Servo voltage stabilizer is a closed loop control mechanism which serves to maintain balanced 3 or single phase voltage output in spite of fluctuations at the input owing to unbalanced conditions. Most of the industrial loads are 3 phase induction motor loads and in real factory environment, voltage in 3 phases is rarely balanced. Say for example if the measured voltages are 420, 430 and 440V, the average is 430V and the deviation is 10V.

The Percentage of unbalance is given by (10V X 100)/ 430V = 2.3% It is seen that 1% voltage unbalance will increase the motor losses by 5%.

Thus voltage unbalance can increase the motor losses from 2% to 90% and hence temperature also raises by an excessive amount which results in further increased losses and reduced efficiency. Hence it is proposed to take up a project to maintain balanced output voltage in all 3 phases.

Single Phase:
It is based on the principle of vector addition of A.C voltage to the Input to get desired output using a transformer called Buck-Boost transformer(T), the secondary of which is connected in series with the input voltage. The primary of the same is fed from a motor mounted variable transformer(R). Depending upon the ratio of primary to secondary voltage, the induced voltage of the secondary comes either in-phase or out of phase based on the voltage fluctuation. The variable transformer is usually fed from the input supply at both the ends while tapping at around 20% of the winding is taken as fixed point for the primary of the Buck-Boost transformer. The variable point of the auto-transformer, therefore, is capable of delivering 20% out of phase voltage which is used for bucking operation while 80% which is in-phase with the input voltage and is used for boosting operation. The wiper movement of the variable transformer is controlled by sensing the output voltage to a control circuit that decides the direction of rotation of synchronous motor fed through a pair of triacs to its split phase winding.

3 Phase Balanced Input Correction:
For low capacity operation say about 10KVA, it is presently seen that a double wound variac is used eliminating the Buck-Boost transformer on variable transformer itself. This restricts wiper movement of variac to 250 degree as the balance is used for secondary winding. Though this makes system economical, it has serious draw backs in terms of its reliability. The industry standard never accepts such combination. In areas of reasonably balanced input voltage, three phase servo controlled correctors are also used for the purpose of stabilized output where as single three phase variac is used mounted by one synchronous motor and single control card sensing the two phase voltage out of three. This is much more economical and useful if input phases are reasonably balanced. It has the draw back that while severe unbalancing takes place the output is proportionally unbalanced.

3 Phase Unbalanced Input Correction:
Three series transformers (T1,T2,T3),the each secondary of which is used ,one in each phase that either adds or subtracts the voltage from the input supply voltage to deliver constant voltage in each phase there by making the balanced output from unbalanced input. The input to the primary of the series transformer is fed from for each phase from one each variable auto transformer (Variac) (R1,R2,R3) each of whose wiper is coupled to an ac split phase (2 Coils) synchronous motor(M1 ,M2 M3). The motor receives ac supply for each of its coils through thyristor switching for either clock wise or anti-clock wise rotation to enable desired output voltage from the variac to the primary of the series transformer, either in phase or out of phase, to perform addition or subtractions as required at the secondary of the series transformer to maintain a constant and balanced voltage at the out put. Feed back from the out put to the control circuit (C1,C2,C3) is compared with a fixed reference voltage by level comparators formed out of op-amps to ultimately trigger the TRIAC as per the need for actuating the motor.

This scheme mainly consists of a control circuit, 1single phase servo induction motor coupled to a variac feeding primary of a series transformer for each phase.

  • Control circuit comprising of a window comparator wired around transistors and RMS error signal voltage amplification by IC 741 is rigged up in Multisim and is simulated for various input operating conditions ensuring firing of the TRIACs that would operate the capacitor phase shifted induction motor in required direction that controls the rotation the variac wiper.
  • Based on the maximum and minimum values of voltage fluctuations, series transformer and the control transformers are designed using standard formula matching to commercially available iron core and super enameled copper wire size before winding the same for use in the project.


In a balanced 3 phase power system, all the voltages and the currents have the same amplitude and are phase shifted by 120 degrees from each other. However it is not possible practically as unbalanced voltages can result in adverse effects on equipment and the electric distribution system.

