AVR for Generator

AVR is an important part of a generator; if it fails, the generator could stop working.

An AVR (automatic voltage regulator) helps keep the generator’s voltage output stable and within acceptable limits.

You can detect faulty AVRs in generators by checking for certain symptoms. If you suspect that your AVR may be faulty, you can replace it by following these simple steps. When selecting an AVR for your generator, make sure to choose one that is compatible with your model. You can buy an AVR for a generator from most major retailers or online stores.

What does an AVR do on a generator?

An automatic voltage regulator is a solid-state electronic device tasked with maintaining the generator output voltage at a preset value despite variable loads and the operating temperature.

They also control the division of the reactive power between the generators running in parallel via the reactive droop, handle overloads, provide frequency compensation against a low-frequency scenario, and provide over-excitation protection. Some advanced AVRs may also provide surge protection, though the majority will not.

Does a generator need an AVR?

The answer to this question depends on the types of load you connect to your generators. Incandescent lamps found in most construction sites and the power tools having universal motors are very forgiving types of loads and require very little power regulation. Unregulated generators can be used to meet the power requirements of these appliances.

However, sophisticated electronic equipment is sensitive to voltage and frequency fluctuations and requires a well-regulated power supply.

With the constant excitation, the addition of any load decreases the terminal voltage of the generating set till a state of equilibrium is reached. This equilibrium value depends on the inherent field strength of the rotor and the load.

Where is an AVR located in a generator?

Normally, you will find generator automatic voltage regulators in one of the three places

  • The main control panel of the generator,
  • In the terminal box of the alternator, or
  • Under the alternator’s rear cover (for very small portable units only).

How does an AVR Work?

While the design and working principle vary substantially among manufacturers and their models, at the most fundamental level, an AVR functions by sensing the output voltage at the output terminals or in a separate sensing coil in the stator. It then compares them to a reference voltage, often a Zener diode, providing the desired voltage level through a voltage adjustment rheostat.

What is an error voltage in a generator?

The difference between these two voltages (actual and reference) is called error signal E. Most automatic voltage regulators carry a Proportional – Integral – Derivative (PID) control system loop. This PID control loop generates a control signal C to adjust the error value E back to zero.

PID Diagram

This control signal C is made up of the following three components.

  1. Proportional Signal Cp: The control output of this component at any time is directly proportional to the value of the error E at that time.
  2. Integral signal Ci: This control output depends on the summation of errors over the total or some part of the time duration the error has existed. Hence, Ci depends both on the magnitude as well as duration of the error and tries to accelerate the elimination of the error, with a possibility of overshooting the set value.
  3. Derivative Signal Cd: This control output depends on the current rate of change of error. If the error is increasing fast, Cd will be higher and vice versa. It effectively predicts the system’s upcoming behavior and acts to improve the stability of the system and decrease its settling time.

These control signals generate an excitation field current to vary the rotor’s magnetic field strength, hence controlling the output voltage within the desired limits. The amount of excitation depends on the load current. An increase in the load current flow requires a higher excitation current.

As stated earlier, following a transient dip due to the imposition of a heavy reactive load, the voltage may overshoot multiple times and follow oscillations of decreasing magnitude before stabilizing to the desired band.

For certain high-end applications,

  • The degree of oscillation is an important criterion when selecting AC generators and their AVRs. A very big overshoot can cause a short circuit on sensitive electronics. On the other hand, a very big transient dip may cause the generator voltage to collapse, particularly if AVR depends on the generator output power for its functioning.
  • The speed of AVR response to the transient loads is critical, as discussed in the next section.
  • The AVR must be able to bring back the voltage output to within (+-) 3% of the set value even after heavy transient loads, eliminating any electrical surges due to switching.

Fast or Slow AVRs

The response time of any AVR is governed by its ability to recover the terminal voltage to a value within the steady state band around the initial value.

The initial voltage dip will normally be greater for a slow-responding AVR, along with a lower record steady-state value. In other words, the faster any AVR adjusts to the load changes and its exciter produces a higher excitation current, the recovered voltage will be nearer to the initial voltage.

