Link 1: Speed
The frequency of the generator is determined by engine speed. When speed is stable, frequency is stable; when speed fluctuates, frequency fluctuates, and voltage naturally becomes unstable.
Common causes of engine speed fluctuation: air ingress or clogged filter in the fuel system, resulting in intermittent fuel supply; clogged air filter, resulting in insufficient air intake; improper governor sensor gap, resulting in inaccurate signal acquisition. To determine whether speed is stable, simply observe the tachometer needle.
Link 2: Carbon Brushes and Slip Rings
Carbon brushes are responsible for transmitting excitation current to the rotating rotor. When carbon brushes are worn short, spring pressure decreases, the contact surface area shrinks, and current transmission becomes unstable. Oxidation or burning on the slip ring surface reduces conductivity, also affecting current transmission.
These two components are the most common sources of voltage fluctuation issues, and they are also the simplest to address—replace the brushes and polish the slip rings, and the work can be completed in half an hour.
Link 3: Rotating Rectifier Diodes
After excitation current is transmitted from the stationary part to the rotating part, it needs to be converted to direct current by rectifier diodes. If a rectifier diode fails or its performance degrades, the excitation current waveform becomes distorted, and the output voltage fluctuates accordingly.
Testing method: use a multimeter in diode test mode; a normal diode conducts in one direction and blocks in the reverse. If a faulty diode is found, it is advisable to replace the entire set, as diodes from the same batch age similarly—replacing only one will likely lead to another failure soon after.
Link 4: Automatic Voltage Regulator (AVR)
The AVR is the core component that controls output voltage. It monitors the output voltage, compares it to the setpoint, and adjusts the excitation current magnitude to keep the output voltage stable.
Common AVR issues: oxidation of the external potentiometer, causing changes in contact resistance that lead to output voltage fluctuation; internal parameter drift, causing slower response speed; blown fuse or damaged components. Potentiometer issues can be addressed by rotating the potentiometer back and forth several times to remove the oxide layer; internal faults require replacement of the AVR.
Link 5: Stator Windings
If the previous four links have been checked and the problem remains unresolved, the issue may lie in the stator windings. Inter-turn short circuits or poor connections in the windings can cause unbalanced output voltage or fluctuation.
Testing method: use a multimeter to measure three-phase DC resistance; the three values should be essentially equal, with a deviation not exceeding 2%. A significantly higher reading in one phase may indicate poor soldering or broken strands; a lower reading may indicate an inter-turn short circuit. Use a megohmmeter to measure insulation resistance; cold condition should exceed 5 MΩ. Values below this threshold indicate moisture ingress or aging of the windings.
Link 6: Load
Some voltage fluctuation is not caused by equipment issues but rather by rapid load changes. Starting large electric motors, operating welders, or connecting impact loads will momentarily drop voltage before recovery. This type of fluctuation is normal as long as recovery time falls within specified limits (typically 3–5 seconds), and no intervention is required.
If load changes are minimal but voltage fluctuation is significant, the excitation system response speed may be insufficient, and the AVR response time settings should be checked.
Troubleshooting Sequence
When addressing voltage fluctuation issues, follow the sequence of the six links above: first check speed to confirm whether the engine is stable; then check carbon brushes and slip rings to address the simplest and most common faults; then check rectifier diodes and the AVR; finally address the windings. The first two links resolve the majority of issues without requiring disassembly, offering the highest efficiency.
Under Hainan's bonded maintenance policy, imported components (rectifier diodes, AVRs, sensors, etc.) identified during generator troubleshooting that require replacement can be sourced through bonded channels, with tariff reductions and short procurement cycles.
About Hainan Longpan
Hainan Longpan Oilfield Technology Co., Ltd. provides repair services for marine engines, diesel generator sets, gas turbines, and other power equipment. Its technical team comprises over 300 members. In 2025, the company was approved as Hainan's first pilot enterprise for bonded maintenance outside the free trade port area. Its business scope covers generator engine overhaul, diesel engine repair services, generator troubleshooting, generator performance testing, and analysis of generator voltage fluctuation causes.
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