4 Types of AC Automatic Voltage Regulators For maximum Operational Reliability

14 Oct 2021

Best Automatic Voltage Regulator

Est Reading time: 11 minutes

The operational reliability of industrial applications is highly dependent on incoming voltage stability. In this sense, it means choosing the right AC automatic voltage regulator for your system. With different types of AC voltage regulators in the market, which is the best in terms of cost-efficiency, reliability, and functionality?


In this guide, we’ll explore 4 commonly used automatic voltage regulators for AC voltage and share our professional opinion on each.

What Is An AC Automatic Voltage Regulator

An AC Automatic Voltage Regulator (AVR), or Automatic Voltage Stabiliser is engineered to ensure that the output voltage remains consistently at a predetermined level irrespective of fluctuations that may occur on the input voltage. 


The automatic voltage regulator is built with control components that senses change in the output voltage and compensate for the difference accordingly. As a result, connected systems receive a constant and stable voltage at all times.

Why Do You Need An AC Automatic Voltage Regulator

Unlike theoretical depiction, incoming voltage for real-life applications is subjected to fluctuation. This may cause the voltage to sag, swell, or flicker, which results in a deviation from the rated nominal voltage of the connected equipment.

Some equipment may have a larger tolerance against voltage differences but others may not operate well even with a slight deviation from the nominal value. For example, a 3-phase AC motor is more tolerant to voltage differences compared to its single-phase counterpart.

Even if the AC motors could operate reliably with occasional voltage sag, the respective DC control modules may not. DC components like actuators, relays, and logic ICs, are highly sensitive to unstable voltage levels. A flicker on the AC mains can affect the regulated DC voltage supplied to electronic control components.

Without an AC automatic voltage regulator, you’re risking the entire system to uncertainties of voltage spikes, sags, and fluctuations. Some facilities may also experience a substantial voltage drop due to wiring impedance. This could lead to disruption in operation, shortened component lifespan, or in less critical scenarios, non-optimal performance.

Voltage regulation saves businesses from costly repercussions, such as production losses, rejects, delayed deliveries, and other indirect issues. Installing an AC automatic voltage regulator is the only sensible option.

Common Types of AC Automatic Voltage Regulators

When searching for an AC automatic voltage regulator, you’ll come across various builds. Here are the top 4 types of voltage regulators and their pros and cons.

1. Servo (Linear /Rotary)

A servo automatic voltage regulator functions to provide stabilized voltage by changing the winding ratio of its transformer based on a negative feedback circuitry. It features a moving mechanism in the form of a servo motor and an attached carbon brush.

Servo voltage regulators are known for their high accuracy. The regulator is accurate up to ±1% for input voltage variations of up to ±50%. They are also fairly reliable and cost-efficient.

In a typical setup, a servo automatic voltage regulator is built with the following parts:

  • A buck-boost transformer, which is partly connected to the autotransformer to enable varying turns ratio.
  • An autotransformer or dimmer – a toroidal-shaped transformer with the fixed tap connected to the buck-boost transformer and the variable tap connected to the servo motor via a carbon brush.
  • Carbon brush – Serves as the moving mechanism that moves the auto-transformer according to the servo position.
  • Servo motor – Receives positioning signal from the control circuit and rotates its arm accordingly.
  • Control circuit – An electronic circuit made up of active and passive components such as a microcontroller, op-amps, and logic ICs. It samples the output voltage, calculates the adjustment needed and sends the respective offset signal to the motor.
How It Works

The control circuit of the servo regulator continuously samples the output voltage. It then compares the value against the desired output and decides if it needs to alter the winding ratio.


When the output voltage deviates from the nominal value, the control circuit signals the servo motor to shift to a new position. The servo motor will then rotate its arm, which is connected to the carbon brush, to a new position across the autotransformer.


When the carbon brush shifts, so does the ratio between the primary and secondary winding of the buck-boost transformer. This directly influences the amplitude of the output voltage on the secondary winding.

The regulated voltage, which is the voltage that falls across the secondary winding, is connected to equipment.



2. Magnetic Induction
magnetic induction automatic voltage stabiliser
magnetic induction automatic voltage stabiliser

When you require a low-maintenance AVR that operates reliably in harsh environments, the magnetic induction regulator is an ideal choice. The magnetic induction voltage regulator can sometimes be confused with an induction motor.


Both the magnetic induction regulator and induction motor are similar in the sense that they feature a stator and rotor. However, an induction motor rotates without limitation, but the magnetic induction AVR rotator’s angle is limited to less than 180 degrees.


The principle of the magnetic induction voltage regulator is to alter the proximity between the primary and secondary winding. By doing so, the magnetic flux coupled across the windings changes in magnitude and orientation. Depending on the relative positioning of both windings, the output voltage can be increased or decreased to a limit.

