The Challenge:

Balancing rotors (length-6m, dia-1m) of generators of capacity up to 500 Mega Watts, by finding an appropriate magnitude and position of compensating weight.

Solution:

Using PXI based system, with PXI 4472B, PXI-6602 counter/timer to meet the requirement of simultaneous sampling with noise reduction features (anti-aliasing filter in 4472B), precisely detecting the edges of pulses in the tachometer signal (6602 Counter/Timer), synchronizing DSA card and counter card using PXI features with the help of LabVIEW. Using LabVIEW, Order analysis toolkit, and Sound and vibration toolkit to find the information from the vibration signal required for balancing.

Introduction:

A leading manufacturer of Generators in India manufactures generators of capacity up to 750 MWatts (fig 1.1).

Before assembling the rotor and stator, it is required that the rotor should be balanced so that the magnitude of the vibration signals produced by the generator during the normal operation, within the operating range, are well within the safe limits.

For balancing the rotor, vibration signals from the rotating rotor and the reference signal from tachometer are used. The system takes vibration signals from the accelerometer, velocity pickup, and eddy current probe, and the reference signals from the tachometer. These signals are acquired and processed using NI hardware and software, and finally the magnitude and the phase information are extracted. The magnitude and phase of the first order component is used to find the amount and location of the compensating weight.

Implementation:

: The vibration signals and the tachometer reference signal from the balancing station goes to the PXI system, which is running on LabVIEW-RT operating system (Fig 1.2).

The vibration signals are acquired using PXI 4472B, while the tachometer signal is acquired using PXI-6602 Counter/ Timer card. All analog input channels of the DSA card and the counter card are synchronized in time using synchronization tools in PXI system and DAQmx drivers in LabVIEW. The analog input is sampled at a rate of 6000 Samples/ Sec. Care is taken for the initial filter delays in the DSA card, and aliasing. The counter card is configured to measure period of the tachometer signal. The vibration signal from accelerometer/ velocity pickup/ eddy current probe is converted into acceleration/velocity/displacement, using scaling vi. The acceleration signal is converted to displacement and velocity and the velocity signal is converted to displacement by integrating the acceleration and velocity signals. From the time domain data (acceleration, velocity, displacement) peak-peak value is calculated. This time signal (acceleration, velocity, displacement) is re-sampled using the re-sampling technique (order analysis toolkit). FFT of the re-sampled data is taken to get the order spectrum. From the order spectrum all the desired order components magnitude values are extracted. These are the RMS values of the order magnitudes.

The challenges in this implementation were- noise in the signal, integrating the signals, and establishing a stable amplitude and more important a stable phase information. To reduce and remove high frequency noise the anti-aliasing filter in the card (4472B) helped. To remove the low frequency noise the cutoff frequency of the HP filter was adjusted to an appropriate value after some interval of the rotating frequency (Delta f =3 Hz= 180 RPM). Care has been taken to overcome the filter resetting effects.

In the balancing procedure the rotor is run for the first time at a particular RPM. From the above analysis 1st order magnitude and phase information are taken out. Form the 1st order magnitude if it is found that the rotor is almost balanced, then nothing is done. If it is found that the unbalance is beyond the tolerable limits, then some known weight is added to some location on the rotor. Second run is made at the same RPM as previous, and again from the above analysis 1st order magnitude and phase are extracted. From the two runs, information about the amount of weight to put and at what location, so as to balance the rotor is determined. After putting this second compensating weight third run is made and it is verified whether the rotor is balanced or not.

Figure 1.4 shows the front panel of the application.

Figure 1.5 shows the comparison of the vibration magnitude (1st order component) obtained from an already existing system and the NI system. Performance of the NI instrument and other instrument are similar towards the higher RPM. Towards lower RPM performance of NI instrument is better (as verified from other instruments). This NI system is tested to be working absolutely fine for 16 vibration channels. There is provision to extend input channels by adding more number of 4472B cards.

Conclusion:

With the help of PXI system, DSA cards (PXI-4472B), Counter/Timer card (PXI-6602) along with LabVIEW, Order analysis toolkit, sound and vibration toolkit we were able to provide a highly reliable, rugged, small in size, and easy to use solution. The synchronization was greatly simplified because of the tools available in the PXI platform. The efficiency of the system was much better in the PXI based system. The simultaneous sampling and the anti-aliasing features in the DSA card helped in achieving correct phase information and noise reduction. The counter card helped in getting exact occurrence if the tachometer pulses.

Besides this the order analysis toolkit helped tremendously, as it removed the need of hardware re-sampling, which would have been a tedious task. Above all this the programming and debugging were very much simplified because of LabVIEW.

Also the solution is nearly quarter times as costly as the system they are using. Thus we, with the help of NI products were able to provide a cost effective solution.