Automatic voltage regulator (AVR)

Introduction: purpose and applications

The rotating rotor field induces voltage in the generator stator. At a given speed, changing excitation current changes magnetic flux and internal generated voltage. Load, power factor and internal voltage drops affect terminal voltage, so fixed excitation alone cannot maintain a stable supply across changing conditions.

The AVR senses voltage, compares it with a reference and changes excitation to reduce the error. This is a closed feedback loop with a finite response time. Insufficient damping may produce oscillation, whereas an overly slow response prolongs recovery after load application. The sensing circuit, the regulator's power supply and its output to the exciter perform different jobs and should not be treated as one undifferentiated supply connection.

Construction and main components

• Sensing circuit — provides credible main-stator voltage feedback; it is not necessarily the regulator’s power source.
• Reference and comparison function — establish the required voltage and its deviation from the measurement.
• AVR power stage — changes direct current in the stationary exciter field.
• Exciter — a smaller machine on the common shaft whose rotating winding supplies energy for the main rotor.
• Rotating diode bridge — converts exciter alternating current into direct current for the main field.
• Main rotor and stator — create the rotating magnetic field and electrical output for consumers.
• PMG, where fitted — a separate permanent-magnet generator powering the AVR independently of main-stator voltage.
• Reactive-sharing circuits and limiters — coordinate parallel operation and permissible excitation; their arrangement is model-specific.

Working principle, step by step

1. The prime mover accelerates the generator. Its specified arrangement establishes initial supply and voltage build-up.
2. The AVR senses main output voltage and compares it with the reference. A load change creates a regulation error.
3. The regulator changes stationary exciter-field current. The rotating exciter winding responds with a change in its alternating output.
4. The rotating diode bridge rectifies this current for the main field winding. Its magnetic flux changes the voltage generated in the main stator.
5. The output is measured again. Feedback progressively reduces error, while the designated limits and parallel-operation mode modify the response under relevant conditions.

For one generator supplying an isolated shipboard bus, the AVR principally maintains voltage, while the diesel governor maintains frequency through rotational speed. Active power (kW) becomes mechanical work, heat or other energy at the consumer; engine torque supplies it. Reactive power (kvar) describes reversible energy exchange through electric and magnetic fields in an AC circuit. Reactive loading changes excitation demand and generator current; it is not equivalent to additional diesel kilowatts.

In parallel operation, machines share bus voltage and frequency. Altering one generator's excitation reference mainly redistributes reactive power, whereas changing engine fuel input redistributes active power. In an isolated group, the regulators collectively determine bus conditions. When connected to a strong external grid, one relatively small generator barely changes grid voltage or frequency. A setting suitable for standalone operation can consequently cause unwanted reactive exchange when machines are paralleled.

Reactive sharing uses the specified compensation method, such as quadrature droop with a current transformer. This introduces a sensing contribution related to the reactive component of load. An incorrect phase or transformer polarity changes the feedback relationship and can produce increasing circulating currents. Rating and connections must be checked against drawings, rather than compensated by random potentiometer adjustment.

Under-frequency roll-off (UFRO) reduces excitation below a defined frequency threshold. It helps limit an unfavourable voltage-to-frequency relationship and coordinate electrical demand with engine recovery. Low voltage accompanying low frequency may therefore be an intentional AVR response. Excitation limits protect relevant machine components, but the AVR does not replace every generator protection: short-circuit discrimination, reverse power and breaker tripping have their own functions.

Key characteristics

Voltages belong to different circuits and are not interchangeable. A self-excited arrangement draws power from the designated main-machine output; PMG excitation uses a separate source, but both depend on healthy rotating machinery.

Maintenance: planning the checks

The calendar below is a planning framework, not mandatory overhaul intervals. The ship’s planned maintenance system (PMS), running hours, condition and maker instructions determine the actual scope and timing.

Troubleshooting: symptom, cause, action

Frequently asked questions

Conclusion

The AVR closes the voltage-control loop through generator excitation. Sound diagnosis separates excitation from mechanical speed and accounts for standalone or parallel operation.

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