A generator must maintain two critical electrical characteristics: voltage and frequency. The AVR regulates voltage, while the governor regulates engine speed and, therefore, frequency.
This guide explains how each control loop works, what causes voltage or frequency to drift, and how to specify the regulation performance your application requires.
Two Control Loops, Two Different Jobs
A generator set combines an engine and an alternator. Together, the engine and alternator support stable voltage and frequency.
The governor meters fuel to maintain engine speed. Because frequency is tied to speed, a four-pole alternator operating at 1,800 rpm produces 60 Hz. When real-power demand in kW rises, the engine momentarily slows, and the governor adds fuel to restore speed and frequency.The automatic voltage regulator, or AVR, controls excitation current in the alternator to maintain terminal voltage. When reactive demand in kVAR rises, terminal voltage can sag. The AVR then strengthens the magnetic field to restore voltage.
Under normal conditions, the two loops have limited interaction because the AVR responds much faster than the engine and governor.
The practical distinction is straightforward: kW loads, such as heaters and fully loaded motors, place demand on frequency. Reactive and inrush loads place greater demand on voltage.
A facility’s power factor reflects the relationship between the two, which is why a generator can maintain frequency even as voltage becomes unstable, or vice versa.
How Frequency Regulation Works: Governor Control
Droop Control
A droop governor allows frequency to fall slightly and predictably as load increases, typically 3% to 5% from no load to full load.
This behavior supports stability and load sharing. When generators operate in parallel, matched droop settings allow them to divide load changes in proportion to their ratings without fighting one another.
Isochronous Control
An isochronous governor maintains the same frequency across the load range, returning to 60.0 Hz after each load change.
A generator serving a facility on its own will almost always operate isochronously, allowing occupants to see stable frequency regardless of demand. Paralleled generator sets can also operate isochronously when their controls actively share load electronically.
Transient Behavior
Steady-state settings tell only part of the story. When a large block load hits the generator, frequency dips as the engine responds, then recovers over the next few seconds.
The depth of that dip depends on engine response, turbocharger performance, rotating inertia, and the size of the load step relative to the generator’s rating.
Deadband also matters. Governors ignore very small deviations to prevent constant hunting, so frequency naturally moves within a narrow operating band.
How Voltage Regulation Works: Generator AVR
Modern AVRs typically maintain steady-state voltage within roughly ±0.5% to ±1% of nominal. The greater challenge is transient response.
A large motor start or block load draws substantial inrush current at low power factor, causing terminal voltage to dip until the AVR increases excitation to the field.
The depth of the dip and the speed of recovery depend on the alternator’s transient reactance, excitation-system design, and the size of the inrush current relative to the machine.
This same voltage-dip behavior drives alternator selection in our guide to sizing a generator for variable power factor loads. Motor starting is as much a regulation event as it is a sizing event.
Excitation design also matters. A permanent magnet generator (PMG) excitation system powers the AVR independently of terminal voltage.
As a result, excitation support does not collapse during a deep voltage dip, improving block-load recovery and fault ride-through compared with self-excited designs.
At the other end of the spectrum, heavily capacitive loads that push the system toward leading power factor can destabilize voltage regulation. Our power factor guide covers the limits of lagging and leading power factor.
Why Standby and Prime Duty Change the Requirements
The four ISO 8528 power ratings define how long and how heavily a generator may operate. The regulation priorities change with the duty cycle those ratings describe.
- Standby duty is a transient test. A standby generator spends most of its life waiting. When the transfer switch operates, it may need to absorb a facility’s load in one or two large steps, often while several motors restart at once.
- Transient voltage and frequency dip, block-load acceptance, and recovery time are the specifications that matter most.
- Prime duty is an endurance test. A prime-rated generator carries variable load for unlimited hours. Steady-state accuracy, governor stability across the load range, fuel-system health, and load-sharing behavior in paralleled installations become the priority. Small regulation errors that might be tolerable during a short standby event can become continuous power-quality concerns when the generator is the only source of power.
- Continuous duty is a consistency test. Constant load at a stable output creates the least demand on transient response but the greatest demand on long-term voltage and frequency accuracy.
