What could be the reason for a diesel generator set running at normal speed but with unstable voltage?

I. AVR (Automatic Voltage Regulator) Faults (Most Common)

1. AVR internal components aging, voltage regulation circuit damaged, capacitors bulging, resulting in ineffective voltage regulation and slight voltage fluctuations with the load.

2. Loose AVR wiring, oxidized terminals, poor wiring connections, resulting in intermittent excitation power supply.

3. Incompatible AVR selection: using a poor-quality AVR on a small generator set results in poor load-bearing voltage regulation.

4. Poor AVR heat dissipation causing overheating and reduced voltage regulation performance during operation.

II. Excitation Circuit Faults

1. Carbon brushes worn too short, insufficient carbon brush spring force, loose carbon brush holder, intermittent slip ring contact, unstable excitation current → voltage fluctuations.

2. Rotor slip ring carbon buildup, oil contamination, oxidation, fluctuating carbon brush contact resistance.

3. Partial short circuit in rotor excitation winding, inter-turn leakage, unstable excitation current.

4. Broken excitation lead wire, poor soldering.

III. Generator Stator Side Problems

1. Minor inter-turn short circuit in stator winding, aging phase-to-phase insulation, output voltage changes with heat generation.

2. Loose stator terminals, overheating and oxidation, continuously changing voltage drop under load.

3. Loose neutral point connection, unbalanced three-phase voltage and overall drift.

IV. Load Side Problems (Voltage Fluctuations Caused by External Power Supply)

1. Frequent load changes: frequent start-stop of motors, welding machines, high-power equipment, inrush current causing voltage drops/surges.

2. Severe three-phase load imbalance: One phase load is too large, resulting in inconsistent three-phase voltages and overall voltage fluctuations.

3. Overly long load lines and undersized cables cause significant voltage drop variations with current.

4. Load leakage or grounding faults lead to unstable current.

V. Issues specific to permanent magnet exciters/auxiliary exciters:

1. Loose permanent magnet poles or demagnetization in the permanent magnet exciter (small exciter), resulting in unstable excitation power supply voltage.

2. Damaged or poorly soldered rectifier diodes (rectifier bridge) in the exciter, causing large pulsations and voltage fluctuations in the rectified output.

VI. Mechanical and interference factors:

1. Worn generator bearings or slight rotor rubbing causes changes in winding parameters and voltage drift after heating during operation.

2. Excessive unit vibration causes intermittent disconnections of the AVR, carbon brushes, and terminals.

3. Harmonic interference from peripheral frequency converters and welding machines disrupts AVR sampling and voltage regulation.

Simplified Troubleshooting Steps (from simple to complex)

1. No-load test: Disconnect all loads and observe if the voltage is stable.

– Unstable no-load voltage: Fault in AVR, carbon brushes/slip rings, excitation winding, rectifier bridge

– Stable no-load voltage, fluctuates with load: Load imbalance, line voltage drop, impact load

2. Check carbon brush length, slip ring cleanliness, and carbon brush springs.

3. Tighten all terminals of AVR, excitation, and stator, and polish oxidized contacts.

4. Replace with the same model AVR for testing to quickly determine the condition of the voltage regulator board.

5. Measure the three-phase output voltage to confirm if there is a three-phase imbalance.

6. Check the excitation rectifier diodes for breakdown or poor soldering.

7. Check if the permanent magnet exciter is demagnetized or has unstable output.

8. Troubleshoot external cables and high-power impact loads.