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.
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
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