18 May 2011

Replacing an AVR

Automatic Voltage Regulator (AVR) is one of the main component of an Alternator. Main functions of AVR is to built-up the rated voltage and maintain the output voltage within the limit at no-load and on-load conditions. There will not be any output or abnormal output when an AVR fails. What are to be checked before replacing an AVR
  1. Disconnect the  AVR. Then check all windings of alternator for continuity and insulation resistance. If any defect is found then rectify it. Never replace the AVR if any winding is having a very low IR value or winding is defective. Connect the AVR after rectifying the defect and then start the machine. If there is no output voltage go to step 2.
  2. Check the condition of diodes in RRA. If found defective then replace the defective diode of Rotating Rectifier Assembly (RRA). Start the DG Set. If alternator does not develop  voltage go to step 3.
  3. Then run the alternator at idle speed without connecting AVR and note the output voltage. The output voltage must be balanced (If unbalance output voltage is found check the armature winding  and lead wires as explained in step 1). Then run the alternator at rated speed and again check the output voltage. Most of the AVRs need 5V output  between phase and neutral (or on the input power termlias of AVR). If the output is very low (<5V) then connect a battery (at least 12V, also check the resistance of exciter field coil. While connecting a battery please confirm current through the exciter field do not exceed the maximum excitation current of alternator. To know the maximum excitation current please refer name plate of alternator or product catalogue) at the exciter field lead wires of the alternator in correct polarity. Leave it for few minutes and then disconnect the battery. Start the DG set and run it at rated speed. If the output between neutral and phase is more than 5V then stop the engine. If the output does not improve then repeat this process for few more time. If the alternator does not develop voltage the we can try with another AVR.
  4. Replace the AVR. While replacing with other make AVR please confirm that this AVR suits with the alternator. Exciter field resistance & rating of the alternator must be with in the limit as prescribed in the catalogue of AVR.
  5. Check the output voltage, frequency and excitation voltage. Output voltage must be balanced and frequency must be slightly (1Hz) more than the name plate value. To set the frequency adjust the engine speed. Also the no-load excitation voltage must be within the limit. Switch OFF DG. While doing so note that  under-frequency indicator on AVR glows and voltage droops.
  6. Re-start the DG. Put load on DG. Check the output voltage, frequency and excitation voltage. Maximum permissible droop in frequency is 3%. If frequency droops more than this value check the engine. Excitation voltage must not be more than the alternators maximum excitation voltage.at trated load (refer name plate to get maximum excitation voltage)
  7. Operate the DG Set for few time. After switching OFF alternator check for any abnormal heating.