The AVR ( automatic voltage regulator ) is suitable for supplying power to the fields of either brushless or commutator type exciters and will maintain the A.C generator voltage within plus and minus 1% of the preset value with paralleling.
When the load varies between no load and rated load and with a frequency changes more than plus minus 4%.Upon application or rejection of rated load,the generator voltage recovers to plus and minus 1% within 0.3 second on most AVR exciter in A.C generator combination.
The AVR is suitable for operation in ambient temperature up to 50 ‘c and will deliver a power output of 150 watts with a max. current of 3 A.Under some circumstances the A.C generator wave shape changes considerably from no load to rated load.
It is due to the type of load,hence the r.m.s average or peak voltage does not stay in proportion.In this situation,i.e : greater than 10% harmonic distortion,the voltage regulation may be outside the limit quoted.
How the Auto voltage regulator (AVR) operate?
When rated load is switched on to A.C generator the voltage fall,typically to approximately 70% of its no load value.If the A.C generator voltage is to be maintained near to its no load value,then its excitation must be increased progressively in proportion to the amount of load or more correctly in proportion to the fall in voltage.The AVR perform this task.
The voltage to be regulated is derived from the line UR and VR of the A.C generator and is potentially divided by the resistance chain R1,R2 and R3.The signal appearing between terminals T2 of potentiometer RV1 and the junction of C1 and R3.
Rectifier diodes D1 and D2 rectify the signal voltage.An error signal is produced by the difference between the signal voltage and a stabilized reference voltage developed across the zener diodes D9 and D10.Capacitor C1 smoothed the signal.This error signal is applied to the base emitter circuit of VT1.The A.C generator voltage is adjusted by the potentiometer RV1.
For more detail about AVR diagram and operation,please refer to my post : Basic of AVR and operation