Coastdown Time as a Mechanical Condition Indicator for Vertical Axis Motors with Grease-Lubricated Ball Bearings

A technique is presented to assess the mechanical condition of vertical axis electric motors equipped with grease-lubricated ball bearings by comparing observed and predicted rotor coastdown times. A theoretical expression for calculating the coastdown time required, as input, a relationship between rotor torque and rotational speed. Torque values derived from existing theory in the literature proved unsatisfactory. Experiments were conducted on four electric motors with varying bearing size and preload, blade configuration and grease lubricant properties to refine the input criteria. Substitution of these empirically derived torque values into the expression then gave acceptable results. Deviation of actual coastdown times from those predicted indicates a probable mechanical problem and need for investigation.