A Dieselgenerator that produces low voltage is unreliable and can damage connected equipment. Niederspannung (Unterspannung) is one of the most common generator faults, and diagnosing the root cause requires systematically working through the excitation system, Spannungsregelung, and engine speed control.
Normal vs Abnormal Voltage
| Zustand | Voltage Tolerance | Aktion |
|---|---|---|
| Normal steady-state | ±1% of rated | Keiner |
| Acceptable transient | ±15% for < 1 second | Monitor |
| Warnung | ±5-10% sustained | Investigate within 24 Std. |
| Kritisch | ±10%+ sustained | Immediate investigation; do not connect sensitive loads |
| Abschaltschwelle | Unten 85% rated or above 115% bewertet | Automatische Abschaltung (if controller enabled) |
8 Causes of Low Generator Voltage
1. AVR (Automatischer Spannungsregler) Failure
The most common cause. The AVR controls the exciter field current to maintain constant voltage. Symptome: voltage collapses to residual level (5-15% von bewertet) or is stuck at a fixed low value. The AVR may have failed internally (burnt components) or lost its sensing input (Sicherung durchgebrannt, broken wire).
2. Rotating Diode Failure
In brushless alternators, rotating diodes on the exciter rotor convert AC to DC for the main rotor field. When one diode fails open, the generator loses 15-25% of its excitation capacity. When a diode fails shorted, it can damage other diodes and cause complete excitation failure. Open-circuit diodes cause voltage droop under load; shorted diodes cause erratic voltage and possible AVR damage.
3. Motorgeschwindigkeit (Frequenz) Too Low
Generator voltage is directly proportional to engine speed and excitation. If the governor fails to hold rated speed (1500/1800 U/min), voltage drops proportionally. A 5% speed drop causes a 5% voltage drop even with a functioning AVR.
4. Loose or Corroded Connections
High-resistance connections anywhere in the excitation circuit (AVR to exciter, exciter to main field) reduce field current. Voltage may appear normal at no-load but drops severely under load. Thermal imaging during load testing reveals hot spots at corroded connections.
Step-by-Step Troubleshooting Procedure
- Safety first: Shut down generator. Lock out/tag out. Warten 5 minutes for capacitor discharge.
- Sichtprüfung: Look for burnt components on AVR board, corroded terminals, loose wires, blown fuses.
- Measure residual voltage: Start generator with AVR disconnected. Should be 5-15% der Nennspannung. If zero, main rotor or stator winding fault.
- Test AVR: Reconnect AVR. Flash field with 12V battery (positive to F+, negative to F-) für 1-2 seconds while running. If voltage builds, AVR startup circuit failed.
- Test rotating diodes: Measure forward and reverse resistance across each diode (disconnect exciter rotor leads). Good diode: low forward, high reverse. Failed diode: open or shorted in both directions.
- Check governor: Verify engine speed under load with optical tachometer. If speed droops > 3%, adjust governor or check fuel system.
FAQ
Q: Can I adjust the AVR to fix low voltage?
A: Ja, if the AVR is functional and the low voltage is due to calibration drift. Most AVRs have a voltage adjustment potentiometer. Do not adjust more than ±5% without investigating the root cause—masking a failed diode or corroded connection with AVR adjustment leads to complete failure later.
Q: Why does voltage drop when I apply load?
A: Some droop is normal (2-3% from no-load to full-load for typical Generatoren). Excessive droop (>5%) indicates: failing rotating diodes, loose connections in excitation circuit, undersized AVR, or engine governor droop (speed drops → voltage drops).
Q: How do I test rotating diodes without a multimeter?
A: You cannot accurately test diodes without a multimeter (diode test function). A 9V battery and small bulb can provide a rough go/no-go test, but this method cannot detect partially failed diodes or measure forward voltage drop. A digital multimeter with diode test is standard equipment for any generator technician.
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