January 2009 Electrical Apparatus

January 2009 Electrical Apparatus

This is a summary of the Electrical Apparatus January 2009 featured technical article,  by Richard L. Nailen, P.E.     

Adapting a fixed-speed, integral horsepower induction motor to a load requiring a different fixed speed commonly involves V-belts, with driving and driven pulleys of two different diameters (the smaller one usually on the motor shaft). The belts exert a pull on that shaft causing both bending stress in the shaft and radial load on motor bearings.

Based on the number and size of belts and the pulley configuration, belt manufacturers provide user guidelines for properly tensioning the belts. Too great a tension overstresses the belts; too little results in slippage and undue belt wear. Tension is easily checked against those guidelines while the drive is at standstill.

However, the motor designer must determine whether the desired tension is safe for the motor, which requires prediction of the actual belt pull during drive operation. That depends upon the direction of pull and its axial location on the shaft as well as the friction and arc of contact between belts and the driving pulley. Most of that information is unknown to the motor manufacturer. An assumed 140 degree arc of contact is conservative. A ratio of tight-side to slack-side belt tension of 3 to 5 is reasonable (the two are equal at standstill).

Actual pull direction will be determined by the relative tight- and slack-side tensions, the position of the driven pulley axis, the driven pulley diameter, and the center-to-center distance between the pulleys. Not knowing most of those details, the motor designer must either take the purchaser’s word for the pull direction or make some assumption. For example, the worst condition for a horizontal motor is a belt pull directed vertically downward.

When that’s not the case, forces on the motor will contain both horizontal and vertical components. These must be combined vectorially. Suiting the motor to the application may then require special bearings, special shaft dimensions, or both. If not satisfied with the cost of such modifications, the purchaser will need to provide more of the actual drive configuration details so that the motor loading can be more accurately calculated.

Also contributing to the forces acting on the motor is the weight of the driving pulley, which may be several hundred kilograms for a large motor. Given the number and size of belts– one item of drive information that is customarily supplied, along with the driving pulley diameter– the motor designer can find an appropriate weight in a pulley catalog.

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