4 Types of AC Automatic Voltage Regulators For maximum Operational Reliability

14 Oct 2021


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, or AC AVR, 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 4 popular AVRs and their pros and cons.

1. Servo (Linear /Rotary)

A servo voltage regulator provides 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.



Our Thoughts


Servo automatic voltage regulators are one of the most reliable types of regulators. They are an economical option that gives you the best value in the long run.

2. Magnetic Induction

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.



Our Thoughts


Magnetic induction automatic voltage regulator is the go-to option when you require an AVR that is ultra-reliable, maintenance-free and works reliably in rugged environments or heavy industrial applications.

3. Static Type (Tap Switching)

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.



Our Thoughts


We wouldn’t recommend static type AC voltage regulators. They are known to be unsafe and have low reliability. It wouldn’t do justice to jeopardize your system for the sake of getting cheaper regulators.

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.

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.



Our Thoughts


Ferroresonant regulators are a great option for applications that demand stable, high-precision regulation.

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.

Solve Your voltage issues

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

automatic voltage stabiliser
Scroll to Top
Form Sent!
Thank you for getting in touch with us.

We will be in contact shortly.

Three Phase

SESL-H-3P-S Model
Large Capacity Rating
200 to 2,000 KVA


Enclosure 335
1000W x 1300H x 580D (mm)
250~300 KVA ± 15%
120~200 KVA ± 20%


Enclosure 336
1280W x 1480H x 660D (mm)
400 KVA ± 15%
250 KVA ± 20%


Enclosure 337H
1880W x 1950H x 880D (mm)
500 KVA ± 15%
300 KVA ± 20%


Enclosure 339
1470W x 1950H x 1340D (mm)
600 ~ 1,500 KVA ± 15%
400 ~ 1,000 KVA ± 20%
300 ~ 750 KVA ± 25%
300 ~ 600 KVA ± 30%

Three Phase

SES-H-3P-S Model
Added I/P Breaker Protection
60 to 1,000 KVA


Enclosure 333
490W x 800H x 990D (mm)
≤ 100 KVA ± 15%


Enclosure 334
540W x 900H x 1000D (mm)
120 – 150 KVA ± 15%


Enclosure 335
1000W x 1300H x 580D (mm)
180 – 300 KVA ± 15%
120 – 200 KVA ± 20%
120 – 150 KVA ± 25%


Enclosure 336H
1880W x 1480H x 660D (mm)
400 KVA ± 15%
250 KVA ± 20%
180 ~ 200 KVA ± 25%


Enclosure 337H
1880W x 1950H x 880D (mm)
500 KVA ± 15%
300 KVA ± 20%
250 KVA ± 25%


Enclosure 339H
2170W x 1950H x 1340D (mm)
600~1,000 KVA ± 15%
400~600 KVA ± 20%
300~400 KVA ± 25%

Single Phase

SES-H-S Model
Added I/P Breaker Protection
1 to 100 KVA

Enclosure 102
270W x 460H x 490D (mm)
≤ 20 KVA ± 30%
≤ 15 KVA ± 20%
≤ 10 KVA ± 25%
≤ 10 KVA ± 15%

Enclosure 103
400W x 580H x 500D (mm)
25 KVA ± 15%
20 KVA ± 20%
15 KVA ± 25%

Enclosure 332
380W x 670H x 780D (mm)
30 KVA ± 15%

Enclosure 333
490W x 800H x 990D (mm)
40 ~ 50 KVA ± 15%
25 ~ 30 KVA ± 20%
20 ~ 25 KVA ± 25%
15 ~ 20 KVA ± 30%

Enclosure 334
540W x 900H x 1000D (mm)
60 ~ 75 KVA ± 15%
40 ~60 KVA ± 20%
30 ~ 50 KVA ± 25%
25 ~ 40 KVA ± 30%