Electrical Apparatus June 2016

Electrical Apparatus June 2016

This is a summary of the Electrical Apparatus June 2015 featured technical article,  by Richard L. Nailen, P.E.    Receive this article with your subscription to Electrical Apparatus

True or false: Clarifying common misconceptions about electric motors
In technical literature, in the interpretation of industry standards, and in misconceptions based on inadequate understanding of a-c motor fundamentals, specifiers and users of motors can make mistakes in applying these machines. Although some of those mistakes are relatively harmless, others may lead to premature failures.
Given the present-day emphasis on energy savings, motor users should be aware that the use of power factor correction capacitors does not reduce the energy requirements of any motor; that stray load loss in a motor cannot be ignored in calculating motor efficiency; that “larger” ball bearings tend to lower efficiency through increased friction loss; that matching motor horsepower rating to the load will usually mean a slightly lower efficiency at that load than using the next larger size motor; and that efficiency levels above those mandated by government regulations or industry standards are not subject to the guarantees implicit in those documents. Nor are industry standards legally binding in any other way unless made part of the motor purchase contract.
They should also realize that a drop in motor voltage under load will cause current (and temperature) to rise, not drop; that hand contact on a motor surface is never a reliable way to judge operating temperature; that stator winding burnout is not the most common cause of motor failure; and that stator winding temperature monitoring alone cannot guard against damage from excessive starting. Phase voltage unbalance will not only cause much greater current unbalance. It will also increase core loss. Explosion-proof motors are designed to contain, not prevent, internal explosions, which may cause motor failure even though no external damage results.
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