Up Time Magazine - December 2008/January 2009 - (Page 41) 1. If it is new and smaller than 50hp, it is very likely concentric wound. 2. If it exhibits the pattern seen in Figure 8, it is very likely concentric wound. 3. If it has been rewound at any size, it may be lap wound. Note: These are not absolute, as several manufacturers design medium and high voltage concentric wound machines. Figures 6 and 7 clearly illustrate the way windings are placed in a concentric wound stator. Note the different depths and locations of the coil groupings. identified or suspected, the RIC needs to be performed over two pole faces. Place the motor in observe and confirm any suspected eccentricity with correlating evidence, such as EMAX eccentricity analysis or vibration analysis. Figure 9 shows an example of dynamic Figure 8 - Concentric Wound Motor RIC Graph eccentricity. Notice how the peak amplitudes of the blue phase vary from pole group be off balance, causing different levels of voltto pole group as the rotor is rotated. This is age to be induced onto the rotor. This results occurring for each of the three phases. Dy- in irregular current flow on the rotor and varynamic eccentricity is the more severe type of ing levels of counter electromotive force, which eccentricity due to the increased chance of a is felt by the stator. These varying forces on the stator winding produce changes in the amrotor/stator rub due to rotor pull-over. plitude of the current similar to a load change. To determine the severity of an eccentricity By displaying the current in a spectrum format, problem identified with the MCE, it is important the modulations can be seen as sideband activto correlate with other technologies, which an- ity around a location known as the Eccentricalyze the motor while it is in a dynamic condi- ity Frequency (FECC). The FECC is the number tion. If the eccentricity is affecting the running of rotor bars multiplied by the shaft frequency characteristics, such as high eccentricity side- (RPM/60) of the motor. The current modulabands in the current spectrum of EMAX and tions are seen as peaks on the spectrum, which higher than acceptable vibration levels at twice will be odd multiple sidebands of the line freline frequency, then action should be taken to quency powering the motor. In a 60 Hz syscorrect the eccentricity as soon as possible. If tem, the 1st and 3rd sidebands will appear as eccentricity is not evident in the RIC test with 4 peaks, 120 Hz apart, and non-synchronous to the motor de-energized, but the running tests line frequency. These peaks are seen in Figure do indicate eccentricity, then soft foot should 10 on the following page. be the first thing investigated. One advantage the EMAX offers when performing CSA is that the technician may use alarm EMAX Analysis set points to estimate the severity of the ecEccentricity analysis using EMAX technology is centricity and act accordingly. However, speed performed through a high frequency spectrum and rotor bar information is necessary for the of current signature analysis. When air gap ec- technician to be able to confirm that the peaks centricity exists in a motor, the air gap flux will identified on the spectrum are indeed eccen- Figure 6 - Concentric Stator Figure 7 - Concentric Random Wound Stator Due to design, concentric wound motors create RIC results that appear as though there may be an air gap eccentricity when the rotor is only positioned through a single pole face. Results of a RIC performed on a concentric wound motor can be seen in Figure 8. Notice on the blue phase indicator, that the peak amplitudes from one pole group to another are essentially the same, but different from the other two phases. The blue phase is always slightly lower than the red phase, but higher than the green phase. In a case where concentric wound motors are www.uptimemagazine.com Figure 9 - Example of RIC 41 http://www.uptimemagazine.com
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