New Danfoss VLT® Decentral Drive E cient, reliable and powerful

BEARINGS, BELTS & CHIAINS
identification of potential
problems.
> Remedial action
The most effective
preventative or remedial
action in the event of
electrical erosion in bearings
will depend on the
characteristics of the
equipment and on the
interventions available to the
engineer. In new installations,
risks can be minimised by
using symmetrical multi-core
motor cables in which the
earth conductor is either
arranged as a concentric
shield around the phase leads
or as three separate leads
positioned between them.
Shielded motor cables can
also provide a short, lowimpedance
path for commonmode
current to return to the
inverter. Likewise, a
grounding brush on the
motor shaft can offer an
easier path to earth than that
provided by the motor
bearings. Effective grounding
between other motor
components can also be
important.
The high-frequency
switching that can be a major
cause of electrical erosion can
sometimes be minimised by
changing the characteristics
of the drive control program,
or by installing dedicated
filters in the drive circuit.
The use of bearings with
insulating housings can
eliminate the electrical
pathway through the motor
bearings, thus preventing
erosion. Insulated bearings are
readily available, but different
coatings and bearing designs
have different characteristics
when exposed to highfrequency
AC currents.
Bearing manufacturers have
tested their proprietary
designs under a wide range of
operating conditions and can
offer advice on the most
suitable bearings for particular
applications. In some cases,
bearings with ceramic rolling
elements can be particularly
useful, providing perfect
electrical insulation between a
motor’s stator and rotor. D&C
Solar plant uses plain
bearings to track the sun
More than 1,000 precision plain bearing
rod ends are playing a key role in the
world’s largest parabolic trough solar
power station – the 50MW Andasol 1 plant in
southern Spain
The €300m plant, which went on line in mid-
2009, is expected to generate about 180GWh a
year – supporting the power needs of more than
200,000 people, and saving around 150,000
tonnes of carbon dioxide. It is the first of three
similar plants being built at the site in Andalusia.
The plant consists of three main parts, the
largest being an array of solar troughs with a
collector area of more than 510,000m 2 –
equivalent to around 70 soccer pitches. There is
also a power plant area and a heat accumulator,
which allows the station to continue generating
electricity even in cloudy weather or after sunset.
The solar field consists of parallel rows of
parabolic collectors arranged on a north-south
axis to follow the path of the sun from east to
west. To track the sun, each collector is fitted
with a hydraulic drive.
The trough-shaped parabolic mirrors focus
the solar radiation onto an absorber pipe,
positioned along the collector’s focal line. A
closed-circuit pipe system uses synthetic oil to
transfer heat at up to 400°C. The heated oil is
pumped to a power block where it flows
through a heat exchanger. The next stage is
similar to the steam cycle in a conventional
power plant – the steam generated in the heat
exchanger drives a turbine connected to a
generator. The steam then condenses back into
water, which is recirculated.
A total of 1,248 Elges hydraulic rod end
bearings supplied by Schaeffler support the
solar troughs, positioning them with millimetre
precision to follow the sun.
“High-precision bearings are required so that
the power plant can operate at maximum
efficiency,” explains Dr Arndt Schweigert, head
of Schaeffler’s power generation sector.
“Moreover, the bearings also have to withstand
loads that should not be underestimated.”
The plain bearings have been optimised for
the slow, precise swivel motion. It is critical that
these movements are smooth, without any
stick-slip effects or jolting on start-up.
At the same time, the hydraulic rod ends
need to support high forces and to handle
alternating loads. This allows the 150m-long
chains of collectors to be adjusted to within a
tenth of a millimetre.
The rod ends are fitted with manganese
phosphate-coated radial spherical plain bearings
with steel-to-steel sliding contact surfaces. The
Parabolic reflectors focus the
sun’s energy on a pipe carrying oil
which is heated to 400°C
surface treatment improves the running-in
characteristics and reduces friction. The inner
ring of the spherical plain bearing is 70mm wide
and has a 110mm cylindrical bore diameter, as
well as a spherical outer slideway. The 160mmdiameter
outer ring has a cylindrical outer
surface and a concave inner slideway with a
diameter of 140mm.
Schaeffler has also provided 7,488 lead-free
plain bearing strips for the Andasol plant. These
are mounted in the supports between the
individual segments of the 150m-long collector
chains, ensuring smooth slewing movements
while tracking the sun.
30 November/December 2010 www.drives.co.uk