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The drive system transmits power from the turbine shaft to the generator shaft or the shaft powering an other device. It also has the function of changing the rotational speed from the one shaft to the other when the turbine speed is different to the required speed of the alternator or device.
The following options can be considered for micro hydropower drive systems:
This system is only for the
case where the shaft speeds are identical because it uses a flexible coupling
to join the two shafts together directly. The advantages are low maintenance,
high efficiency (>98%) and low cost. The only disadvantage is that the
alignment is far more critical than with an indirect drive.
Flat belt and pulleys
Modern flat belts run at high tension and are made of a strong inner band coated with a high friction material such as rubber. They have higher efficiencies than V-belts drives and run cleaner (ie with less rubber dust). One pulley must have a slightly convex profile (crowned) which together with good alignment, keeps the belt in position in either vertical or horizontal use.
The main disadvantage is that a higher tension is needed than with other drives (two tons is not unusual) which means that the bearings suffer high loads, sometimes requiring additional layshafts to be used or standard alternators to be fitted with heavier duty bearings. Also their availability in some areas is less good than that of V-belt drives.
Flat belts generally require narrower pulleys
than the equivalent multi V-belt with advantages in cost and reduced overhang.
Their maximum speed ratio is around 5:1.
"V" or wedge belts and pulleys
This is the most common choice for micro hydropower schemes up to 100 kW. A major advantage is that these belts are very well known because of their extensive use in all kinds of small industrial machinery, hence they are also widely available.
V-belts differ from flat belts in that the frictional grip on the pulley is caused by wedging action of the side walls of the belt within the pulley grooves. Therefore less longitudinal tension is required to maintain the grip and less radial load is imposed on the shaft and bearings.
Usually a number of V-belts are run side by side in multiple-governed pulleys. Matched sets of belts are required to ensure even tension and these sets can be difficult to obtain in some countries. At higher powers and torques multiple V-belt installations can become cumbersome with eight or more large belts and very wide pulleys.
The tolerance of misalignment of V-belts
is very good but efficiency is low than other types of belt at around 85-95
%. At very low powers the low efficiency can be a problem and timing belts
are often preferred. Maximum speed ratio is around 5:1.
Timing belt and sprocketed pulley
These drives are commonly found on vehicle camshaft drives and involve toothed belts and pulleys. They are very efficient (about 98%) and clean running. The belt tension is lower than in any other belt drive, giving reduced bearing loads, but the belts do not slip on overload so cannot protect the shafts and bearings.
The main disadvantages are the cost of
belts and pulleys and the low availability. They are specially worth considering
for smaller drives (less than 3 kW) where efficiency is at a premium. Speed
ratio can be up to 10:1.
Chain and sprocket
Chains can have a very high efficiency
but only at some sacrifice of lifetime. Long-life chain drives tend to
be similar in efficiency to belt drives. Chain drives are not recommended
because of their high cost, poor availability, the need to replace sprocket
wheels periodically and the difficult lubrication requirements. Very high
speed ratios of greater than 20:1 can however be achieved.
Gearboxes are used with larger machines
when belt drives become too cumbersome and inefficient. Problems of specification
alignment, maintenance and cost rule them out except in cases where they
are specified as part of a turbine-generator set.