Up Time Magazine - December 2008/January 2009 - (Page 42) tricity related. If the number of rotor bars and the speed are known, the MCEGold and WinVis software automatically places a red X at the four peak locations that identify eccentricity. Obtaining the speed from the Advance Spectral Analysis (ASA) current demodulation software, Low/High Resolution rotor tests, or via a strobe light is the easy part. The rotor bar count, however, is another matter. First, at the earliest opportunity you should verify that a rotor bar count request exists on each of your motor repair specifications. The report you get back from the shop should include how many rotor bars and stator slots exist in the motor. Second, utilize the vibration department to assist in the rotor bar count. They may have previously identified the number of rotor bars through spectrum analysis of the vibration signal. If neither of these methods work, reverse calculation as described later in this article may be required. One of the most often overlooked tools is the caution and alarm set points, which are preprogrammed into the software. We often get information sent in for review on possible eccentricity and neither the eccentricity related peaks, nor any other peak for that matter, is close to the yellow caution line. Also remem- ber when dealing with a VFD, no longer does 60 Hz line frequency necessarily apply. If the drive is operating at 40 Hz, rather than four peaks 120 Hz apart, you are now looking for four peaks 80 Hz apart and nonsynchronous to 40 Hz rather than 60 Hz. The MCEGold and WinVis auto frequency adjusts automatically and corrects for VFDs, and it will correctly identify peaks based on the measured fundamental frequency, as seen in Figure 11. Whether across the line or powered from a VFD, Figure 10 - Side Band Peaks on the Spectrum Figure 11 - Eccentricity Current Spectrum uptime delivers. With content that is focused 100% on Maintenance & Reliability, each issue of Uptime® delivers practical knowledge to over 50,0000 maintenance & reliability professionals around the globe. Are you in? www.uptimemagazine.com sales@uptimemagazine.comwww.www. uptimemagazine.com/subscribe.htm if speed is known, but the number of rotor bars is unavailable, the following information will assist you in analyzing the eccentricity test’s frequency spectrum. Eccentricity related peaks usually exist between 600 Hz and 2000 Hz on the current spectrum. This frequency range is based on the fact that the eccentricity peaks are odd multiple sidebands of the line frequency around the product of the number of rotor bars and the speed of the shaft (# of rotor bars x shaft speed). Commonly, the faster the motor, the lower the number of rotor bars. The slower the motor, the higher the number of rotor bars. When multiplied, the product is usually less than 2000 Hz. However, larger two pole motors may exceed the 2000 Hz mark. Eccentricity peaks cannot be harmonics of line frequency. As a result of slip being involved, the calculation of eccentricity prevents it mathematically from being a multiple of the line frequency. Therefore, running the harmonics marker, which places a small green x at each of the line frequency harmonic peaks, allows you to quickly remove the identified peaks from the suspect list. Line frequency harmonics are very common and often can look like eccentricity peaks. If in a 60 Hz system you identify that four or more peaks exist between 600 and 2000 Hz, which are 120 Hz apart and non-synchronous december/january 2009 42 ® http://www.uptimemagazine.com http://www.uptimemagazine.com http://uptimemagazine.com/subscribe.htm
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