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Home›Repair guides›Cummins/Onan›Automatic Voltage Regulator Voltage and Stability Adjustments

Cummins/Onan — Automatic Voltage Regulator Voltage and Stability Adjustments

Adjustments · Voltage regulator VR21

From a Cummins/Onan service manual — check it applies to your exact model before use.

Applies to: Cummins/Onan MDGDA, Cummins/Onan MDGDB

Before you start

  • High voltage. Touching uninsulated high voltage components and terminals on the voltage regulator can result in severe personal injury or death.
  • The sealed adjustment pots on the voltage regulator board are factory preset. Attempts to adjust them can lead to serious voltage instability and overheating.

Procedure

  1. Use the voltage trimmer for small voltage adjustments; measure voltage across any line and neutral with the set at no load and nominal frequency.
  2. If trimmer adjustment is insufficient, lock it at midpoint and turn the VOLTS pot fully counterclockwise and the STABILITY pot to its midpoint.
  3. If the red LED lights, refer to Jumper Reconnections and UFRO Adjustment.
  4. Turn the VOLTS pot clockwise until rated voltage is obtained.
  5. If voltage becomes unstable with a large load, turn the STABILITY pot clockwise until stable; readjust the VOLTS pot each time.
  6. Set the RESPONSE jumper A to C for rated output less than 90 kW or B to C for rated output greater than 90 kW.
  7. Set frequency jumper connections to correspond to the application frequency.

Tools required

Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.

Specs & values

  • Output voltage trimmer adjusts plus or minus five percent of nominal voltage
  • Response jumper A to C for under 90 kW; B to C for over 90 kW
  • 50 Hz frequency jumper setting
  • 60 Hz frequency jumper setting
  • DROOP preset at factory for five percent droop at full load and zero power factor

Full service-parts list: Cummins/Onan MDGDA →, Cummins/Onan MDGDB →

Source: Cummins/Onan service manual. Steps and values are as documented; always cross-check against the official manual and any later revisions for your exact serial number before servicing.

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