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Vibration Monitoring, Trending Analysis and Fault Detection
Published in Jyoti K. Sinha, Industrial Approaches in Vibration-Based Condition Monitoring, 2020
Rotor systems in many machines have many fluid bearings and if a fluid-induced instability is observed during machine operation it important to identify the bearing that is causing such instability. The phase relation at the frequency of instability (λ×) of the shaft relative displacement between the measurements at all the bearings can be used to identify the bearing. For the bearing which is suspected as the source of instability, its shaft relative displacement at the λ× frequency component will have a phase lag with respect to the shaft relative displacement at other bearings.
Electronic Systems
Published in Trevor Linsley, Electronic Servicing and Repairs, 2014
Instability can occur in any closed loop system because the output of each black box in the system forms the input to the next box. When a controlling action is required, the necessary changes can take some time to work their way through each block in the system from input to output. Let us suppose a reference sine wave was applied to one of the blocks in a system as shown inFig. 9.5. With negative feedback applied, the feedback signal will be negative with respect to the reference signal at any instant in time. The feedback signal will, therefore, be subtracted from the reference signal, resulting in stability of the output. If there is a time lag in the system, resulting in a phase lag of say 180°, the feedback signal will be in-phase with the reference signal, resulting in positive feedback and instability of the system. An example of this effect is the howling of a loudspeaker when it is brought close to a microphone. The sound from the loudspeaker is picked up by the microphone but a time delay occurs as the sound waves travel through the air between the speaker and the microphone. This signal lag causes some frequencies to be amplified, resulting in the howl. Greater separation of speaker and microphone resolves this particular problem.
Investigation and damping of electromechanical oscillations for grid integrated micro grid by a novel coordinated governor-fractional power system stabilizer
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2021
Narayan Nahak, Omkar Satapathy
Usually, conventional governor impart negative damping to electromechanical oscillations by shifting mechanical modes to right half plane. In this case, the phase lag is more than 90° as shown by Pm0 and the damping effect shown by damping torque Pd0 in Figure 8. Pm0 can be resolved in to two components, the damping component in phase with Δɷ and synchronizing component in phase with Δδ. This phase lag can be compensated by providing additional damping torque. By this the phase lag can be decreased to less than 90°. So by additional damping, the prime mover output power is now shifted to Pm1 by additional damping Pd1 with positive synchronizing component Ps1.