Up Time Magazine - December 2008/January 2009 - (Page 39) Ratio X and 2X Force at X is 4 times stronger X NX that allows air gap eccentricity analysis. each of the three phases. Figure 2 is an example for a two-pole motor. The rotor was positioned mechanically 180 degrees producing a 360-degree sinusoidal graph of inductance. Analysis of phase-to-phase inductance for air gap eccentricity focuses on: Alignment, Minimum & Maximum Peak-to-Peak Inductance over two or more pole faces and Inductive Imbalance. Air gap evaluation with the MCE is accomplished X and 3X 9 times stronger utilizing the Rotor InfluX and 4X 16 times stronger ence Check (RIC) test. The RIC is a graphical repFigure 1 - Effect of Magnetic Pull resentation of the magnetic coupling between rub that appears as random marks on both the rotor and the stator. the stator bore and surface of the rotor. The RIC test will be most successfully applied Pull-over describes the bending of the in troubleshooting and determining a course motor shaft allowing the rotor to come into of action if pre-existing RIC data is available. contact with the stator. Pull-over most However, even without a baseline test, the RIC often occurs during start up when the test will give you definite indications of existmagnetic forces pulling on the rotor are ing eccentricity. greatest. The magnetic pull acting upon the The RIC test is conducted by positioning of rotor varies as the square of the difference in the air gap (Figure 1). For example, when the rotor in incremental steps through one or more pole groups and recording each of the the air gap at the narrowest point between three phase-to-phase inductance values at each the rotor and the stator is one half that of increment. These readings of inductance are the air gap at its widest, the magnetic force graphed allowing for analysis of the motor’s at the narrowest will be four times stronger winding, rotor, and air gap. than the widest point. Couple that with the in-rush current and the magnetic pull could flex the shaft leading to a rotor/stator strike. A pole group is the amount of mechanical movement of the rotor required to produce a 360-degree sine wave of the stator winding Analysis inductance values. For example, in a four-pole Testing a motor for air gap eccentricity can motor, the rotor would need to be positioned be easily accomplished using Motor Circuit incrementally through 90 degrees mechanicalAnalysis (MCA) or Current Signature Analysis ly to complete a 360-degree electrical sine for (CSA). The hardest part seems to be deciding what to do about it, after confirming that an air gap eccentricity exists. When performing CSA there is no absolute standard on how much eccentricity indicated on a current spectrum is too much. The same can be said for the results of a Rotor Influence Check (RIC). The graph may indicate the presence of an air gap eccentricity, but without historical results for comparison you really cannot develop a sense of how much ecFigure 2 - Graph of Inductance for One Pole Group centricity has developed and if it is getting worse with time. Alignment: Is there a level alignment of the graph across the chart? To determine if the motor has an eccentricity in the rotor/stator air gap, evaluate the alignment of the RIC graph. Unevenness in the space between the rotor and stator will affect the alignment of the Rotor Influence Check (RIC) as the rotor is positioned during the test. An uneven alignment such as what is seen in Figure 3 indicates eccentricity exists between the rotor and stator. The more severe the air gap eccentricity, the greater the misalignment. Comparison to a previous RIC test allows for determining if the condition is worsening over time. Peak-to-Peak Inductance: What are the minimum and maximum inductance values? When air gap eccentricities exist in a motor, peak-to-peak inductance values for each individual phase will vary from one pole face to the next. To see this change in peak-to-peak inductance values, position the rotor through at least two pole faces. Figure 4 on the following page demonstrates how the peak-topeak inductance values vary from one pole group to another for an individual phase when there is an air gap eccentricity. Remember, in a concentric wound motor each individual phase will have it’s own minimum and maximum inductance values. An example of how a concentric wound motor develops uneven minimum and maximum inductance values between individual phases is shown later in this article. MCE Analysis Motor Circuit Analysis utilizes a combination of phase-to-phase resistance and inductance measurements to detect abnormalities in the motor’s winding, rotor, and air gap. Positioning the rotor in specific mechanical increments and graphing the phase-to-phase inductance values results in a graph www.uptimemagazine.com Inductive Imbalance Inductive Imbalance is a percentage calculated by dividing the average of the three phase-to-phase inductance readings into the value Figure 3 - RIC Graph Confirming Air Gap Eccentricity 39 http://www.uptimemagazine.com
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