Under unbalanced conditions the distribution system will incur more losses and heating effects, and be less stable. The effect of voltage unbalance can also be detrimental to equipment such as induction motors, power electronic converters, and adjustable speed drives (ASDs). A relatively small percentage of voltage unbalance with a three phase motor results in a significant increase in motor losses, which entails a decrease in efficiency as well. Energy costs can be minimized in many applications by reducing the motor wattage lost because of voltage unbalance.

Percentage Voltage Unbalance is defined by NEMA as 100 times the deviation of the line voltage from the average voltage divided by the average voltage. If the measured voltages are 420, 430 and 440V, the average is 430V and the deviation is 10V.

The Percentage Unbalance is given by (10V * 100 / 430V) = 2.3%
Thus 1% voltage unbalance will increase the motor losses by 5%.
Hence Unbalance is a serious power quality problem, mainly affecting low-voltage distribution systems and it is therefore proposed in the project to maintain balanced voltage with respect to magnitude in every phase, thus maintaining balanced line voltage.

Static Voltage Stabilizer Vs Servo type stabilizer

1) Voltage Correction speed:
It does not contains any moving part. Static voltage stabilizer has pure electronic circuit to achieve correction in voltages. Hence static stabilizer has exceptionally high voltage correction speed than servo voltage stabilizer. Voltage correction speed to SVS can be in range of 360 to 500 V/sec. On other hand Servo stabilizer has moving servo motor with help of which it achieves correction in voltage. Servo stabilizer is electromechanical device hence its voltage correction speed is slower than static voltage stabilizer.

2) Correction Time:
Due to high voltage correction speed static voltage stabilizer has low correction time of 20 to 30 millisecond as compared to servo stabilizer correction time of 50 milliseconds to 5 Seconds.

3) Maintenance:
Since Servo stabilizer has moving servo motor hence it has regular wear and tear which needs maintenance. Due to static nature of Static voltage regulator it don’t need maintenance.

4) Reliability:
In servo voltage stabilizer correction in voltage achieve by increasing or decreasing no of winding in autotransformer with help of shaft of servo motor. This increases or decreases voltage across primary of buck boost transformer in turn secondary of buck boost transformer and hence correcting output voltage. Hence reliability of servo voltage regulator is mainly depend on reliability of servo motor. Similarly reliability of static voltage stabilizer depends on reliability of IGBT power stage. In General static IGBT power stage is more reliable than electromechanical servo motor hence SVS are more reliable than servo regulator.

5) Auto Bypass facility:
In static voltage stabilizer providing auto bypass is very easy. And due to fast electronic nature, SVS can transfer to bypass automatically and without break in output voltage (zero transition time) in case of even of fault. In servo voltage stabilizer it is complicated to provide auto bypass mechanism and even it provided it becomes very costly proposition and transition to bypass ill be with break in output voltage (needs transition time).

6) Protection from over current fault due to short circuit:
In static voltage stabilizer, DSP control board continuously senses input voltage, output voltage, IGBT current and load current as part of working principle. In event of short circuit at output of static voltage stabilizer, load current increases exponentially high which automatically get sense by DSP controller and it cuts output & switches of IGBT power stage instantaneously to clear over current fault. Hence over current fault get clear in SVS very quickly and without adding any extra hardware. In case of servo voltage stabilizer, over current protection can be achieve with help of extra hardware (MCCB, CB etc.) and that of clearing of fault is not instantaneous.

Other Differences between Static type Voltage stabilizer and Servo voltage Stabilizer:

SpecificationsStatic Voltage StabilizerServo Voltage Stabilizer
EMI/RMI FilterAs standard with no extra costOptional at extra cost
SizeVery CompactCompact
WeightVery low weightHigh weight
Input Voltage WindowWider as 170-290VACless wider 185-260VAC
Voltage Stability1%2%
Working PrinciplePWM method, IGBT basedServo motor controlled
SoundSilent operationHigh sound over age
Output waveform DistortionNot distortionDistortion
Overload cutoffstandardoptional