Doosan G90 Generator Failure and Diagnostic Pathways

[MikePhua]

Doosan’s Generator Lineage and the G90’s Role
Doosan, a South Korean industrial powerhouse founded in 1896, entered the generator market through its acquisition of Ingersoll Rand’s portable power division in 2007. The G90 series, part of its mid-range diesel generator lineup, was designed to deliver reliable three-phase power for industrial, mining, and construction applications. The G90-WJ DWJD-2A-T3 (C24) variant is powered by a Cummins engine and paired with a Leroy-Somer alternator, a combination known for durability and global parts support.
With thousands of units deployed worldwide, the G90 series has become a common sight on job sites requiring 60–90 kVA output. Its popularity stems from its balance of compact design, fuel efficiency, and compatibility with soft-start motor systems and auto-start controls.
System Architecture and Key Components
The G90-WJ DWJD-2A-T3 (C24) integrates several subsystems that must work in harmony:

• Engine: Cummins diesel, typically 60–80 hp, Tier 3 compliant
  
• Alternator: Leroy-Somer brushless, with auxiliary and sensing windings
  
• AVR (Automatic Voltage Regulator): Regulates output voltage via field excitation
  
• Exciter: Supplies current to the main field windings
  
• Swivel and junction box: Houses critical wiring and diode bridges
  
• Safety interlocks: Door switches and breaker logic to prevent unsafe operation
  

The generator is designed to produce 480V three-phase power, with selectable output modes including 240V single-phase. It uses a soft-start system to reduce inrush current when powering motors, and includes a digital auto-start module for remote operation.
Terminology Clarification

• AVR (Automatic Voltage Regulator): A device that controls the voltage output of the generator by adjusting the excitation current.
  
• Exciter: A small generator or coil that provides the initial magnetic field to the main alternator.
  
• Residual magnetism: The leftover magnetic field in the iron core of the generator, necessary for startup voltage generation.
  
• Auxiliary windings: Secondary coils in the alternator that supply power to the AVR.
  
• SCR (Silicon Controlled Rectifier): A semiconductor device used to control DC output from AC input.
  

Failure Scenario and Initial Observations
During a routine test of a trommel system powered by the G90, the generator abruptly ceased producing power. The engine continued to run smoothly, but output voltage dropped to near zero across all phases. The load at the time was modest—approximately 27 amps—and the system had been operating without issue for over 40 hours.
Initial checks revealed:

• Main breaker did not trip
  
• All fuses intact, including the 8A fuse on the AVR board
  
• No signs of overheating or arcing
  
• Residual voltage of ~25V present across the bus
  
• AVR replaced with no improvement
  

These symptoms suggested a failure in the excitation or regulation system rather than a mechanical fault.
Diagnostic Strategy and Field Tests
Experienced technicians recommend a structured approach to generator diagnostics:

1. Battery Excitation Test
   Disconnect the field wires from the AVR and apply 12V or 24V DC directly to the exciter terminals. A healthy generator should produce 180–240V AC output under this test. This confirms the integrity of the exciter, main field, and stator windings.
   
2. Measure AVR Output
   With the generator running, check the DC voltage at the AVR’s field terminals. If the AVR is receiving proper AC input but fails to deliver sufficient DC output, it may be faulty or underpowered.
   
3. Inspect Auxiliary Windings
   These windings supply power to the AVR. Measure AC voltage on the red/black or yellow/green wires. If absent, the AVR cannot function properly.
   
4. Check Voltage Sensing Circuit
   The AVR adjusts excitation based on sensed output voltage. If the sensing wires are disconnected or damaged, the AVR may misregulate or shut down excitation.
   
5. Examine Diode Bridges and Rectifiers
   Rotating rectifiers convert AC from the exciter to DC for the main field. A failed diode can prevent excitation, even if the AVR is functioning.
   
6. Inspect Junction Box Wiring
   Loose or corroded terminals, especially in the eyelets, can interrupt excitation or sensing circuits. A single broken wire can disable the entire system.
   

Operator Anecdotes and Practical Fixes
In one case, a technician discovered a broken eyelet buried beneath other wires in the junction box. Replacing the crimp restored full output. Another operator noted that the door safety switch, if misaligned, could remotely trip the main breaker—an issue that caused multiple service calls.
A mechanic in Illinois reported a similar failure on a Doosan unit where the rotating rectifier had degraded under light load. Replacing the diode bridge resolved the issue, though the fault was initially masked by residual voltage.
Maintenance Recommendations
To prevent similar failures, consider the following:

• Inspect and clean junction box connections quarterly
  
• Test AVR output under load and no-load conditions
  
• Replace diode bridges every 3,000–5,000 hours or if voltage irregularities occur
  
• Verify door interlock function and breaker logic annually
  
• Use dielectric grease on sensing and auxiliary terminals to prevent corrosion
  

For units in high-humidity or dusty environments, additional sealing and ventilation may be necessary to protect sensitive electronics.
Modern Comparisons and Lessons Learned
Newer generators like the Doosan G100 or Caterpillar XQ100 feature digital regulators, CAN-bus diagnostics, and remote monitoring. While these offer enhanced control, they also introduce complexity. The G90’s analog systems, though less sophisticated, allow for field-level diagnostics and repairs without proprietary tools.
The battery excitation test remains one of the most effective methods for isolating faults in traditional generator systems. It bypasses the AVR and directly tests the generator’s core components, offering a clear path forward when digital readouts fail.
Conclusion
The Doosan G90-WJ DWJD-2A-T3 (C24) is a robust generator with a proven track record, but like all machines, it depends on the integrity of its electrical pathways. Failures in excitation, sensing, or regulation can mimic more serious faults, but with methodical testing and a solid understanding of its architecture, most issues can be resolved in the field. For operators and technicians alike, the G90 offers a valuable lesson in balancing simplicity with performance.