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Introduction to Stabilizer:

The embedding of microprocessor chip technology and power electronic devices in the design of intelligent AC voltage stabilizers (or automatic voltage regulators (AVR)) led to produce high-quality, stable electric power supply in the event of significant and continuous deviation of mains voltage.

As advancement to the conventional relay type voltage stabilizers, modern innovative stabilizers use high performance digital control circuits and solid state control circuitry that eliminates potentiometer adjustments and allows the user to set voltage requirements through a keypad, with output start and stop facility.

Stabilizers are necessary because the voltage delivered to your home and mine may sometimes fluctuate which can cause serious damages to your appliance; if not completely destroying it. This AVR has a servomechanism that hunts the accurate voltage when surges happen in order to maintain the needed voltage level that ultimately protects your appliance.



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. This regulation is carried by buck and boost operations performed by internal circuitry.

There are huge varieties of automatic voltage regulators, Stabilizers, available at our office (Real Summit Network). These can be single or three-phase units as required by the type of application and capacity (KVA) needed. Three-phase stabilizers come in two versions as balanced load models and unbalanced load models.

These are available either as dedicated units for appliances 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.

Types of Voltage Stabilizers

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.

However the three main types of stabilizers as available in our office are: Servomechanism Stabilizer, Relay Type and Static Voltage Stabilizers.

Servomechanism Stabilizer

Single phase servo motor ac automatic voltage stabilizer; The product has the low energy consumption, the over-voltage protection, the low voltage protection, the over- current protection, the over-power protection, the over-temperature protection, the leakage protection and so on. It boasts for many kinds of protection functions integrated to one product.


It also has the following functions:

  • The selective time delay, the anti- impact, the LCD multi- demonstrations.
  • The Servo Motor type AVR uses a magnetic core with a motorized servo mechanism to regulate the voltage.
  • The servomechanism is the one responsible in switching connections in order to maintain the acceptable voltage.
  • Durability wise, a Servo Motor type AVR is better than a Relay type as it can withstand voltage fluctuation better than the solid state Relay type which may easily get damaged. Occasions´╝Ü Computer, Home appliances, Test equipment, Lighting equipment, Alarm and security system, X-ray equipment, Medical equipment, Duplicator, Broadcasting equipment, Numeric control machine tool, Industrial robot, Photographic processing equipment, Laboratory instrument, Hi-Fi equipment

Components of Servo stabilizer or Automatic Voltage regulator.

Servo motor controlled Automatic voltage stabilizer consists of following components (Please refer servo stabilizer circuit diagram shown below):

  1. Buck/Boost Transformer: Buck/Boost transformer connected between mains input and out of stabilizer of load terminals. One terminal of primary of buck boost transformer permanently connected to fixed tapping of autotransformer while another end connect to motor shaft.
  2. Auto transformer: It has toroidal shape and auto transformer connected between neutral point and phase of input power supply.
  3. Motor: One end of Primary of Buck Boost transformer connected to shaft of this motor with arm and brush mechanism. When motor moves then this arm shaft moves across winding of autotransformer to increase or decrease number of winding. Motor is generally AC Synchronous motor or DC servo motor which is connected and it is fitted on top of autotransformers centre point.
  4. Motor Driver: It is purely electronic circuitry which controls movement of motor. It consisting of PCBs consisting of solid state circuitry made up of capacitor, register, transistor Amplifier, microprocessor and ICs.
  5. Control Circuitry Power supply: Control Circuitry PCB boards needs constant DC power supply. Control circuitry Power supply make available power for PCB boards. It consists of Rectifier (to convert AC to DC) and step down transformer.

Working of Servo stabilizer or stablizer.

  • Voltage received from main at input of automatic voltage regulator is continuously sensed by accessories circuitry and gives feedback to Main control circuitry which consists of Microprocessor. This microprocessor continuously receives values of input voltages. Whenever there is high or low voltage at input of AVR automatic voltage regulator, the microprocessor gives trigger to motor driver.
  • Based on amount of high voltage or low voltage observed at input, “motor driver” moves servo motor across winding of autotransformer so as to increase or decrease no of winding and hence voltage across primary of buck boost transformer. It is since Servo motor shaft is connected to primary of buck boost transfer. And hence when there is change in voltage across primary of buck boost transfer then induce voltage across its secondary also changes. The motor moves in such a way that proper voltage should be observe across primary of Buck Boost transformer so that voltage across secondary should be equal to set or desired output voltage of servo stabilizer. And voltage across secondary of Buck Boost transformer is output voltage of stabilizer.
  • This process takes place continuously so as to correct input voltages.
  • In case of 3 Phase automatic Voltage Regulator is independent phase controlled then it will have above arrangement for all three phases separately. It means it is as good as 3 number of Single phase servo stabilizer in one unit.


Static Voltage Stabilizers

As the name suggests, static voltage stabilizer doesn’t have any moving parts as a servo motor mechanism in case of servo stabilizers. It uses power electronic converter circuit to achieve voltage regulation rather than a variac in case of conventional stabilizers. 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.

It essentially 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.

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.

These stabilizers are very popular compared with tap changing and servo controlled stabilizers because of the wide variety of advantages such as compact size, very fast correction speed, and excellent voltage regulation, no maintenance due to the absence of moving parts, high efficiency and high reliability.



Relay Type Voltage Stabilizers

In this type of voltage stabilizers, voltage regulation is accomplished by switching the relays so as to connect one of a number of tappings of the transformer to the load (as in the manner discussed above) whether it is for boosting or bucking operation. The figure below illustrates the internal circuitry of relay type stabilizer.

It has electronic circuit and set of relays besides the transformer (which can be toroidal or iron core transformer with tappings provided on its secondary). The electronic circuit comprises rectifier circuit, operational amplifier, microcontroller unit, and other tiny components.

The electronic circuit compares 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. However, these are suffering with several limitations such as slow voltage correction speed, less durability, less reliability, interruption to power path during regulation, and unable to withstand high voltage surges.



Need of Voltage Stabilizers

Voltage fluctuations are nothing but the change in magnitude of voltage, of which normally exceeding or below the steady state voltage range prescribed by some standards.

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.

  • In case of lighting equipment, low voltage drop reduces the lumen output (illumination) that will further reduce the life of the lamp.
  • AC motor produces less torque and hence the speed under low voltage, and they produce more speed than desired during overvoltage. This degrades motor life and also causes insulation damage under high voltages.
  • In case of induction heating, low voltage reduces heat output which causes the load to operate at inappropriate temperatures than desired.
  • In TV and radio transmission, voltage drop will reduce the quality of transmission and also cause the malfunction of other electronic components.
  • Refrigerators are AC motor driven appliances that draw large currents during voltage drop conditions which may lead to overheating of windings.

To overcome above mentioned effects of voltage variations, voltage stabilizers are needed.

Basic Principle of Operation of Voltage Stabilizer

The voltage regulation is required for two distinct purposes; over voltage and under voltage conditions. 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.

These two main operations are essential in each and every voltage stabilizer.

As discussed above, the components of voltage stabilizer include a transformer, relays, and electronic circuitry. 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.


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