Tensioning motor belts by ampere draw can be misleading. Doing so on a motor/fan combination until the current matches the nameplate seems to make sense until you realize that most belt systems are only about 50% loaded. The result is lots of bad bearings and machining and motors lasting days instead of years.
Belt tensioning, in fact, has a definite impact on system efficiency. Even slight over or under tension will reduce system efficiency by more than 2%. Looking at energy consumption also provides immediate and measurable data that demonstrates the impact. The loss in efficiency is an increase in friction that directly relates to unnecessary wear on components, including bearings, belts and sheaves.
A more accurate measurement of condition can be made by using electrical signature analysis equipment to record kilowatts before and after adjustments. This takes the place of calculating the demand and makes the measurements far more accurate.
Gleaned from MotorDoc newsletters with permission from Dr. Howard Penrose, CMRP, Success by Design.