Cummins/Onan — Overvoltage Sensor Calibration
Adjustments
From a Cummins/Onan service manual — check it applies to your exact model before use.
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Before you start
- Rear of control accessory panel and transfer switch are energized. Do not touch due to serious shock hazard!
Procedure
- Open the cabinet door of the automatic transfer switch.
- Move the operation selector switch to STOP.
- Turn the % DROP-OUT DIFFERENTIAL knobs to minimum and the PICK-UP VOLTAGE knobs to the clockwise limit (maximum).
- On the sensor to be calibrated, turn PICK-UP VOLTAGE knob very slowly counterclockwise until you hear relay K6 close.
- This reading times the multiplying factor should equal the line voltage. Check line voltage with a voltmeter, divide by the factor to see if the reading is correct.
- If setting is too low: turn % DROP-OUT DIFFERENTIAL knob counterclockwise to minimum; turn PICK-UP VOLTAGE knob to desired setting; insert small screwdriver through CALIBRATE hole and turn countercloc
- If setting is too high: turn PICK-UP VOLTAGE knob to its clockwise limit; insert small screwdriver through CALIBRATE hole and turn to its clockwise limit; turn PICK-UP VOLTAGE knob to the desired sett
- Repeat Steps 3 through 5 for each overvoltage sensor.
- For more accurate calibration and calibration of the % DROP-OUT DIFFERENTIAL knob, use the Onan Multi-Tester or a variac and perform the following steps.
- Remove the twist-lock disconnect plug and open the control accessory panel.
- Remove the transformer leads from T2-1 (COM) and T2-2 (120-V) from left side of terminal strip and connect them to output leads of a 120-volt variac.
- Connect a voltmeter to the output leads of the variac and connect the variac to a 120-volt AC source.
- Reconnect the disconnect plug, turn % DROP-OUT DIFFERENTIAL knobs to minimum and PICK-UP VOLTAGE knobs to maximum.
- Adjust the Multi-Tester or variac to give a 120-volt output; turn PICK-UP VOLTAGE knob counterclockwise very slowly until relay K6 closes; knob should indicate 120 volts.
- Set PICK-UP VOLTAGE knob to 132 volts, relay K6 should drop out; increase output voltage until relay K6 picks up at approximately 132 volts; check % DROP-OUT DIFFERENTIAL and adjust if needed; after c
Tools required
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Specs & values
- Sensor PICK-UP VOLTAGE knob range is 90 to 140 volts for a nominal 120-volt system.
- At 120 volts output, relay K6 should pick up at 120 volts.
- At 132 volts pick-up, with 5 percent differential, relay K6 should drop out at approximately 95 percent of 132 volts = 125 to 126 volts.
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.