In a typical setup, the magnetic induction regulator has the following components.

  • A primary winding, which is wound multiple turns across the stator.
  • A secondary winding wound across a movable shaft, or rotor.
  • A servo that turns the rotor to a particular angle.
  • Control circuitry, which samples and sends the appropriate output to the servo.

Magnetic induction AVR is available for single and 3-phase AC voltages. For 3-phase applications, the regulator features 3 primary and secondary windings which are spaced 120 degrees apart.

How It Works

The intelligence of the magnetic induction voltage regulator stems from its control circuitry. The presence of a microprocessor, as well as an accompanying sampling circuit, enable the regulator to compare the output voltage to the desired value.


When the microprocessor detects an offset between the sampled output and the desired value, it moves the servo to compensate for the difference. As the servo is fed with the appropriate signal, it rotates the secondary winding to the calculated position.


As the rotor shifts, the distance and orientation from the primary winding change. This results in either an increase or decrease in the magnetic field coupled to the secondary winding and thus, the output voltage.



3. Static Type (Tap Switching)
Static automatic voltage regulator

You would have thought that the static tap switching regulator is a great option because it is fully electronic and has no moving parts. Besides, static tap regulators are also considerably cheap compared to its counterpart.

Before you decide on tap switching, you need to be wary of its limitations, particularly the Full Power Semiconductor (FPS) type, as it’s not the safest nor most reliable choice around.

An FPS tap switching regulator has the following components:

  • A multitap transformer.
  • An array of SCR (Silicon Controlled Rectifier), connected in series to each of the taps.
  • A controller circuit for activating the SCR based on the sampled output voltage.

Most static tap regulators are offered in 3 or 6 taps configurations. If you require precise voltage regulation, this isn’t the right technology. A static regulator with 3 taps will offer around 10% of tolerance. Even with 6 taps, you’ll get at most a ±5% tolerance from the nominal value. For applications that demand precise regulation, static tap regulators are not good enough.


The response time for static tap regulators is dependent on the microcontroller’s algorithm. A majority of static tap regulators use the error signal feedback method or ESBM.


Unfortunately, ESBM can be inefficient when stabilizing input voltage that is 15% higher than the nominal voltage and introduces latency in stabilization time. 


Besides delay, static tap regulators also face safety and reliability issues. The SCRs are prone to damage from inrush current, making them unsuitable for harsh electrical environments. This brings our attention to the Series Transformer (ST) variant of the static tap regulators.


Instead of being directly connected to the load, the SCRs are connected to a secondary transformer, which provides isolation from the direct current surge. However, the additional transformer increases the regulator’s cost.  

How It Works

The microcontroller on the sensing circuit samples the output voltage and compares it to the desired value. If there’s a discrepancy, the microcontroller will activate one of the SCR that will connect the tap on the secondary winding. 

Depending on the algorithm, it may take more than one cycle to reach the required output voltage.



4. Solid State (Ferroresonant)

Ferroresonant regulator, also known as ferro or constant voltage transformer (CVT), leverages an interesting principle of magnetic saturation to produce high-precise voltage regulation.

The ferroresonant regulator was invented in 1938 by Nicholas Solar and remains a popular option for applications that demand near-flawless regulation. For example, they are commonly used in film developments, cable TV and battery charger, where there’s very little tolerance for supply voltage deviation. 


A ferro typically regulates voltage to within 1% of its nominal value. The ferroresonant regulator is designed for single-phase supply but it’s possible to place separate CVTs for 3-phase systems. 


Unlike most regulators, the CVT is categorically a passive device. It features a magnetic core driven to saturation and a tank circuit to negate the potential side effects of a transformer operating in saturation. With no moving parts, the ferroresonant regulator is not subjected to mechanical wear and tear. 

How It Works

In order to understand how a ferroresonant regulator works, you’ll need to refer to the ferroresonant saturation curve.

Ferro Automatic Voltage Regulator

A transformer operating in the normal range will produce an output voltage that is proportionate to the input voltage. A ferroresonant regulator, however, operates on the non-linear part of the curve, where the output voltage remains consistent despite huge changes in the input voltage.


There are no sensing circuits, mechanical components or a feedback loop involved as ferros operate on the basis of magnetic flux saturation. 