ISO 8528 Performance Classes: Where the Two Meet
ISO 8528 does more than define power ratings. Its performance classes, G1 through G4, set numeric limits for the behaviors discussed in this article, including steady-state voltage and frequency deviation, transient dip during load acceptance, and recovery time.
G1 covers basic loads, such as lighting and simple resistive equipment. G2 approximates utility-quality power and suits most commercial standby applications. G3 applies tighter limits for telecom and process loads that are sensitive to deviations. G4 reserves specially agreed limits for the most demanding critical and data applications.
The performance class shown on a data sheet offers a fast way to compare regulation performance between candidate units. However, always confirm the specific numeric limits against the manufacturer’s documentation for the unit under consideration.
Specifying Regulation Performance: A Short Checklist
When evaluating a generator for purchase, request the following information on the data sheet or in the proposal:
- Steady-state voltage regulation as a percentage of nominal voltage.
- Steady-state frequency regulation and governor type, (droop or isochronous, including droop percentage where applicable).
- Transient voltage and frequency dip, plus recovery time, at a stated block-load step.
- ISO 8528 performance class.
- Excitation system type: self-excited or permanent magnet generator.
- Governor type: mechanical or electronic, along with paralleling capability if the installation may expand.
Depco Power Systems can match these specifications against available inventory and your application requirements.
Browse our industrial diesel gensets or contact us to review the regulation requirements for your facility.
Frequently Asked Questions
What does an AVR do on a generator?
The automatic voltage regulator controls the alternator’s excitation current to maintain terminal voltage at its setpoint.
When load increases and voltage begins to sag, the AVR strengthens the magnetic field to restore voltage. When load drops, it weakens the field.
Modern AVRs typically maintain steady-state voltage within roughly ±0.5% to ±1% and respond within cycles, far faster than the engine side of the generator set.
What is the difference between droop and isochronous governor control?
A droop governor allows frequency to decline slightly and predictably as load rises, typically 3% to 5% from no load to full load. This helps paralleled generators share load stably.
An isochronous governor returns to the same frequency at any load. It is the normal mode for a single generator serving a facility on its own.
The appropriate choice depends on whether the unit operates independently or in parallel with other power sources.
What causes generator voltage fluctuation?
- Large motor starts and block loads that draw inrush current.
- A failing or misadjusted AVR.
- Loose or corroded excitation-circuit connections.
- Worn brushes or slip rings on machines that use them.
- Heavily nonlinear or capacitive loads that interfere with regulation.
Voltage fluctuation under a steady load can point to the excitation system. Fluctuation that occurs only during load changes usually reflects normal transient response and should be evaluated as a sizing and specification issue.
What causes generator frequency fluctuation or hunting?
Hunting is a rhythmic swing in engine speed and frequency. It usually traces to the fuel and governing side of the system: unstable governor settings, air or contamination in the fuel system, a failing actuator, or cyclic loads that repeatedly affect the engine.
A persistent frequency offset, rather than oscillation, typically indicates a droop setting or speed-setpoint issue instead of a fault.
What is block-load acceptance?
Block-load acceptance is the largest single load step a generator can pick up while keeping voltage and frequency dip within specified limits and recovering within a specified time.
It is the defining transient test for standby applications, where a transfer switch can place much of a facility’s demand on the generator in one step. Acceptance capability depends on engine response, alternator design, and excitation support, and is typically listed on manufacturer data sheets.
Do standby- and prime-rated generators regulate voltage and frequency differently?
The control hardware is the same, but the operating demands differ.
Standby duty emphasizes transient performance, including deep block loads at transfer, motor-restart surges, and fast recovery. Prime duty emphasizes sustained accuracy and stability over unlimited operating hours, as well as load-sharing behavior in paralleled systems.
Matching the performance class and excitation design to the intended duty is as important as matching the generator’s kW rating.
What ISO 8528 performance class do I need?
Most commercial and industrial standby applications are well served by G2, which approximates utility-quality power.
Telecom, broadcast, and process loads that are sensitive to deviations typically specify G3. G1 suits basic resistive loads, while G4 applies when the owner and manufacturer agree on special limits for critical facilities.
When in doubt, identify the most sensitive equipment the generator must serve, then work backward from that equipment’s tolerance for voltage and frequency deviation.
