Diagnostics and testing of rotating electrical machines - DNV Kema

12
ENERGY
Non-destructive testing & plant diagnostics
Mechanical testing of rotors
and stators
Safe operations with flaw indications
Retaining ring inspections
Replacing retaining rings is quite an expensive operation, it may
even cost up to EUR 1,000,000. The DNV KEMA Inspection
system of Retaining Rings (KIRR) offers a perfect solution
to these problems. This system detects and characterizes flaw
indications in the retaining ring and will enable you to determine
whether the retaining ring needs to be replaced. Furthermore,
you will be able to continue monitoring newly detected flaw
indications that are still acceptable to be worked with. This
means that you can regularly keep an eye on the condition of the
retaining rings. Initially DNV KEMA will try to obtain as much
information as possible about the ring’s geometry and the actual
access to the rotor, sometimes there is hardly any information
available. The ring’s geometry is then first measured using an
ultrasonic C-scan mapping technique. Ultrasonic crack detection
is focused on the shrunk connections and wall thickness steps.
The ring’s coating is left in place. Geometry and flaw data
are then fed into a computer model. The residual life span is
embedded in the analysis.
Bump testing of stator end windings
With bump testing the natural response of an end winding is
measured upon excitation with a mechanical impulse (hammer).
The response is fully determined by the properties and
mechanical structure of the stator end winding. This principle is
comparable to the excitation of piano strings by their hammers
when vibrating in distinct frequencies and thus producing a tone.
A relative simple method when compared with the end winding,
but nevertheless the natural vibration frequencies of a complex
part can be measured accurately. An acceleration transducer
is mounted successively on each end winding unter test. The
measured signals are fed into a data analyzer and the response
frequencies and magnitudes, the so-called Power Spectrum
Density (PSD), is recorded, showing the energy distribution of
vibrations. An extra acceleration transducer is mounted at the
hammer. The most important frequency range for analysis lies
between 95 and 110 Hz. Depending on temperature resonance,
modes can shift slightly during operation. Electromagnetic
forces at the windings have a frequency of 100 Hz. When natural
vibrations lie close to 100 Hz, resonance may occur resulting in