Improper selection of Electric Motor starters for energy efficient motors.

For this building construction problem one must understand the difference between "inrush" and "locked rotor" current.  Locked rotor current is the value which begins at the moment of contact closure.  It declines rapidly as the motor accelerates up to running speed.  The dissipation of locked rotor current takes place in a mere matter of seconds.  Locked rotor amperage normally runs between six to ten times full current. 

Inrush current, which can be up to twenty times the full load current, actually occurs for approximately a hundredth of a second.  Although its value far exceeds the locked rotor amperage, since it occurs for such a small period of time, it does not affect thermal protectors.  However, energy efficient motors utilized with older magnetic circuit protectors can often experience nuisance starting trips. Hal Finkelstein has done a lot of research on these motor starter problems. Learn more