Breezing into Power Generation
Coupling design
aids development of giant turbines for windpower
In this current age of raging
blackouts caused by the halted development of traditional
power plants, environmentally-friendly wind farms are gaining
popularity. Increasingly larger, more efficient wind turbines
have gradually gained a competitive edge as an alternative
energy resource. Each mammoth turbine can deliver up to 2000
kilowatts, serving the needs of 300 average residences. However,
there are many design challenges introduced by these behemoths.
Wind generators operate at varying speeds, between 12 and
22.5 rpm, driving generator speeds between 1000 and 1800 rpm.
Constantly varying wind forces and changing climatic conditions
present extreme torsional and structural demands. Additionally,
the turbine's location about 50 meters above ground isolates
it from routine observation and maintenance. The entire system--propeller,
shaft, gears, coupling and generator--must be designed to
function continuously without failure for a projected 20 years
or more. And, if the farm is located anywhere near residential
areas, it must operate quietly.
Coupling Components
Among the highly engineered components, is the coupling that
connects the high-speed gearbox shaft and the asynchronous
generator. The gearbox is mounted on rubber bushings that
absorb the heavy oscillating aerodynamic loads relative to
the generator. These movements are in turn transmitted to
and absorbed by the couplings.
Elastomeric couplings have the flexibility to absorb the
oscillating loads, however, the material tends to soften over
time. Fretting corrosion, especially around bolt locations,
plagues steel disc couplings, particularly where intermittent
torque is experienced. Constructed from composite materials,
CD Couplings from Zero-Max, Inc., Plymouth, MN, have been
customized from smaller standard models for use on the NM2000
turbines produced by Denmark-based NEG Micon.
Using two disc packs in a spacer coupling configuration,
the hubs mount on the 150-mm shafts. Overall disk diameter
is 460 mm with a coupling length of 578.5 mm. The coupling
disks use eight bolts to connect to the exterior mounting
hubs and a steel centering shaft. The open arm design, along
with the rugged composite materials provide high misalignment
capacity found in elastomeric couplings, but with higher torsional
stiffness. This open arm disc allows up to 1 mm parallel misalignment
and up to 3-degree angular misalignment. The ply-fiber orientation
of the composite material lends rigidity while dampening shock
(promoting quieter operation).
Additional Design Considerations
Early in the design process, engineers had to modify the
distance between the gearbox and turbine generator by 100
mm, thereby reducing the overall length of the coupling correspondingly.
Using a linear variable displacement transducer, they determined
the misalignment conditions. The change increased misalignment
from 1 to 1.2 degrees, but fell well within the specifications.
In over one-and-a-half years of testing, these couplings
have operated during winter and summer extremes with no discernible
affect. Visual inspection and bolt-tension checks performed
during routine service of the turbine have indicated no need
for maintenance. No sign of fretting corrosion has been detected.
Currently in the offing are even larger wind turbines for
offshore installation, designed to operate at similar speeds,
utilizing the asynchronous generators, but at higher torques.
Engineers are confident that the same coupling design can
be modified to handle these higher torques. Other applications
for these couplings include large commercial workboats, off-high
vehicles, paper converting machines, printing presses and
mining equipment.
--SG
For more information:
Circle 703 - Zero-Max Inc, or connect directly
to their website via the Online Reader Service Program
at http://www.OneRS.net/105df-703
Circle 704 - NEG Micon, or connect directly
http://www.OneRS.net/105df-704
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