SENSORS | CONDITION MONITORING
OFFSET MEASUREMENT
OF THE COUPLING RING
Above 100m high loads act on
the rotor blades, housing and tower,
caused by wind. The gearbox and
generator have an elastic bearing.
This is why couplings in wind
turbines must balance the relative
movement of the gearbox and
generator. When measuring the offset
of the coupling ring, non-contact eddy
current displacement sensors can be
used to determine the distance, while
measuring onto the metallic coupling
ring. This leads to the determination
of the load profile.
Monitoring the measured
values is required in order to
avoid unnecessary wear of
couplings, bearings and
shaft seals or, in extreme
cases, serious damage
to the wind turbine. Eddy
current sensors allow
measurements to be
carried out in different
directions – in the axial, radial
and tangential axes. Some
suppliers offer temperaturecompensated
versions that provide
high measurement stability, even
in strongly fluctuating ambient
temperatures. The sensors are
factory-calibrated for ferromagnetic
and non-ferromagnetic materials,
which eliminates the need for field
calibration of the sensor.
Compared to inductive switches
and sensors, eddy current sensors
provide a higher bandwidth and so
are suitable for precise detection of
high-speed movements.
Hydrostatic bearings are used in
wind turbines. The task is to monitor
the gap size between the bearing
surface and the shaft.
In the lubricating gap is an oil
film, which prevents direct contact
between the bearing surface and
the shaft. In the case of a malfunction
in the hydraulics, the oil pressure
can rise, and in extreme cases, the
gap will close. This would lead to
damage of the bearing and in turn to
a possible turbine failure.
“This sensor
technology helps
customers by
preventing repairs,
reducing failures,
reducing costs and
helping to predict
maintenance
Jones says: “Our sensor is mounted
horizontally to the bearing shoe. It
measures through the oil film and
the plant bearing directly onto the
shaft. Here, non-contact eddy current
displacement sensors can be used,
which are robust, compact and have
an integral controller.”
As well as offering
resistance to high
pressures, lubricants and
extreme temperatures,
these sensors are also
said to enable rapid
commissioning and can be
retrofitted to existing wind
turbines.
With very large generators and
electric motors, it’s important to
determine the radial run out of the
rotor inside the motor compared to
the stator.
Due to imbalances during
operation, which could be due to
wear caused by extreme wind and
weather conditions, the rotor might
touch the stator, which could lead to
catastrophic failure. Therefore, noncontact
optical and capacitive sensors
are used to measure the distance
between the stator and rotor and to
monitor the rotor gap whilst the motor
is running.
During air gap monitoring
in a wind turbine generator, the
average temperature is 120°C.
Some capacitive sensors are
specially adapted to measurements
in a generator, they are resistant
to vibration and are protected by
a special housing. Their triaxial
design enables flush installation into
electrically conductive materials
as the guard ring electrode and
grounding are also located on the
front edge of the sensor alongside the
measurement electrode
The gearbox temperature is
measured during operation. Heating
up of gearbox components may
indicate a potential problem with a
component. Subsequent remedial
action can be started, or maintenance
planned.
To control air supply, the air
flaps automatically open and close
depending on the temperature. Drawwire
displacement sensors can monitor
the position of the open-air flap.
In order to monitor the supporting
moments, eddy current displacement
sensors measure the distance
between the wind turbine nacelle
and the tower, which enables early
recognition of any fluctuations
Non-contact laser triangulation
sensors are ideal for measuring the
distance between the tower and
the foundation. High measuring
rates enable the sensors to detect
any changes reliably. Depending
on the number of sensors installed,
detailed evaluations about the
vibration behaviour of the mast can
be determined.
Test rigs have been developed
for load tests on wind turbine rotor
blades, which simulate the real loads
caused by high winds and extreme
weather conditions. The tip of the
rotor blade can be distorted by up to
10m due to mechanical loads.
Draw-wire sensors are mounted
on these test rigs for measuring this
distortion. Two sensors per traction
point measure the deflection and
torsion of the rotor blade. The drawwire
sensors operate with measuring
ranges between 3m and 10m. The
digital signal output is provided for
further simulation and analyses. !
cycles”
24 WWW.EUREKAMAGAZINE.CO.UK | JULY 2019
/WWW.EUREKAMAGAZINE.CO.UK