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What is the difference between Relay Type Voltage Stabilizer & Servo Voltage Stabilizer?

Today, Voltage stabilizers have become mandatory for electrical equipment of home, offices & industries. It helps to protect electrical equipment and machine from over & under voltage and other voltage surges. Voltage stabilizer delivers a constant voltage at its output terminals regardless of changes in the input or incoming voltage. In the starting, manually operated voltage stabilizers used to boost or buck the voltage. These manually stabilizers ware built with electromechanical relays to give an output voltage within a desired range.

Later on, additional electronic circuitry came in to the force and became tap changer automatic voltage regulators. At present the latest technology of voltage stabilizer is servo stabilizer in which voltage correction is carried continuously. That can protect your appliances more effectively. Before we point out the differences between relay type and servo voltage stabilizer, let’s have a look on type of technology used in voltage stabilizers. Through this we can clearly notify you about features & components difference between both types of voltage stabilizer.


1. Relay Type of Voltage Stabilizers:

Voltage Stabilizer

In this type of stabilizer, electronic circuit and set of relays besides the transformer, comprises rectifier circuit, controller 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, the control circuit switches the corresponding relay to connect a desired tapping to the output voltage. Relay types of voltage stabilizers usually change the output voltage accuracy of ±10 %. It is used for low rating appliances in homes, offices and industries. Many electrical equipment & machines are suffering with several limitations such as unaspected performance, less durability, interruption to power path etc. Therefore it helps to provide reliable voltage for protecting much equipment. At the same time researcher has developed a latest technology called servo controlled system to stabilize the voltage fluctuation rate very quickly. Let’s know the second and most popular technology used for stabilizing the voltage.


2. Servo Voltage Stabilizers:

Servo StabilizerIn this stabilizer, a phase of buck boost transformer primary is connected to the fixed tap of the auto transformer and other one connects with moving arm which is controlled by the servo motor. Apart from it the buck boost transformer primary of servo stabilizer gets connected in the series of incoming voltage. These stabilizers called servo stabilizer because it uses a servo motor to enable the voltage correction. It is mainly used for high output voltage accuracy. Usually, ±1 % with input voltage changes up to ± 50 %. It is most popular for low high rating appliances used in homes, offices and industries to provide correct voltage rate for protecting equipment and their desired performance. Demand

These stabilizers can be single-phase, three-phase Oil cooled and Air cooled balanced unit type or three-phase unbalanced. In a single phase servo stabilizer, motor coupled to the variable transformer to achieve voltage correction. Meanwhile, in 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. Whereas in unbalanced servo stabilizers, there are three independent servo motors coupled with three auto transformers.

If we compare servo stabilizers with relay type stabilizers we can find various advantages of servo voltage stabilizer such as higher correction speed, high precision of stabilized output, capable to withstand inrush currents and most importantly the high reliability. We can highlight the different between both types of voltage stabilizer are as follows:


Differences between a relay type of voltage stabilizer and servo voltage stabilizer can easily find under given table:


Features Relay type of voltage stabilizer Servo voltage stabilizer
Voltage Accuracy output of ±10 % Output Voltage ±1 % input voltage change up to ± 50 %.
Technology Relay Type Controlled by Servo Motor
Voltage correction Correcton in steps of of 20Volt Stepless correction. 25Volt/second
Models Only Air Cool Model Available Available in both, Air & oil cool models, depend on capacity also
Load Capacity Up to 10 KVA only Up to 5000KVA and above as per the demand
Functioning criteria Set of relays connected besides the transformer Buck-Boost transformer connected with a fixed tap


Difference Between Aircooled & Oilcooled Servo Stabilizer

Servo transformers provide better utility of power and handle current fluctuation better than other type of equipments.  A company’s need in controlling power can vary according to its choice and feasibility of the unit.  Basically, there are two types of Servo models that prevail in the market today.  The one is a Servo oil cooled stabilizer and the other is Servo air cooled stabilizer.  Both these types of apparatus work on the same principle, i.e. stabilization of voltage and regulating the output.  When the voltage comes low, the unit will bring it up to the required level and in case there is a high voltage, obviously the output will be cut off.  In both scenarios, the working induces a comprehensive study on the architecture of these two models so that we can compare which one to prefer on our buyer’s list.

Servo Stabilizer Oil cooled Servo Stabilizer Air cooled
Cooling oil of coiling Cooling by air of coiling
Capillary for oil rotation Louvers in air cooled
Heavy load in summer also, Low load, heats up in summer
30 kVA oil cooled, 75 kVA can bear
Weight 1.5% Weight 1%
  Expensive due to design
0.8 power factor 0.9 power factor-for 30 kVA 40 kVA is designed
Life oil cooled Less air cooled life
  Fans for cooling above 50 kVA
Transformer motor vibrates oil in Fins  


We have a Servo air cooled stabilizer that is adorably reminiscent of the metallic almirah we fathomed in the corner of the room.  This stabilizer has quite an appeasing box like structure neatly packed like a cabin and the front panel shows the meter reading with a few switches that incorporate its operational values.  There are louvers in the side walls that transmit heat out of the chamber through a motorized fan when long hours of working generate heat in the coils.  However, still this type of equipment is necessitated where less load is borne, hitherto, although people prefer using it beyond 30 KVA.  Having oil-free operation definitely makes it lighter in weight and the cooling mechanism is done by the fan inside the unit.  But an oil cooled Servo stabilizer is filled with an oil coolant that runs around the fins which is vibrated by a motor.  So this makes it weigh 1.5% more than an air cooled one. But it can bear heavy loads in summer also as its heat is exhausted in the oil. However, a Servo Stabilizer air cooled is a bit expensive than the oil transformer because of its design and the power factor that necessitates to build a 40 KVA unit to give an output of 30 KVA.

Ergo, both stabilizers are important in their sense. Still for heavy power loads, we can go in for an oil cooled Servo transformer and make life a little extra smoother.

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