Automatic Voltage Regulators

All modern ship alternators are equipped with voltage regulators. The regulators permit to obtain constant voltage of generators under various amount of load and power factor. Here we shall speak about AVR for brushless alternators.

Block diagram (Fig. 1) shows the regulator principle in an autonomous alternator system. The alternator voltage is fed to the AVR unit via fuses and exciter switch. The reference level of the alternator voltage is set by the voltage adjusting potentiometer. The difference between the actual voltage and reference value is amplified and converted to triggering pulses to the thyristor bridge. A-c voltage is fed from the alternator via supply transformer to the thyristor bridge.

Under normal circumstances, the voltage is kept constant. If speed drops of more than 5% occur, the AVR unit will automatically decrease the voltage proportional to the frequency (U/f-constant).

To obtain correct load sharing between parallel running alternators, the alternator voltage is made current dependent. This is obtained by current transformer and a resistance network inside the AVR unit.

A voltage build-up circuit which uses the alternators residual magnetism provides automatic voltage build-up.

The compounding circuit and a compounding transformer provide the necessary excitation current during short circuit conditions when voltage generator is equal to zero. It is necessary for immediate voltage build-up after elimination of short circuit.

The main unit of regulator is AVR circuit. The AVR circuits consist of the following parts: voltage measuring circuit, amplifier, frequency measuring circuit, synchronizing and triggering circuit, stabilized power supply.

Fig. 1. Automate voltage regulator. Block diagram: 1 — current transformer; 2 — fuses; 3 — excitation switch; 4 — AVR unit: 5 — voltage adjuster; 6 — voltage build-up circuit; 7 — thyristor bridge; 8 — compounding transformer; 9 —rotating diode bridge; 10 — compounding circuit; 11 — supply transformer

All the above mentioned circuits are mounted on a printed board.

The alternator voltage is transformed before it is fed to the printed circuit board via terminals. The voltage is rectified and a proportional voltage is fed to an operational amplifier where it is compared to the reference voltage. The difference voltage is amplified and controls, via a triggering circuit, the firing angle of the thyristors and thereby the exciter current.

In case of alternator voltage change, the regulator generates a signal which changes the firing angle of the thyristors until the desired voltage is obtained. A stabilizing filter ensures stable and alternator voltage.