The effects of magnetic unbalance and mechanical misalignment on the vibration of a D-C motor

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Chirillo, Louis D.
Berude, John B.
White, D.C.
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Massachusetts Institute of Technology
Six parameters may affect the vibration of a directcurrent motor: (a) air-gap eccentricity (b) motor loading (c) relative strengths of main field poles (d) asymmetry of armature construction (e) dynamic balance of the armature (f) condition and type of bearings The object of this thesis was to investigate the effects of air-gap eccentricity on the vibration of a direct-current motor operating at no load and partial load. Previous attempts to determine the effect of air-gap eccentricity were nullified by end-bell removal, which introduced another variable. In earlier investigations, motor load applied by means of a prony brake introduced vibration frequencies which could not be attributed definitely to the prony brake. Both difficulties have been overcome in this investigation. Eccentric sleeves fitted in each bearing housing permitted controlled variation of air-gap eccentricity without disturbing the motor end-bells. An integral-mounted eddy-current brake assembly permitted load application without introducing unrecognizable frequencies. Vibration data were recorded and analyzed for no-load and partial-load motor operation at armature positions varying between centered and 0,0386 inch off-center. Air-gap flux-density distribution data were recorded using a search loop on the armature surface. These data were used in predicting the vibration level of the slotfrequency component of total motor vibration. Conclusions drawn from this investigation are: (1) Measurement and analysis equipment and technique have a high degree of precision. (2) Over-all vibration level varies directly with armature displacement. (3) Slot-frequency vibration level varies directly with armature displacement, and is caused by total flux pulsations under the pole faces. (4) Correlation between observed and predicted slotfrequency vibration levels is excellent at small armature displacements, but as armature displacement increases, the difference between predicted and observed levels increases. Recommendations for future study in this field are: (1) An improved eddy-current brake assembly and airgap search-loop system should be used to obtain more accurate data under full motor load operation. (2) Investigations of the effect of unbalanced main field-pole strengths, acting alone and in concert with air-gap eccentricity, should be made. (3) Armature tooth-slot configuration should be investigated to determine the optimum for vibration reduction purposes . (4.) A phase-sensitive circuit should be used to evaluate the type of motion experienced by the mounted motor to yield information valuable in the design of resilient motor mountings. Any improvement in the concentricity of armature and pole surfaces will reduce the vibration level of an existing motor if adjustment of poles is achieved with the use of high-permeability shim stock.
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Naval Architecture and Marine Engineering
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