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Servo (from Latin: servus “slave”) Voltage Stabilizer is a servo motor controlled voltage stabilizer that provide almost stable voltage with output voltage variation of around ±1%. Servo Motor is controlled and operated by Electronic Circuit, which in turn drives Auto-transformer, followed by controlling the output of Buck Boost Transformer.

Servo Stabilizer’s capacity varies from 1 kVA to 3500 kVA or higher and with efficiency of 98 – 99%, it stabilize the fluctuating voltage thus can be used anywhere whether household, commercial, public or private institutions and industrial.

History
In the Year 1962, during Indo-China War, there was a massive failure in defense equipments due to voltage problem. As a solution in 1963, Automatic Electric Ltd. (AE) under the guidance of its Founder Chairman cum Managing Director, Late Mr. M.G. Bhatt designed and developed first Servo Voltage Stabilizer in India using Mercury Sensing Relays.

As voltage fluctuation was common phenomena all over India, this product was introduced further in 1965 to open market for industrial application.



Basic Design and Operation
The major components involved in manufacturing of servo voltage stabilizer are Autotransformer, Buck-Boost Transformer, AC Synchronous Motor and Controller Circuit, which can further be explained as: Autotransformer– Also called Dimmer Transformer / Continuous Variable Transformer/ Variac / and has a toroidal shape and is connected between input phase and neutral and control the output of the buck boost transformer. Only the Correction Load is on this transformer.

Buck Boost Transformer– The primary winding of this transformer is connected to input phase, while the secondary winding is attached to the tapping and carbon. Total load is on this transformer.

AC Synchronous Motor– Also called Servo Motor which drives or rotate the arm of Auto-Transformer.

Electronic Circuit– Also called Servo Motor Circuit which operates the Servo Motor to move clockwise or anticlockwise direction depends upon correction we require when compared to input voltage.

Carbon Brush– Carbon Brush is the only wear and tear part involved in servo stabilizer which is fitted in the fingers of arm of Auto Transformer.

The basic design is same for single as well as three phase connections.

Types
In case of single phase servo stabilizer, depending upon cooling method, manufacturers distribute it into air cooled and oil cooled.

In case of three phase servo stabilizer, it is primarily diversified as Balanced type and Unbalanced Type, this is on the basis of load. Further in both of them Linear type Servo Stabilizer and Dimmer type Servo Stabilizer is well renowned. Additionally these two further distributed on the basis of cooling method i.e. air cooled and oil cooled. It can be well understand by showing the diagrammatic representation.

Diversification on the basis of Load Application
Loads can be single phase operated i.e. with line and neutral, and can be three phase operated i.e. with three line and one neutral. When the maximum load is three phase operated then the preference can be for Balanced Type Servo Stabilizer. If the load is variable from single to three phase then preference is for Unbalanced Type Servo Stabilizer.

Balanced Type Servo Stabilizer
The life of a machine can be long lasting if it is getting a stable operating voltage, which may be 380V / 400V / 415V. Industries own their 33kV or 11kV distribution transformer dedicatedly with proper distribution further, but then also the requirement of stable voltage is not fulfilled. To operate such industrial loads where each phase has almost similar voltage and require almost similar level of correction then the preference shall go for Balanced Type Servo Stabilizer. Here controlling is single so cost effective and very popular among industrial belt.

Unbalanced Type Servo Stabilizer
Due to its compatibility with almost all type of conditions it is very popular among Household, Commercial, Public and Private Institutions. This type controls all three phases separately with respect to neutral. For common understanding we can say, three Single Phase Servo Stabilizer kept in a single cabinet. This type of Stabilizer is suitable where both single and three phase loads are available in equal proportion. Here controlling is separate so cost is higher when compared to balanced type.

Diversification on the basis of carbon movement over coil track
Carbon (Graphite), either in the form of brush (having flat surface) or in the form of roller (roll) move over the coil track of Auto Transformer to control the output voltage and this movement over the coil track decides the type of Servo Stabilizer.

Dimmer Type Servo Stabilizer
When carbon in the form of brush rotates around a single axis of Auto Transformer then it is known as Dimmer Type Servo Stabilizer.

Linear Type Servo Stabilizer
If carbon in the form of roller rolls and covers the linear track of Transformer then it is known as Linear Type Servo Stabilizer.

Cooling Method
Servo Stabilizer comprises of transformers which generate heat during its operation, and require cooling, which can be naturally through air or with force through cooling fans or through transformer oil, on the basis of which it is divide into Air Cooled and Oil Cooled.

Air Cooled Servo Stabilizer
Servo Stabilizer can be cooled naturally or can be provided with forced cooling. These types are light weight and are generally preferred in case of lower voltage range and lower capacity. Now a day’s some big giants have started manufacturing air cooled Servo with higher capacity too.

Oil Cooled Servo Stabilizer
Transformer Oil is filled inside the container where transformers are kept, and transformers are in direct contact with oil which acts as a cooling agent by taking away the heat from the heated transformer windings. Oil flows through cooling fins or radiators thus normalizing the temperature. The body weight and transformer oil together contribute to the total weight of the Stabilizer. This is preferred in case of voltage range with higher variations and higher capacity. This type is also a choice where air ventilation is not proper and or less space. In special cases it can be kept similar to transformers in open air without any shade.

Applications
From defense it has now been used in Industries, Commercials, Public and Private Institutions. Its popularity among household is yet in the nascent stage. Its general application can be summarized as, with CNC Machines, Air Compressors, Textiles Machines, Lightings, X-Ray and Medical Equipments, Engineering Units, Computers, Pump Sets, Transmitters and many other similar applications [2]. The Add-on feature provision of Low High Voltage Cut Off, Over-Load & Short-Circuit Protection, Single Phase Prevention / Protection, Single Phase Correction has increased its utility from stable voltage to safe voltage [4].

Contribution in Social Cause
It is described that the usage of Servo Stabilizers are not only involved in bringing the electricity bill down but also supporting people who have still not access to the continuous or no supply of electricity. Additionally it has also been seen to support in lowering the emission of CO2, by around 0.9 kg per unit electricity saved.