Digital and Analog AVRs

The main features and functions of an analog AVR were stated in the section “What does an AVR do on a generator?” A digital AVR can perform all the functions of an analog AVR.

In addition, they carry features to protect the generator, log the operation and performance-related data, in-depth stability control features, PID compensation, RTD monitoring, PLC functionality, shutting down excitation, and a variety of other protections to prevent severe damage. All of this is, of course, at an additional cost.

These additional functions include:

  1. Under voltage monitoring (UVM)
  2. Under excitation monitoring (UEM)
  3. Overvoltage monitoring (OVM)
  4. Over flux monitoring (OFM)
  5. overexcitation monitoring (OEM)
  6. Over temperature protection (OTP)
  7. Power factor monitoring (PFM)

DVR samples the main stator voltage with sampling rates in microseconds and applies the true RMS calculations to the samples. It requires a separate power supply independent of the main stator coils to operate effectively.

Different Power Supply Arrangements for AVRs

The three different arrangements in which AVRs are connected to the generators are

  1. Shunt Excitation Systems – It uses the stator output voltage and compares it to the reference. Its performance deteriorates on high generator overload, and the system has low short circuit current capability.
  2. Permanent Magnet Generator Excitation System – This allows the AVR system to work independently of the stator output voltage. PMG system provides the fixed voltage power supply required for its functioning. This system has a high overload capacity.
  3. Auxiliary Winding Regulation Excitation Principle (AREP) – The system has two separate auxiliary windings installed on the main stator with their voltages depending on the stator output voltage and current drawn by the load, respectively. This arrangement provides a high short circuit capability to the alternator.

These systems have already been discussed in detail in our article on “How Do Portable Generators Work?along with the diagrams.

What happens if AVR fails?

The failure of an AVR will lead to the loss of excitation, resulting in a loss of output voltage and consequential shutting down of the generator on an under-voltage fault.

If your generator does not carry under voltage protection, the generator may continue to run and damage the connected electrical appliances.

How can I detect faulty AVRs in generators?

If your automatic voltage regulator has trouble, you may be experiencing faults like

  • No output voltage,
  • The low output voltage,
  • Higher voltage output,
  • The unstable output voltage,
  • The voltage dropping under load

You should first satisfy yourself that the generator is not overloaded and the stator and all circuit breakers are functioning normally. After eliminating problems in these devices only, go on to check the AVR.

In most cases, you will find problems in the sensing, the power circuit wiring, and their components as the most likely cause. Next, readjust the trimmers as indicated in the owner’s manual and test the generator again.

If your generator is not producing any voltage output, you may carry out the rotor excitation test. This test works for the brushed type of generators with limited solid-state components in the AVR. It is not recommended for machines with PCBs and integrated control circuitry.

To carry out the test,

  1. Disconnect the AVR from the exciter circuit.
  2. Start your generator and verify its no load rpm to be around 3750.
  3. Connect one lead of matching polarity from your battery set to the rotor brush.
  4. Flick the second lead into contact with the other brush and watch the voltmeter connected to a 120 V receptacle. If you get a voltage reading, the rotor and its downstream components are functional, and the exciter circuit carries the fault.

If still, you did not get any voltage, you may have to replace the AVR, as even with the factory data, the resistance tests often do not provide a clear indication in case of an AVR fault.

How can I replace my generator’s AVR?

The replacement of AVR has been dealt with in our article “The Generator Not Putting Out Full Power.” You can refer to the procedure there.

Who provides automatic voltage regulators?

It is a common practice in the industry for the alternator manufacturer to supply the AVR based on the components, accessories, and the power supply configuration like PMG, AREP, etc. Some of the bigger manufacturers of the alternator are Stamford, WEG, Leroy summer, AVK, etc.


AVR or Automatic Voltage Regulator is an important part of a generator. It regulates the voltage and keeps it within the prescribed limits. If there is any fault in the AVR, it can damage other parts of the generator and also lead to failure. You can buy an AVR for your generator, but you should select the right one depending on your generator’s specifications. I hope this article was helpful. Let me know in the comments section if you want any clarifications.

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