Natural Frequency Test (NFT) on Stator Winding and LP Turbine
Natural Frequency test on End Winding:
To assess the mechanical integrity of the Generator end winding, Natural frequency test
is performed in order to determine the natural frequency response of individual loops in
the radial and axial directions. If the measured natural frequency values are out of the
resonance frequency band 95-100 Hz, it can be delared that the mechanical integrity of
the stator end winding is in satisfactory condition. If it is out of resonance
frequency, end winding has to be strengthened till the measured frequency values are out
of resonance frequency.
Natural Frequency test on Turbine Blades:
Any rotating component exhibits a natural frequency of motion at which offers minimum
resistance to forces. The geometry, material properties and fixity conditions dictate
the natural frequency of any component. Important components like Turbine Blades
particularly the blades of larger lengths are so designed that the natural frequencies
are away from the prevailing excitation frequencies during normal operating ranges of
the machine. This is done for increasing the fatigue life of the component to avoid
dangerous resonance conditions. The natural frequency which is dependent on root
contact, increases with the fixity of the blade attains a constant value. For the
purpose of testing, the fixity condition is simulated in the stationary root by fixing
specially wedges in the slot at the root of the blade on the disc. NFT testing is
usually done on Turbine free standing blades every time when the blades are removed and
refitted during over hauling period after long operation.
In the earlier conventional method of NFT , blades are tightened with technological
wedges and suitable taper plates with long handles of length 600 mm to 700 mm are
used. To tighten and removing the technological wedges, force is applied by manual
hammering on the taper plates with long handle which is also repeated some times on
the same blade during testing.
Powertest Asia developed a novel method to carry out NFT on Steam Turbine LP Rotor
freestanding blades. In this novel method, tightening & removal of the wedges,
pneumatic tool with short handle adaptors for technological pieces and specially
designed taper plate wedges with shorter handles suit to the pneumatic tool are
used. For this novel state of the art test method, Powertest Asia applied for the
patent, No. 201641039558.
By adopting this new method of NFT testing, the advantages are
In the absence of manual hammering, there is no chance of damaging the Blades,
Rotor and Labyrinth rings due to accidental slippage as in the old conventional
method while tightening and removing the technological pieces.