Force Impact hammer-based system response testing has been in use for decades and this has remained the quick & concise approach to evaluate physical systems that have higher stiffness and smaller by size.
Experimental Modal testing & analysis, the full-size procedure to determine eigen values of a given system is either conducted using modal shakers or by using impact hammer approach, merits decided by the size of the test object and other parameters.
In some instances, only the impact hammer method is recommended, considering its adaptability to the case and the test specifications that need a specific method of injecting forces.
Large electrical power generators, by design, possess considerable size of stator windings and this project out of the core slots on either side of the system, called the end windings. These are the dynamically vulnerable system components that can get into high instability upon excitation due to electromagnetic forces and other rotational contributing factors in their operational usage. Natural frequencies of these end-windings coinciding with excitation frequencies can render detrimental consequences on the entire generator.
Depiction of Stator Windings and Impact Hammer Excitations
Since the winding is built in stages and layers with combinations of conducting bars and insulation bindings, each segment of the winding and its composition is likely to have varying assembly integrity thus resulting in a build that can exhibit varying natural frequencies.
Bump test is the qualified and governed methodology deployed to evaluate the behaviour of these end windings during the assembly stage of the generator. Typical bump test comprises of a large force hammer (up to 1kg head weight) to sufficiently excite each of end winding component and a series of tri-axial modal accelerometers to measure responses at all the designated locations all over the end-winding structure.
After a good pre-test procedure that includes reciprocity and linearity checks, impact force is induced into the specific points / direction on the end windings and vibration response data is recorded onto a multi-channel acquisition unit for further assessments. Combining all the response, sets of FRF's are compiled and modal curve fitters are run through to derive the frequencies, mode shapes and damping coefficients.
There is a twist to the way the output data is interpreted in this test process. While the key observations are around the line frequency of generator operations, harmonics up to 4th order play a role in understanding the overall behaviour of the generator in its operations. One such mode is termed as the 4-node mode, let us understand this more.
The 4 Node Mode - explained
A node is a location on the structure where the vibration displacement is essentially zero for that mode. In a 4-node circumferential mode, there are four stationary points around the circumference, between these nodes, the end winding moves alternately inward and outward; thus, as the end winding vibrates with the following behaviour
- 4 locations remain approximately stationary, at nodes
- 4 regions move significantly, at anti-nodes
- Adjacent antinodes move in opposite directions
The regions between the nodes move radially/axially, while the four node locations have very little movement, as this progresses, half a cycle later, the moving regions reverse direction thus creating characteristic lobed mode shape.
The importance and criticality of this specific mode is since electromagnetic forces associated with the generator's operating magnetic field can excite particular circumferential modes. If an electromagnetic excitation frequency matches closer to an end-winding natural frequency, resonance and large vibration responses shall occur that can induce a failure.
Cumulative FRF graphs from all the response points
One subtle but important point to note is that the "4-node mode" is not simply the fourth natural frequency of the end winding but is the representation of the spatial mode shape around the circumference, whose existence and severity has a direct bearing on the operational performance and operational life of the generator.
Over the years, NV Dynamics has perfected this specific test and assessment methodology conducting multiple assignments across renowned industry names. The tests are conducted on 660MW generators with high degree of compliance and results obtained are well accepted by the client and their global consultants.