Natural Frequency Test (NFT)
The operating conditions and start up stresses on the endwindings could result in loosening of the support ties and blocks and may cause the coils on the endwinding to vibrate or move. This will result in a change in the stiffness of the endwinding supports, which will lead to a shift (lowering) of the natural frequency of the endwinding structure.
It may so happen that this shift in natural frequency might come dangerously close to the exciting frequency of the 2 X line frequency forces that are generated by the currents flowing in the endwindings, and result in large movement of the endwinding coils. The natural frequency of endwindings should not be matching with the system’s natural frequency, which may otherwise cause high amplitude vibrations due to resonance, increasing stresses on the machine. This test is thus conducted to identify the natural frequency of endwindings. The response of the end winding to the tap of a hammer is recorded using an accelerometer pickup strategically positioned, and analyzed to characterize the relevant end winding resonant properties.
We will able to distinguish the different types of NFT problems
- End winding looseness
- Wedge Looseness
- Slot Corona due to Vibrations