, or ChangFa. |
| Generally unable to touch moving parts | Moving parts are exposed and care must be taken with safety of personnel and animals when system is running. |
| Reconfiguration of equipment not generally possible or easy. The way you bought it is the way you have to run it. | Reconfiguration of equipment is fairly easy. The engine and/or alternator can be swapped for other devices if required. Easy to switch between 1-ph and 3-ph alternator as required and retain the same engine. You might want to switch the alternator ‘head’ between and ST, STC or Stamford types. |
| Can use a ‘single bearing’ alternator. The alternator relies on the engine bearings to support the driven end of the alternator rotor | Requires ‘two-bearing’ alternator to fully support the rotor at each end |