FREQUENCY-RESPONSE ANALYSIS (AKA: BUMP TESTING)
Just as every bell has a unique tone when struck by a hammer, so do all generator winding components. Every structure, in fact has its own characteristic modes of mechanical response when deformed after being struck. While not very musical in nature, the response waveform gives the trained Technician valuable information on the integrity of the windings under test.
Bump testing can be done discretely (giving specific results for each specimen struck), or as a composite modal wave shape function where some of the specific information about individual specimens gets lost in favor of getting system-level resonant response information. The two major design philosophies are split between GE & Westinghouse, which dates back as far as the companies respective founders, Edison and Westinghouse.
End winding resonance testing determines the mechanical resonant frequency of each of the series and phase connections. Each of the series and phase connections are struck with an instrumented mallet equipped with an accelerometer, and the responding oscillation is detected by an accelerometer mounted on the specimen winding. The data are sent to a Fast Fourier Transform processor where the stimulus and response are analyzed in the frequency domain. Y(s) = H(s)*X(s) is a standard transfer function relationship where Y(s) is the output in the frequency domain (response), X(s) is the forcing function (how much energy you put into the system and at what frequencies), and H(s) is the transfer function that is the “fingerprint” function of the system under test. This transfer function is displayed graphically as amplitude versus frequency, so that the resonant peak frequencies can easily be identified from a graph.
The resonant frequency of an object is the frequency where the system most efficiently accepts energy from an outside stimulus. In the case of generator windings, the “system” is composed of the end windings, and the stimulus the twice-per-revolution mechanical force that acts upon the windings. In the 60 Hz grid the forcing function will be 120 Hz; similarly for 50 Hz the forcing frequency will be 100 Hz.
In general, the current-carrying components of the generator must be mechanically designed in such a way that their resonant modes are not near this forcing frequency. The word “near” means avoiding the band of -5% to +10% of the forcing frequency, or (115 – 135 Hz, for a 60-Hz unit).
Each OEM has certain parameters for determining acceptable levels of resonance. Sidewinders applies the most sensitive criterion of all the OEMs, which is 0.20 g/Lb-f (read as: g’s per pound force). Working through Newton’s Second Law of motion, F=ma, if you solve for units of [acceleration/force] you arrive with units of reciprocal mass (1/m). The higher the mass, the lower this fraction; the lower this goes, the lower the amount of acceleration you will see. This is a perfect moment to segue into the topic of tuning.
If you’ve ever played with a guitar, you quickly saw that the thicker, longer strings vibrated with the lowest frequency. You probably also saw that if you tightened any string, the frequency went up. You also noticed that if you put your finger down somewhere on one of the frets, the frequency went up. We apply these ideas to bump testing as follows:
If a unit displays resonance, we first must characterize it as being above, or below the desired frequency band. If above, we may apply “low tuning”, and as you might expect, if the results are below the desired range we would apply high tuning.
A way to low-tune a generator would be to either add mass, or increase the spacing between unsupported spans of the winding. Sidewinders generally prefers to “high tune” machines, as this generally involves adding additional means of structural rigidity. Ways to high-tune a winding system include: adding additional blocking, applying wicking & flooding resins, replacing failed ties, adding nose rings, and in the case of certain Siemens-Westinghouse and MHI units, we can re-tension radial banding or radial studs. All of these measures tend to drive frequency response upward.
Screenshots before (L) and after (R) Sidewinders made repairs due to bump test findings
Sidewinders has attended to many cases where a serious in-service failure occurred due to an unmitigated resonance issue that could have been detected and corrected by means of this testing modality.
Bump testing is a fast, economical way to gather valuable information about the condition of your generator and help predict and avoid costly failures! Contact Sidewinders for a bump test quotation for your next planned outage.