Servo Controlled Voltage Stabiliser

The IE Rules 1956 stipulated that the supplier shall supply voltage at the commencement of supply limiting the variation to 6 % of the declared voltage in case of LV/MV distribution system and +6 % / -9 % in case of HV distribution system. But generally it is observed that very low voltage during the day & normal voltage during night in residential area and very low voltage during day and very high voltage during night in industrial areas. As most of the equipment are designed to operate for a voltage variation of ±10 %, automatic voltage regulator is required.

Generally two types of regulators are available:

  1. STEP RELAY CONTROL: This type of AVR can generally maintain output voltage within ±10 % of the standard output voltage. For wide variation in the voltage, many steps are involved and it may complicate and may also result in voltage surges to the connected equipment.

    The losses in an electronic stabilizer can be reduced by use of toroidal core transformer.

  2. STEP-LESS SERVO CONTROLLED STABILISER: This type of stabilizer can regulate the output voltage within ±1 % of the standard voltage and here the variation in the voltage is smooth without steps / interruptions. This type of stabiliser can be designed for wide variation of the voltage with ease. It also have reasonably quick correction time.

    The servo stabilizer find use in telecom equipment, photocopier machines, offset printing presses, AC plants, moulding and machine tools machine, Scientific equipment etc.

The voltage regulators are selected based on the following requirements:

  1. Input Voltage Variation: Standard ranges are 300 - 460 V, 290 - 470 V, 245 - 470 V for three phase and 160 - 260 V, 170 - 270 V and 140 - 270 V for single phase systems.
  2. Output Voltage Variation: Generally taken as 240 ± 1 %.
  3. Capacity: Stabilisers are normally designed to take 20 % overload for 60 minute, 50 % overload for 30 minutes and 100 % overload for few minutes. This overloading can take care of short time overloads, transformer inrush currents, motor starting currents and handling of thyristor loads.
  4. Type of load: It may be balanced three phase or unbalanced three phase. In the Telecom Department generally unbalanced load condition and unequal input voltage is considered.


OPERATING PRINCIPLE: The schematic diagram of the servo is given in figure below. It consists of

  1. Step-less variable toroidal wound a motor driven auto-transformer also known as dimmer-stat or variac.
  2. Reversible, instantaneous start synchronous motor (Servo motor)
  3. Buck-boost transformer (Series transformer)
  4. Power contactor
  5. Electronic control system
  6. Metering and indication panel
  7. Protection and cutoff system ( if any)
  8. Housing suitable for air/oil cooling.

The buck-boost transformer is a series control transformer interposed between input and output. The primary is fed through the Auto transformer output. Depending upon the variation in the input voltage variation, a voltage is induced in this transformer in-phase or out of phase with reference to the input voltage so as to regulate the voltage at a steady value. Thus both step up and step down voltages are obtained through the same transformer tapping. The electronic control system monitors the output voltage continuously and based on the variation of output voltage the servo motor is driven in the forward and reverse direction to vary the output of the Auto transformer which in turn changes the voltage compensation at the buck/boost transformers. The control system monitors the voltage between the phase and neutral for each phase and regulates the voltage on each phase. However the regulation of voltages between the phases may be slightly different based on the actual loading of the individual phases.

The single phase stabilizer employs one variable Auto transformer driven by a servo motor and one sensing circuit.

The balanced type three phase stabilizer also employs single control circuit. Here the three variable auto transformers are mechanically coupled and are driven by a single motor. The sensing is done in one of the three phases and the other two are corrected automatically. These stabilizers can be used with loads where unbalance is not much and lesser accuracy in output is acceptable.

For the unbalanced load and unequal input voltages, the three phase servo voltage stabilizer consists of three sets of identical components each set corresponding to a single phase system with the corresponding control system and with a common output contactor housed in the same enclosure for smaller capacities and in separate enclosure for very high ratings. The main electrical systems are connected in Star to form a three phase system with a common neutral between input and output. In case of oil cooled system, the Auto transformer and buck-boost transformers are immersed in oil.


  1. Output voltage regulation within ±1 % of the load voltage. A provision is also available to vary the settings to adjust the output voltage at the desired value within ±5 V of the rated voltage.
  2. To cutoff the output at high and low input voltages beyond the designated voltages with audio and visual alarm.
  3. Overload protection by isolating the load in the stabilizer with visual and audio alarm.
  4. Tripping arrangement in case of any phase failure.
  5. Auto/Manual provision for restarting the stabilizer after tripping due to any reason.
  6. Micro limit switches to cutoff the power supply to the servo motor while it reaches the extreme low voltage and extreme high voltage points.


