Page 464 - Wind Energy Handbook

438 COMPONENT DESIGN

• the churning of oil in the bath, which can result in a net efficiency loss, is avoided,
• the oil circulation system enables heat to be removed much more effectively from
the gearbox by passing the oil through a cooler mounted outside the nacelle,

• it allows for intermittent lubrication when the machine is shut-down if a standby
electric pump is incorporated.

With a pressure fed system, it is normal practice to fit temperature and pressure
switches downstream of the filter to trip the machine for excessive temperature or
insufficient pressure.
Guidance on the selection of lubricant, which has to take into account the ambient
temperatures at the site in question, is given in the AGMA/AWEA (1996) docu-
ment. Sump heaters may be needed to enable oil to be circulated when the turbine
starts up at low temperatures.

7.4.11 Gearbox efficiency

Gearbox efficiency can vary between about 95 percent and 98 percent, depending
on the relative number of epicyclic and parallel shaft stages and on the type of
lubrication.

7.5 Generator

7.5.1 Induction generators

The induction generators commonly used on fixed-speed wind turbines are very
similar to conventional industrial induction motors. In principle the only differ-
ences between an induction machine operating as a generator and as a motor are
the direction of power flow in the connecting wires, whether torque is applied to or
taken from the shaft and if the rotor speed is slightly above or below synchronous.
The size of the market for induction motors is very large and so, in many cases, an
induction generator design will be based on the same stator and rotor laminations
as a range of induction motors in order to take advantage of high manufacturing
volumes. Some detailed design modifications, e.g., changes in rotor bar material,
may be made by the machine manufacturers to reflect the different operating
regime of wind turbine generator, particularly the need for high efficiency at part
load, but the principles of operation are those of conventional induction motors.
The synchronous speed, which is determined by the number of magnetic poles
will be in the range of 1500 r.p.m. (4 pole), 1000 r.p.m. (6 pole) or 750 r.p.m. (8 pole)
for connection to a 50 Hz network. For commercial reasons it is common to use a
voltage of only 690 V even for large generators and in some very large wind
turbines the resulting high currents have led to the decision to locate the turbine
transformer in the nacelle. The physical protection of the generator is arranged to
avoid the ingress of moisture, i.e. a totally enclosed design, and in some wind