Many maintenance technicians have encountered this problem: the no-load voltage of diesel generators is normal, but it drops sharply when a load is applied, and sometimes there is no voltage output at all. Repeatedly replacing the AVR voltage regulating plate only treats the symptoms but not the root cause. Today, let's talk about the core vulnerable part of the brushless generator excitation system: the rotating rectifier assembly.
I. What is a rotating rectifier assembly and what exactly is it responsible for? The rotating rectifier assembly is a dedicated accessory for brushless generator sets. It is mainly compatible with mainstream generator sets ranging from 20 to 850kW and is installed at the rotor end of the generator.
Core working logic:
1. Receive the three-phase alternating current sent by the exciter.
2. Perform three-phase full-wave rectification through six high-power bolt diodes and convert it into direct current.
3. It supplies power to the main rotor excitation coil to establish a magnetic field and, in conjunction with the AVR regulator, stabilizes the output voltage. It can be said that it is the power supply heart of the rotor magnetic field. Once damaged, the voltage control will directly fail.
II. Rotating Rectifier Assembly Damage: Typical Fault Comparison Table
✅ Fault 1: The unit cannot generate electricity at all. One or more diodes are open-circuited, and the excitation DC power supply is directly interrupted. Replacing the AVR is ineffective. The problem can only be found by disassembling the rotor.
✅ Fault 2: The no-load voltage is normal, but the voltage drop under load is severe. Some diodes are half-broken, the rectification power is insufficient, and the excitation current cannot keep up when the load increases, causing the voltage to drop sharply.
✅ Fault 3: Severe voltage fluctuations, frequent burning of the AVR voltage regulator, reverse breakdown and short circuit of the diode, generating a large reverse current on the rotor side, which is very likely to burn out the expensive voltage regulator as a result. This is a high-frequency interlocking fault.
✅ Fault 4: Abnormal noise during unit operation, loose connection bolts of local heating diodes, excessive contact resistance and overheating. Over time, this may erode the insulating base and even pose a risk of scraping the chamber.
III. Key Taboos for Replacement and Installation
1.The polarity must never be reversed. The rectifier diode is divided into two groups, positive and negative. Once they are connected in reverse, the entire excitation component set will be broken down instantly upon power-on, causing significant secondary damage.
2. Under high-speed rotation conditions, uniform tightening of bolt torque can cause loose connections and heat generation, which is the number one cause of repeated burning out of diodes.
3. After assembly, check the dynamic balance. The rectifier wheel is a high-speed rotating component. After replacing the parts, the weight of the gasket and fasteners may change. Imbalance can cause the entire machine to shake.
4. It is recommended that after the rectifier wheel is found to be punctured, the insulation of the AVR voltage regulating plate must be inspected simultaneously to prevent the newly installed parts from being burned out again.
Ⅳ. Daily Maintenance Tips
During the regular trial operation of the standby unit, the no-load and full-load voltage curves can be recorded. If abnormal voltage fluctuations occur, the unit can be disassembled in advance to check the conductivity of the rectifier diodes, eliminating faults during the shutdown stage to avoid power outages and work stoppage during construction. The troubleshooting of electrical faults in generators should be carried out from the outside to the inside. The AVR is merely the voltage regulation actuator; the rectifier wheel hidden inside the rotor is the true source of many difficult voltage faults.