For a given excitation,if speed of an A.C. generator is reduced then the output voltage would reduce also.In practice,this reduction in output voltage is sensed by the AVR ( Automatic voltage regulator ) which increases the excitation current to compensate.

However,the field forcing capabilities of the AVR which are necessary to maintain output voltage during rapid load changes,will permit an “over-excitation” condition to exist.

As the regulator is not speed ( or frequency) sensitive,field forcing would occur damage field windings and rotating diode.In order to safeguard the machine against this condition,particularly where low speed duty or idling is anticipated a LSP ( low speed protection ) unit is available to sense prolonged over-excitation and automatically insert current limiting resistance in the field circuit. read more