  1. Reverse phase protection.
  2. By pass arrangement to connect the output directly to the input in case of failure of the stabilizer.
  3. Line filter for suppressing the radio frequency components.
  4. Surge suppressor to suppress the voltage spikes.

Installation of Servo Stabiliser

For installation of AVR, provisions of IE rules must be kept in mind. In general, a minimum space of 600 mm should be available all around the AVR and room should be of sufficient height so that variacs etc can be taken out with the help of chain-pulley.

The physical data of the stabilizers are tabulated below (Source: M/s Vinitec):

5750350780 10 mm CTS connectors
8900400850 - do-
10900450850 - do -
161050450900 M10 × 90 Brass Bolts
201050450900 - do -
251260480950 - do -
3013005201000 - do -
4011507301100200M12 × 115 Brass Bolts
5011507301100200- do -
6011807301110200- do -
7512007501150300- do -
100166010301330450- do -
125196012001430600- do -
150196012001430700- do -
20019601300166085040 × 6.3 mm Cu strip
25019601300166090050 × 10 mm Cu strip
3002030135017801000- do -
4002160144019801350- do -
500222027501750180075 × 10 mm Cu strip
60022200275017502000- do -

Testing of Servo Stabilizers

FACTORY TEST: IS 9815 : 1994, Part I indicates the various routine tests to be conducted on the stabilizer before dispatch from the factory. The brief description of these tests are as below:

  1. INSULATION TEST: The insulation between the terminals and body of the stabilizer shall not be less than 5 MΩ when measured with a 500 V megger at the ambient not exceeding 45 °C.

  2. HIGH VOLTAGE TEST: 1.5 kV (RMS) test voltage shall be applied at the rated frequency for one minute between the windings and the body of the stabilizer after connecting the body of the stabilizer to the earth. There shall not be any disruptive discharge or collapse of test voltage during this test.

  3. OUTPUT VOLTAGE TEST: The output voltage shall be for different input voltages from no load to the full load. For ratings above 5 kVA, the test can be limited to no load if it is not feasible to load actually. The output voltage by this test shall not differ by more than 1 % of the designated rated voltage.

  4. NO LOAD CURRENT TEST: The no load current shall not exceed 5 % of the rated full load output current.

  5. NO LOAD LOSS TEST: No load loss shall be measured similar to the OC test on the transformers.

  6. LOAD TEST: This test shall be conducted similar to the SC test of the transformer. The total losses shall not be more than 10 % for capacities up to 30 kVA and 5 % for higher capacities.

  7. INDUCED VOLTAGE TEST: This test is conducted to test the inter turn insulation of winding by applying AC voltage equal to output voltage of frequency not less than twice the rated frequency at the output terminal of the windings for periods not less than 15 seconds. After this test there shall not be any appreciable differences in the no load current with reference to the no load current at normal frequency.

  8. TEST FOR CONTINUOUS OPERATION (JIG TEST): By this test the stabilizer is subjected to the continuous run for 24 hours at no load while the input voltage is automatically varied from minimum to the maximum of the designated input voltage and back to the minimum in not more than one minute. After this test the stabilizer shall pass the output voltage test.

  9. TEMPERATURE RISE TEST: This test is similar to the temperature rise test of transformer and is done under actual plant condition.

  10. LOCKED ROTOR TEST: This is to check the withstanding capacity of the servo motor at the time of mechanical defect of the rotating system. The current drawn by the motor during the locked condition shall not be appreciably high taking into account the average normal current in the forward and the reverse direction.

  11. RATE OF CORRECTION TEST: The input voltage is varied from minimum to maximum suddenly by a variac. The time taken for stabilizing the output at rated value is measured. The desirable rate of correction is around 20 V/s, while the IS stipulates a value of 8 V/s.


  1. PHYSICAL INSPECTION: Inspect all the components and see that they are of approved make and proper rating.

  2. INSULATION TEST: The insulation between the terminals and body of the stabilizer shall not be less than 5 MΩ when measured with a 500 V megger at the ambient not exceeding 45 °C.

