Application Summary
Nuclear reactor power plants in Great Britain use large variable-speed electric motors on an underwater trolley to move new fuel rods through a two-foot wide tunnel from one storage section of the plant to the reactor, and then transfer the spent rods from the reactor to another storage area. But the process didn’t necessarily run this flawlessly before the plant was fully commissioned.

Occasionally, during pre-operational checks when a trolley encountered a tight spot in the tracks, the increased friction demanded higher motor torque, which, in turn, produced a short-duration current rise. The transient currents repeatedly activated the overtorque protection device on the drive shaft for the fuel trolley. Obviously, technicians could not start moving the fuel elements until this problem was solved. Engineers needed a way to record the transient current as the trolley moved along the tracks to provide them with data to measure and analyse the transients and locate the source of the problem.

Potential Solution
First, an expensive, dedicated analyser was tried, but it didn’t have the sampling rate and resolution necessary to fully capture these brief events so they could be studied adequately. The engineers then decided to investigate PC-based data acquisition methods, considering them a superior way to capture, display, and easily store large amounts of data over several days. Although the original analyser could connect to a PC, its RS232 interface was much too slow. GPIB was an available alternative interface, but a GPIB card had to be fitted to the PC, and even with that, the engineers reported only a 70-Hz sampling rate, much too slow to be effective.

IOtech’s Solution
In comparison, immediately after an initial test, it was obvious that the IOtech WaveBook not only provided high resolution, but it also delivered all the speed that was required, 1 MHz, and could be expanded to measure many more channels.

The voltage and current levels at the motor were too high to measure directly, so a Hall-effect current probe was installed with a 0 to 5 VDC output. The sensor connects directly to the WaveBook input. The Hall-effect current probe connects around the motor supply cables, electromagnetic brake, and limit switches. In this way, technicians can observe the status of the system over the course of a complete test run. Also, because they knew the running speed of the fuel trolley, they were able to correlate the tight spots on the rail to peaks in motor current. Because the sample rate of the original data logger was slow, much of the valuable detail was lost. In the end, it was a poorly calibrated mechanical torque protection device that caused the problems. But the IOtech WaveBook is now continuously on line to monitor the motor current and prevent any similar future occurrences.

The IOtech’s WBK family of modules also handle a wide range of signal-conditioning options. More significantly, the recently introduced Ethernet version of the WaveBook lets the PC be remotely located. For this reason the engineers also were eager to try an Ethernet extender to amplify, condition, and transmit the data signal in full-duplex mode for at least one mile.

In addition, the software for this application was of great value. Ideally, it has the following capabilities:

• Allows full-speed, one-MHz capture rates on a single channel (or an aggregate of this on multiple channels)
• Automatically searches for transients on the 50-Hz carrier and saves them to a file with time stamps
• Saves several days of data to a hard drive, complete with waveforms
• Transmits an external signal to activate a camera through an RS232 interface or a PC-resident sound card

Finally, the choice of software came down to either the eZ-Analyst or DASYLab^®. The eZ-Analyst has view screens with ready-made features for time domain and frequency analysis. However the engineers preferred that software for vibration and acoustic work, so they selected DASYLab for monitoring the current transients. The plant operators were impressed with the ease with which DASYLab software could build applications as well as its flexibility to handle other needs. In addition to monitoring current, the WaveBook and the WBK15 signal-conditioning modules monitored temperatures with thermocouple and RTDs, while WBK16 modules monitored strain gages.

Conclusion
The Sizewell nuclear reactor power-plant engineers are satisfied that the WaveBook and both software packages, WaveView and eZ-Analyst, are well-justified purchases for their Control and Instrumentation Department. The flexibility of this system makes it especially useful, not just for the original task to analyse transients, but also for many other projects around the reactor plant. An integrated WBK11A simultaneous sample and hold card is also useful for determining reactive and true power by measuring phase shift and providing a wider set of voltage gain ranges.

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