NUT (SEM) 5224355 - Caterpillar

Initial Operating Procedure After Repairs
The engine should not be operated at rated RPM immediately after the cause for the fault condition has been repaired. The following procedure may prevent additional failures if the actual cause of the fault condition was not detected.1. Remove build-up relay (K1) and disconnect wire (6) from regulator assembly (A1).2. Start the engine and operate it at low RPM. The residual magnetism in the field should produce approximately 2 to 10 volts (generator output). Shorted controlled rectifiers (CR1 and CR2) will allow the field to produce higher output voltage.3. Stop the engine and install build-up relay (K1).4. Start the engine and operate it at low idle RPM. The output voltage will vary (up and down) when the points in the build-up relay are properly operating. If the output voltage is 2 to 10 volts and steady, either the build-up relay points are open or the controlled rectifier is open [or the field rectifier (CR5) is open]. If the build-up relay points remain closed, generator voltage will be higher and steady.5. Stop the engine and reconnect wire (6) to regulator assembly (A1).6. Start the engine and operate it at low RPM. The output voltage should be approximately 66% of generator rated voltage. If the output voltage is either too high or is not steady, the reference circuit may be open or the regulator assembly may be defective.7. With the engine operating at low idle RPM and the generator producing a steady voltage (approximately 66% of rated voltage), slowly increase the RPM to rated RPM. Adjust the voltage level control to generator rated voltage. The generator is now ready to use.Testing Power Rectifiers And Control Rectifiers
The power rectifiers and control rectifiers, used in the SRCR generator sets, are of the stud mounted type. Extra care must be taken in the installation, testing and replacement of these components.
SRCR RECTIFIERS
1. Build up diode CR6. 2. Control rectifier CR2. 3. Control rectifier CR1. 4. Power rectifier CR4. 5. Field Rectifier CR5. 6. Power rectifier CR3.Failure of these units can result from: 1. Excess Current2. Excess Voltage3. Excess Heat4. Incorrect Installation TorqueRectifier failure normally is a short, rarely an open. However, their operating characteristics may change to the extent that performance in the excitation circuit is unsatisfactory. This type of rectifier fault can not be detected with ohmmeter checks.
CHECKING POWER RECTIFIEROhmmeter checks may be performed on these rectifiers, but only to determine open or shorted conditions. To perform these tests, the positive and negative leads of the ohmmeter are alternately connected to the anode and cathode. A shorted diode will indicate zero or equally low resistance in both directions, while an open diode will indicate an infinite resistance in both directions. A "good" rectifier will indicate a reverse resistance that is considerably higher than the forward resistance. Reverse resistance is 30,000 ohms or more. Forward resistance is approximately 10 ohms.A "good" control rectifier will have both forward and reverse resistance of 30,000 ohms to 3,000,000 ohms,