Voltage Stabilizer

Voltage Stabilizer


Nowadays, stabilizers became an optimized power solution to many electronic appliances which are sensitive to voltage fluctuations.

DRex Electronics

It is very common to have a voltage stabilizer with refrigerators, air conditioners, televisions, furnace equipment, telecommunication equipment, medical equipment, micro oven, music systems, washing machines, etc. The main intention behind the usage of voltage stabilizers is to protect the devices against voltage fluctuations.

voltage stabilizer

Fig.1: Voltage Stabilizer

Every electrical appliance is designed to operate under a specific voltage to give the desired performance. Hence, if this voltage is below or above a certain value, the appliance would malfunction or might operate at worse condition or even it might get damaged.

In home and industrial applications, generally automatic voltage regulators are employed to keep the voltage constant to the particular equipment.

What is a Voltage Stabilizer?

A voltage stabilizer is an electrical appliance which delivers a constant voltage to a load at its output terminals regardless of the changes in the input or supply voltage.

It protects the equipment or machine against over voltage, under voltage, and other voltage surges.

It is also known as automatic voltage regulator (AVR).

Voltage stabilizers regulate the fluctuating input voltage before it could be fed to the load (or equipment which is sensitive to voltage variations).

The output voltage from the stabilizer will stay in the range of 220V or 230V in case of single phase supply and 380V or 400V in case of three phase supply, within given fluctuating range of input voltage.

These stabilizers can be available either as dedicated units for appliances such as air conditioners, LCD/LED TV, refrigerators, music systems, washing machines or as a big stabilizer unit for whole appliances in a particular place, say whole house. In addition, these can be either analog or digital type of stabilizer units.

Fig.2: Voltage Stabilizer

The common types of voltage stabilizers include manual operated or switchable stabilizers, automatic relay type stabilizers, solid state or static stabilizers, and servo controlled stabilizers.

In addition to the stabilizing function, most stabilizers come with additional features such as input/output low voltage cutoff, input/output high voltage cutoff, overload cutoff, output start and stop facility, manual/auto start, voltage cutoff display, zero voltage switching, etc.

Why do we need a Voltage Stabilizer?

Generally, each and every electrical appliance is designed for a wide range of input voltage. Depending on the sensitivity, the working range of the equipment is limited to a specific value, for instance, some equipment can tolerate ± 10 percent of the rated voltage while others ± 5 percent or less.

In some countries, electric power distribution is 230 volts for single phase and 415 volts for three-phase. In such case, all electrical appliances (especially, single phase) designed to operate in the voltage range of 220 to 240V.

The acceptable range of voltage in some countries (also in India) is 220 ± 10V as per the electricity standards. And also, many appliances can withstand this voltage fluctuation range.

But in most places, voltage fluctuations are quite common and typically, they are in the range of 170 to 270V. These voltage fluctuations can be significant in adverse effects on appliances.

The most common reasons for voltage fluctuations are lighting, electrical faults, faulty wiring and periodic turning off the device. These fluctuations create mishap to the electrical equipments or appliances.

Long time over voltage will result  the following adverse effects, such as :

  • Permanent damage to the equipment
  • Insulation damage to the windings
  • Unwanted interruption in the load
  • Increased losses in cables and associated equipments
  • De-rating life of the appliance

Similarly Long time under voltage will result the following adverse effects :

  • Malfunctioning of the equipment ( TV, radio transmission equipment)
  • Longer working periods (as in case of resistive heaters)
  • Reduced performance of the equipment
  • Drawing large currents which further lead to overheating (Refrigirators)
  • Computational errors
  • Reduced speed of motors

So the voltage stability and accuracy decide the correct operation of the equipment. Voltage stabilizers therefore ensure that the voltage fluctuations at the incoming power supply does not affect the load or electrical appliance.

How Voltage Stabilizer Works?

Basic Principle of Operation of Voltage Stabilizer

In a voltage stabilizer, voltage correction from over and under voltage conditions is performed through two essential operations, namely boost and buck operations.

These operations can be carried manually by switches or automatically through electronic circuitry.

The process of increasing voltage from under voltage condition is called as boost operation, whereas reducing the voltage from overvoltage condition is called as buck operations.

The concept of stabilization involves in adding or subtracting the voltage to and from the mains supply.

For performing such task stabilizer uses a transformer which is connected in different configurations with switching relays.

Some stabilizers use a transformer with taps on winding to provide different voltage corrections while servo stabilizers use an auto transformer to have wide range of correction.

If the stabilizer senses the voltage drop in incoming voltage, it enables the electromagnetic relay so as to add more voltage from transformer so that the loss of voltage will be compensated.

When the incoming voltage is more than normal value, stabilizer activates another electromagnetic relay such that it deducts the voltage to maintain the normal value of voltage.

