Usage:
D349, D353C, D353D, D353E, D379A, D379B, D398A, D398B and D399 Generator Sets Which Have Statically Regulated Controlled Rectifiers
The troubleshooting charts in the Service Manuals for the above Generator Sets are excluding a short circuit in the rotor field windings as a possible cause for low AC (alternating current) voltage.
Check for short circuit in the rotor field windings if low AC voltage is a problem.
To test the circuit, use a direct current, 0-100 amp ammeter installed as shown in the illustration. If the exciter field current at no load, rated voltage and rated frequency is more than 60% higher than the rated no load current, there is a short circuit in the windings.
If the exciter field current is in the acceptable range at no load, check the current with the generator as close to the rated condition (.8 power factor, rated voltage and current) as possible.
NOTE: See the Service Manual for the rated specifications. A small change in the power factor will change the field current necessary for the given voltage. An increase in the power factor of the load will decrease the necessary field current.
If it is not possible to operate the generator at rated load conditions, operate the generator for four hours at a known load condition. Measure the field current. To find the acceptable field current for load conditions other than rated, call Caterpillar Tractor Co.
Schematic to check the exciter field current for troubleshooting low AC voltage.