Recently, we did torsional vibration measurement on a disk-pack coupling of 315 kW induction motor driven flue gas blower. A strain gauge was attached to coupling spacer surface as shown below. Telemetry transmitter was then installed in opposite angle with battery pack to cancel unbalance mass. A receiver is connected to vibration analyzer which is configured and controlled data collection by host computer.
The test results show that, this machine train has torsional natural frequency at 43 Hz. Fluctuating torque amplitude is within designed rated value, hence, the coupling is unlikely to be failure.
However, additional tests were carried out in details and the results revealed that torsional vibration amplitude was significantly changed when induction motor is operated with different control modes of variable frequency drive (VFD) such as direct torque control (DTC) with variable constants and scalar modes. This is very important information to answer why this coupling disk was frequently damage. Optimum operating condition can then be derived.
Waterfall plot shows dynamic torque spectrum versus time when induction motor was operated with different VFD control modes.