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.
When prime mover is likely to be run at speeds below 96% of the nominal speed,strongly recommend that a slow speed protection device is fitted to ensure taht the excitation current does not exceed its steady state value for period long enough to cause damage to the winding or rotating diode.
How the Low Speed Protection work?
The thermostat is selected so that the bi-metal contacts do not open under steady state conditions plus a tolerance for change in ambient temperature.If,however the speed of the generator is reduced e.g. idling speed of a diesel engine,the bi-metal contact will open and insert the field current limiting resistance.
The operation has a minimum time lag of 10 – 15 second allowing for load switching and induction motor starting.Example :-
- Using unit patt. LSP 1
- steady state current : 2.6 amp
- tripping time on 3.0 amp : 100 sec
- tripping time on 5.5 amp : 15 sec
For A.C. generator using medium frequency exciter pack,field forcing is 5-6 amps,approximately twice the steady state current.If the field current is limited under low speed or idling condition,the main A.C. generator field and diode will be protected.
The thermostat is self resetting and will trip in and out,limiting the average heating of the field.Finally resetting itself a short time after generator is restored to its correct speed.