However, you’ll find a secondary winding in parallel with one or more capacitors that acts as an LC resonant circuit. The resonant tank prevents distortions and harmonics, which are byproducts of core saturation, from affecting the regulated voltage. It also serves as temporary energy storage that helps to smoothen the output voltage.



automatic voltage regulator

How To Choose The Right AC Automatic Voltage Regulator

One of the primary objectives of installing a voltage regulator is to ensure stable and consistent voltage output. Therefore, it’s crucial to choose an AVR that meets your regulation requirement. Servo regulators, with accuracy within the 1% range, are a better option than static tap switching types.
Input Voltage
In some setups, there could be a substantial drop in the incoming AC supply. You’ll need to choose a voltage regulator that could operate reliably within the input voltage range.
You don’t want to choose an automatic voltage regulator that breaks down the slightest hints of surges. A regulator should require as little maintenance as possible, and remain functional in heavy usage. This rules out static tap switching regulators as the SCRs may fail after exposure to in-rush current.
Electrical Load
Different applications require different types of voltage regulators. Determine if you need to power up a linear, non-linear or high-current load and choose an automatic voltage regulator best suited for the job.
Response Time
It goes without saying that the quality of voltage regulation is as good as the response time. A decent voltage regulator shouldn’t take too much time to stabilize the output voltage. This is particularly true if the input voltage is subjected to constant fluctuations.


AC automatic voltage regulators are an indispensable part of electrical applications. Installing one ensures that equipment operates optimally and with a prolonged lifespan.


We’ve explored 4 of the most common AVRs; servo, magnetic induction, static tap switching and ferroresonant. We’ve given our thoughts on which strikes the balance of reliability, accuracy, cost and versatility.


Servo and magnetic induction regulators are undoubtedly the best options for various applications.

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See which of our range of products can effectively serve your needs.

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Stay Safe With Greater Protection

As safety is our utmost priority, Ashley Edison prepares for every scenario possible
—taking the puzzle out of your protection.

Designed And Engineered With Safety In Mind

We leave nothing to chance when it comes to larger capacity units (≥400KVA). By installing high-quality protective acrylic shields in front of live parts, we ensure the safety of your personnel at all times. For enhanced protection during installation, all distances between uninsulated metal parts, busbars, and cable sizes are compliant with IEC61439-1 and IEC 61439-2 guidelines.

Outfitted with fireproof cable trunking, this protective solution safeguards against long fire exposure even in hazardous areas and explosive atmospheres—providing maximum safety for mission critical applications.

Fuss-Free Installation. Seamless Operations.

To ensure ease of installation, Ashley Edison addresses the concerns of contractors and installers by keeping it simple.



We keep the installation process simple and streamlined by allowing sufficient working space for installers. Taking the nut and bolt intrusions into consideration, we ensure that the distance between the uninsulated metal parts, the busbars, and cable sizes are compliant with IEC61439-1 and IEC 61439-2 guidelines.


Even in units with larger capacities, all busbar terminals are clearly marked out, and well positioned in line for easy identification of individual input and output terminations. Installation of these terminations are similar to those practised in high voltage transformers.

Easy Maintenance

Annual Ocular Maintenance
Visual Inspection 230

Conduct Visual Inspection On Voltmeter:

Ensure reading is set to your desired set output voltage value (e.g. 230V).

LED Status

Ensure LED Status Indicators are at “Normal”.

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Visual Observation Of Variable Transformer Surface:

Use airbrush for dusty environments if necessary.

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Visual observation if carbon brush is not worn out.

Moving Motor

Conduct visual inspection to check variable transformers are all regulating (moving).


Accuracy. Beyond Imagination.

The digitally enhanced Ashley Edison MBB Card ensures pinpoint voltage stabilisation against the most erratic anomalies, allowing you to protect precious uptime, effortlessly. Harnessing ultra precision capabilities to produce some of the tightest output voltage tolerance available in the industry, the Ashley Edison MBB Card delivers ±0.5% voltage accuracy, enabling your load equipment to enjoy optimal voltage supplies, regardless of load change in your electrical system.

Expert Digital Control

Utilising the advantageous measurements of true RMS, the Ashley Edison MBB Card’s reads both perfect, sinusoidal waves, as well as complex, distorted non sinusoidal waves—up to 0.1V accuracy. Coupled with lightning fast response time of 1.5ms, this expert digital control feature equips your load with total voltage protection that is precise, continuous and ultra responsive.

AE Digital Control Display_230v

Universal Presettable Function

The Universal Presettable Function onboard the Ashley Edison MBB Card allows seamless control of your desired output voltage value on all 3 Phases—with just a touch of a button. This multifunctional feature enables each AVR to fully operate with just 1 card, instead of separate cards for each individual phase. Fitted LED Output Voltmeter Display and Alert Indicator better facilitates easy status monitoring while performing maintenance on your AVR.


Microcontroller Unit (MCU)

MicroController Unit (MCU) helps you to deliver high speed and reliable efficiency in the AVR’s operations with less heat generation and reduced power consumption. Used in Supercomputers, Surface Mount Technology (SMT) allows components in the Ashley Edison MBB Card to be aligned closer together, creating a more compact and lightweight end product. Additional safety wire-to-board feature establishes fail-proof connectivity between circuits—so you can work safely on your AVR.