Boost Operation

The principle of boost operation of a voltage stabilizer is shown in figure.1 below.

Fig.3: Circuit Diagram of Boost Operation

Here, the supply voltage is given to a transformer, which is normally a step-down transformer.

The polarity of the secondary winding here is oriented in such a way that its voltage is directly added to the primary voltage.

Therefore, in case of under voltage condition, transformer (whether it can be tap changing or autotransformer) is switched by the relays or solid state switches such that this added supply (incoming supply + transformer secondary output) is applied to the load.

Buck Operation

The principle of buck operation of a voltage stabilizer is shown in figure.2 below.

buck operation

Fig.4: Circuit Diagram of Buck Operation

In buck operation, the polarity of secondary coil of step-down transformer is connected in such a way that secondary output voltage is deducted from incoming voltage.

Therefore, in over voltage condition, the electronic circuit switches the relay that switches deducted supply voltage (i.e., incoming voltage –transformer secondary voltage) to the load circuit.

These buck, boost and normal operations are same for all stabilizers whether they are normal type or servo mechanism type stabilizers. In addition to these two main operations, voltage stabilizer also performs lower and higher voltage cut off operations.

stabilizer operation

Fig.5: Circuit Diagram of Automatic Boost and Buck operation in Voltage Stabilizer

The figure above shows two stage voltage stabilizer which uses two relays ( Relay 1 and Relay 2) to provide constant AC supply to the load during overvoltage and under voltage conditions.

By switching the relays, buck and boost operations for two specific voltage fluctuations (one is under voltage, for instance, say 195V and another for overvoltage, say 245V) can be performed.

In case of tapping transformer type stabilizers, different taps are switched based on the required amount of boost or buck voltages. But, in case of auto transformer type stabilizers, motors (servo motor) are used along with sliding contact to obtain boost or buck voltages from the auto transformer as it contains only one winding.

Types of Voltage Stabilizers

The Voltage Stabilizers can be broadly categorized into three types. They are:

  1. Relay Type Voltage Stabilizers
  2. Servo Controlled Voltage Stabilizers
  3. Static Voltage Stabilizers

1. Relay Type Voltage Stabilizers

In Relay type Voltage Stabilizers, the voltage is regulated by switching relays. The relays are used to connect the secondary transformer(s) in different configurations to achieve Buck & Boost function.

The figure below shows the internal circuitry of relay type stabilizer.

relay type voltage stabilizer

Fig.6: Inner view of Relay Type Voltage Stabilizers

It has a transformer (which can be toroidal or iron core transformer) with tappings on its secondary, relays and an electronic circuit board.

The electronic circuit comprises rectifier circuit, operational amplifier, microcontroller unit, and other tiny components.

The purpose of the electronic circuit is to compare the output voltage with a reference value provided by built-in reference voltage source.

Whenever the voltage rises or falls beyond reference value, the control circuit switches the corresponding relay to connect a desired tapping to the output.

These stabilizers usually change the voltage for input voltage variations of ±15 percent to ±6 percent with output voltage accuracy of ±5 to ±10 percent.

This type of stabilizers is most popularly used for low rating appliances in residential, commercial and industrial applications as they are of low weight and low cost.

Advantages of Relay type Voltage Stabilizers

This stabilizer is mostly used for low power rating appliances/ equipment in Residential/Commercial/Industrial use.

  • They cost less.
  • They are compact in size.
Limitations of Relay type Voltage Stabilizers

These type of stabilizer has several limitations such as :

  • slow voltage correction speed
  • less durability
  • less reliability
  • interruption to power path during regulation
  • unable to withstand high voltage surges

2. Servo Controlled Voltage Stabilizers

As the name suggests, this type of stabilizer uses a servo motor to enable the voltage correction.

They are also known as Servo Stabilizers and they are close loop systems.

These are mainly used for high output voltage accuracy, typically ±1 percent with input voltage changes up to ± 50 percent.

The figure below shows the internal circuit of a servo stabilizer which incorporates servo motor, auto transformer, buck boost transformer, motor driver and control circuitry as essential components.

servo stabilizer

Fig.7: Inner view of Servo Based Voltage Stabilizer

In this stabilizer, one end of buck boost transformer primary is connected to the fixed tap of the auto transformer, while other end is connected to the moving arm that is controlled by the servo motor.

Secondary of the buck boost transformer is connected in series with incoming supply which is nothing but stabilizer output.

servo stabilizer working

Fig.8: Circuit Diagram of Servo Based Voltage Stabilizer

Working Principle

The electronic control circuit detects the voltage dip and voltage rise by comparing the input with built-in reference voltage source.

When the circuit finds the error, it operates the motor that in turn moves the arm on the autotransformer.

