For both AREP & PMG excitation systems,the alternator voltage regulator is the R 448. With AREP excitation, the R 448 electronic AVR is powered by two auxiliary windings which are independent of the voltage equal circuit. The primary winding has a voltage in ratio to that of the alternator (characteristic Shunt), the next has a voltage in proportion to the stator current (compound characteristic: Booster effect).The power supply voltage is rectified and filtered before being used by the AVR monitoring transistor. With PMG excitation, a permanent magnet generator (PMG) added to the alternator supplies the AVR with voltage which is free of the main alternator winding. This is built-in at the rear of the machine and connected to the R 448 AVR, and the ST9 jumper must be disconnected. These two principles provide the machine with a short-circuit current capacity of 3 IN for 10 s, and good protection to distortions generated by the load. The AVR monitors and corrects the alternator output voltage by adjusting the excitation current. In SHUNT excitation, R 448 AVR.is supplied by main windings (140V – 50/60 Hz) to X1, X2.
On application of a load, the rotation speed of the generator set decreases. When it passes below the preset frequency threshold the LAM causes the voltage to drop by approximately 13% or 25% and consequently the quantity of active load applied is reduced by approximately 25% to 45%, until the speed reaches its rated value again. Hence the LAM can be used either to reduce the speed variation (frequency) and its duration for a given applied load, or to increase the applied load probable for one speed variation (turbo-charged engine).
To avoid voltage oscillations, the trip threshold for the LAM function should be set approximately 2 Hz below the lowest frequency in steady state. It is advisable to use the LAM at 25% for load impacts Š 70% of the genset rated power.LAM, action eliminated by cutting the ST5 jumper. During load impacts, the function helps the genset to return to its rated speed faster thanks to a gradual increase in voltage according to the principle:
> If the speed drops between 46 and 50 Hz, the rated voltage follows a fast gradient as it is restored.
> If the speed drops below 46 Hz, since the engine needs more help, the voltage follows a slow gradient as it returns to the reference value.
The new functions of the R448 are :
– Parallel operation between alternators with C.T.
– Parallel operation with the mains with C.T.and R 726 module
– 3-phase sensing (R 731 module)
– LAM function integrated in the R 448 AVR.