  3. LOAD TEST: The stabilizer may be provided with actual equipment load in addition to the artificial load so that the full load condition is available for a minimum of six hours with the available input voltage. During this test, the output voltage and temperature rise shall be recorded to see that they are within limits.

FAILURE OF THE STABILISER: The main failure of the servo stabilizer occurs on account of Auto transformer and the servo motor. So they should be of the reputed makes. The selection of variac with the proper rating plays an important role in the satisfactory performance of the stabilizer. It is advisable to derate the name plate rating by 10 to 15 % for air cooled systems, as many stabilizers fail on account of the failure of the variac operating under the adverse condition of very low input voltage and high load current. The rating of variacs for various capacities are tabulated below:

245 – 470 V295 – 470 V

Note: Above 28 A, we should go for oil cooled variac.

The other important reason for the failure of the voltage stabilizer is due to the wear and tear of the carbon brushes in the variac or the reduced spring tension of the carbon brushes which requires periodic inspection for remedial action.


The IS 9815 : 1994, Part I gives specification requirements and other relevant information for servo stabilizer suitable for single phase.

A typical specification of a stabiliser is given below:

1. Rating:22.5 kVA 3 phase unbalanced.
2. Input Voltage Range:245 to 470 V.
3. Output Voltage Range:415V ± 1%.
4. Power Factor:0.8
5. Voltage Correction:Step less with no overshooting or hunting during automatic operation.
6. Speed of Correction:15V / second. (Between phase and neutral)
7. Wave Form Distortion:Nil.
8. Ambient temperature:Up to 50 °C.
9. Cooling:Air (Up to 40 kVA) / Oil cooled. (whichever applicable).
10. Winding:Copper wound transformer duly vacuum impregnated.
11. Operation:Automatic with provision for manual operation.
12. Housing:The frame made of MS angle iron of 35mm x 35mm x 5mm. Enclosure shall be 16 SWG MS sheet duly painted in a single container with sufficient ventilation arrangement. (For Air cooled).
13. Mounting:Indoor floor mounting.
14. Other Features:a). Output voltage of AVR can be increased or decreased by an increase/decrease switch in manual position.
b). Output Voltage adjustment at the desired value at the desired value within ± 5 % of the rated output voltage.
c). Trip bypass arrangement with auto-manual switch.
d). Provision of double earthing and lifting hooks.
15. Protection:a). 100 A MCCB with thermal release range of 48-63 A for O/L and S/C protection.
b). Overload/High/Low input voltage cutoff by means of contactor not less than 60 A (ML4).
c). Auto switch off against phase failure/phase reversal.
d). Manual reset push button to switch on the AVR after cutoff. (if Auto restart is not provided).
16. Metering and Indication:a). Indicating lamp for each incoming phase with fuse protection in respective colours.
b). Separate high/low voltage indication for each phase.
c). 0-500 V voltmeter with selector switch to monitor input/output voltage of phase to phase, phase to neutral and OFF position.
d). 0-60 A ammeter with CT in each phase.
17. Components:a). Power Transformer: 3500 VA capacity conforming to IS 2026.
b). Auto transformer of 28 A capacity.
c). Servomotor of 10 kg⋅cm torque (min).
d) Control relay 16 a double pole.
18. Testing:Voltage stabilizer shall be tested at site for 6 hours with available voltage and the load will be arranged by the department.
19. Factory Test Certificate:As per IS 9815 : 1994, Part I .
20. By Pass Arrangement:By means of On load changeover switch. (Optional).
21. Guarantee:Stabilisers are guaranteed for manufacturing defects for a period of 12 months from the date of installation or 18 months from the date of supply.
1. MCCB:L&T, EE, Siemens, Crompton.
2. Power Transformer:Own make or procured from approved source with certificate to IS 2026.
3. Auto Transformer:Automatic Electric, Bombay Dimmer Stat, Green Dot.
4. Servo Motor:Step Syn (Lonawala, Pune), All Steps (Bangalore).
5. Meters:AE, Bombay LMP.
6. Contactors:L&T, EE, Siemens.
7. Selector Switches:Kaycee, Salzer.
8. PCB:Plug in type with reputed components.
9. Relays:PLA.
10. HRC Fuses:EE, L&T, Standard.
11. Indicating Lamps:EE, Easun, Salzer.


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