Up Time Magazine - December 2008/January 2009 - (Page 38) motor testing upload Is Your Motor Getting Eccentric? A Closer Look at Air Gap Eccentricity by Douglas E. Swinskey & Peter M. Bechard T he first step in evaluating test data is understanding the relationship to the circuit’s Fault Zones and how abnormalities in a specific Fault Zone affect the performance of the motor. The six Fault Zones (Power Quality, Power Circuit, Stator, Insulation, Rotor, and Air Gap) are derived from the most common electrically related motor failures in industrial environments. The Air Gap Fault Zone describes the measurable distance between the rotor and stator within the motor. Air gap eccentricity is a condition that occurs when a non-uniformity in the air gap between the rotor and stator exists. During operation, several stresses are brought to bear upon key components of the motor. An air gap eccentricity results in increasing these stresses during operation. A motor operated with an eccentric air gap results in increased mechanical vibration, accelerated insulation degradation due to increased coil movement, and possible rotor/stator rubbing due to unbalanced magnetic pull. Types of air gap eccentricity are: Static Eccentricity - which occurs when the centerline of the shaft is at a constant offset from the centerline of the stator. An example is a misaligned end bell. Dynamic Eccentricity - which occurs when the centerline of the shaft is at a variable offset from the centerline of the stator, such as a wiped bearing. of either the rotor or stator will lead to an air gap eccentricity. Industry standards recommend that measurements for total indicated roundness (TIR) should be performed at different locations along the length of each of these components. Couple these measurements with the circumferences of each component, and depending on the speed and size of the motor, there are recommended tolerances from 5 to 20 percent variation in the air gap. • Eccentricity can develop due to improper tensioning of drive belts coupled to a motor. One customer working with PdMA’s technical support staff said that while they were releasing the tension on one set of belts, they measured the deflection of the motor shaft, only to find it had moved 1/8 an inch! Improper alignment could also lead to a situation similar to this with both leading to a bowing of the rotor during operation. • Distorted end bells, cocked bearings, or a bent shaft will all cause an air gap eccentricity. During the manufacturing of the rotor, uneven mechanical stresses could be introduced into the cage and lamination stack leading to bowing of the completed rotor. • An air gap eccentricity results in increased levels of vibration due to the uneven magnetic pull it creates between the circumference of the rotor and stator bore. Over time, these elevated levels of vibration can result in excessive movement of the stator winding, which could lead to increased friction and eventually a turn-to-turn, coil-to-coil, or ground fault. Increases in mechanical vibration accelerate bearing failure, which could seize the shaft and overheat the windings or allow additional movement of the shaft leading to a rotor/stator rub. The uneven magnetic stresses applied to the rotor, coupled with the increased vibration, will also contribute to mechanical looseness developing in the rotor assembly. Risk of rotor pull-over increases exponentially with the amount of air gap eccentricity. • Rotor pull-over is an example of rotor/stator december/january 2009 Failure Mechanisms By definition, air gap eccentricity is a mechanical fault with the motor. There are several possible causes for the presence of variances in the distance between a rotor and a stator. The five basic types of air gap eccentricities that can occur are: • Rotor O.D. is eccentric to the axis of rotation • Stator bore is eccentric • Rotor and stator are round, but do not have the same axis of rotation • Rotor and shaft are round, but do not have the same axis • Any combination of the above The following are only a few of the possible causes of an air gap eccentricity: • Improper mounting of the motor to its bedplate can lead to an air gap distortion. A loose or missing bolt allows shifting of the motor’s mounting foot during thermal expansion of the frame. This shifting over time could lead to a distortion of the frame and possible distortion of the stator bore. The common term for a motor incorrectly mounted is “soft-foot.” • During construction of the motor, out-of-roundness 38
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