Vibration study of a rotating machine

The Challenge

Metravib was consulted a few years ago about a problem of cracks on an electric motor fan. We carried out measurements on the equipment, which made it possible to give axes of improvement to our customer so that the cracks would not appear any more for the next series of fans produced.

Recently, the customer came back to Metravib with a new project. The problem of vibration was taken into account upstream of the production of the fan. That is exactly the right timing to have the best opportunity to tackle vibration issues and thus apply design modification if needed. Metravib thus proposed to instrument the customer’s prototype, identify potential resonances and possibly modify the fan design in order to prevent the appearance of cracks during its lifespan.

The Solution

The technical approach consisted in performing:

• an Experimental Modal Analysis which allowed us to characterize the dynamic behavior of the equipment, in other words to identify the critical torsional modes of the equipment in terms of frequency, deformation and damping.
• Tests in operating conditions: after an instrumentation phase consisting of sticking strain gauges and accelerometers on the fan, the motor was run on a test bench simulating real operating conditions. To obtain the most representative measurements, the gauges are conditioned in a rotating part with miniature gauge conditioners to have little mass on board. Such conditioning of the instrumentation is essential to have a good signal to noise ratio. The acquired signals are transmitted to the acquisition system through a rotating collector.

Following the measurements, we processed the data to perform a Time-Frequency or Regime-Frequency analysis by correlating 2 series of tests. Once the analysis work performed, we provided our client with a detailed analysis report identifying and quantifying the resonances found. The corresponding databases were part of the deliverables as well.