This could feed the primary of buck boost transformer such that a voltage across the secondary should be the desired voltage output.

Most servo stabilizers use embedded microcontroller or processor for the control circuitry to achieve intelligent control.

These stabilizers can again be catagorized into single-phase, three-phase balanced type or three-phase unbalanced units.

In single phase type, a servo motor coupled to the variable transformer achieves the voltage correction.

In case of a three-phase balanced type, a servo motor is coupled with three auto transformers such that stabilized output is provided during fluctuations by adjusting the output of the transformers.

In an unbalanced type of servo stabilizers, three independent servo motors coupled with three auto transformers and they have three separate control circuits.

Advantages of Servo Based Voltage Stabilizer

The advantages of servo stabilizers over relay type stabilizers are:

  • higher correction speed
  • high precision of stabilized output
  • capable to withstand inrush currents
  • high reliability
Limitations of Servo Based Voltage Stabilizer
  •  need periodic maintenance.
  • To nullify error, servo motor needs to be aligned. Alignment of Servo motor needs skilled hands.

3. Static Voltage Stabilizers

As the name suggests, static voltage stabilizer doesn’t have any moving parts as in case of servo voltage stabilizers.

It uses power electronic converter circuit to achieve voltage regulation.

It is possible to produce greater accuracy and excellent voltage regulation by these stabilizers compared with servo stabilizers, and typically regulation is of ±1 percent.

Fig.9: Static Voltage Stabilizer


It  consists of buck boost transformer, IGBT power converter (or AC to AC converter), and microcontroller, microprocessor, or DSP based controller.

Microprocessor controlled IGBT converter generates the appropriate amount of voltage by pulse width modulation technique, and this voltage is supplied to the primary of the buck boost transformer.

The IGBT converter produces the voltage in such a way that it can be in phase or 180 degrees out of phase incoming line voltage, in order to perform adding and subtracting voltages during fluctuations.

static voltage stabilizer

Fig.10: Circuit Diagram of Static Voltage Stabilizer

Working Principle

Whenever microprocessor detects the voltage dip, it sends the PWM pulses to the IGBT converter such that it generates the voltage which is equal to that of the deviated amount from nominal value.

This output is in phase with incoming supply and is supplied to the primary of buck boost transformer.

Since the secondary is connected to the incoming line, the induced voltage will be added to the incoming supply and this corrected voltage is supplied to the load.

Similarly, the voltage rise causes the microprocessor circuit to send PWM pulses in such a way that converter will output a deviated amount voltage, which is 180 degrees out of phase with incoming voltage.

This voltage at the secondary of the buck boost transformer gets subtracted from the input voltage so that buck operation is performed.

Advantages of Static Voltage Stabilizers
These stabilizers are very popular as compared to tap changing and servo controlled stabilizers because of the wide variety of advantages such as :
  • compact size
  • very fast correction speed
  • excellent voltage regulation
  • no maintenance due to the absence of moving parts
  • high efficiency
  • high reliability
Limitations of Static Voltage Stabilizer

They are costly as compared to their counterparts.

How to Choose a Suitable Voltage Stabilizer for your needs?

We need to consider several factors before buying a suitable voltage stabilizer for any appliance.

The various factors that need to be considered before selecting a Voltage Stabilizer are:

  • wattage required by the appliance
  • level of voltage fluctuations that are experienced in the installing area
  • type of appliance
  • type of stabilizer
  • working range of stabilizer (to which stabilizer going correct voltages)
  • overvoltage/under voltage cutoff, type of control circuit
  • type of mounting

and many other factors.

Here we will discuss the basic steps to consider before buying a stabilizer for our application.

  •  Check the power rating of the appliance for which you need a stabilizer.The power rating is available on the back of appliance in the form of a sticker or nameplate.It will be in Kilowatt (KW).
  •  Since the stabilizers are rated in kVA, it is also possible to calculate the wattage by simply multiplying voltage of the appliance by maximum rated current.
  • It is recommended to add a safety margin to stabilizer rating, typically a 20-25 percent. This could be useful for future plans to add more devices to the stabilizer output.
  • If the appliance is rated in watts, consider a power factor while calculating kVA rating of stabilizer. On the contrary, if stabilizers are rated in kW instead of kVA, multiply the power factor with voltage and current product.

For example :  Suppose if the appliance (air conditioner or refrigerator) is rated as 1kVA.

Therefore, the safe margin of 20 percent is 200 watts. By adding these watts to actual rating we get 1200 VA wattage.

So 1.2 kVA or 1200 VA stabilizer is preferable for the appliance.

For home needs 200 VA to 10 kVA stabilizers are preferred. And for commercial and industrial applications, single and three phase of large rating stabilizers are used.