Servo Dot Technical Note

GREEN DOT's State - of - the - art in SERVO DOT - High Performance Voltage Stabilizers, Enables to Provide Highly Reliable & Efficient A.C. Power Suppliers, Meeting Majority of Voltage Control application for Electrical & Electronic Equipments.

The widest choice available from any Indian and the most advanced source. With their integrated design & superb solid state control system they set enviable standards in performance and reliability. Many years of manufacturing experience have lead to requirement of production techniques that keeps this renowned range ahead of others in competitive pricing and country wide availability. In a continuing program of development "Green Dot" has introduced one innovation after another, bringing most significant advancement in stabilizer performance. Design-value engineering is constantly injected into production and is a promise to keep this product operational for a longer period with far lesser maintenance. All major & specialized constituents of Voltage Stabilizers are designed, engineered & manufactured with In-house facilities, under closely monitored quality control measures. Providing one source reliability & convenience for service.


SERVO DOTS would control the supply voltage to virtually any equipment which is operated from the mains. Applications include Computers, Calculating Machines, Teleprinters, Photographic Processing Equipments, Printing Presses Radio Transmitters, Television Cameras, Video recorders, Radar & Navigation devices, Communication Equipment of all types, Medical Electronic Equipment such as X-Rays, Intensive Care Units, Electrocardioscopes, Sterilizers, General Laboratory Equipments as Testing Equipment, Heating Lighting, Production & Process control Equipments, etc. Defence Railways and P & T installations use such stabilizers quite extensively.

Other applications include use with electric lifts, Elevators, Plastic extruders, Induction welding Plants, Numerically controlled Machines, Control Panels of sophisticated machines as Pressure Die Casting, Jig Boring & Jig Grinding machines Spectrometers, Electron Microscopes, Spectrophotometers, Chromatographs etc.

They also provide optimum & pleasing voltage environments in commercial flats & establishments & luxurious residential buildings where large number of Air Conditioners Heating Refrigerators, Geysers, Lighting & loads are centrally regulated without piecemeal type of step-regulators. The large stabilizers may be used to control the entire supply to a laboratory, communications installations, or a medium size industrial plant.

Operating Principle 

  • A typical "Servo Dot" would include main elements as
  • A Variable Transformer occupied to a Servo Motor
  • A solid state control unit converting each fluctuation of output voltage into an error signal to actuate the servo motors.




  • One toroidally wound double layer or extended winding construction, differential copper gauged,
  • Variable Auto Transformer with limted sectorial movement.
  • One Servo Motor
  • One Control Unit.


In smaller units covered under type ISD-10 a special type of Variable Transformer is used wherein the input voltage is applied to the Carbon Brush Arm and the fixed winding end of the transformer. The servo motor, coupled to the brush arm gets actuated from the



  • Toroidally wound Variable Auto Transformer.
  • Double wounded Buck/Boost Transformer.
  • One Servo Motor say A.C. Step synchronous reversible type.
  • One Electronic Control Unit.


fluctuations of the output voltage and the arm adjusts itself, so that rated & set value of Output Voltage is restored, by adjustment of the brush arm matching with the input voltage at that time. The Diagram 1 (a) would explain this principle.

In other units, covered under Type OSD-10, a third element called a buck-boost or a series transformer is incorporated and this has the function to add to or subtract from the line, a voltage, to restore nominal output voltage values.

The three phase units of Type ISD-30 & OSD-30 would consist of a 3 phase Variable Transformer driven from one Servo Motor coupled to the common shaft of three ganged assembly of coils, actuated from one control. In these types, three stabilizer units operate simultaneously with sending from one phase, and these are recommended only where supply & load are balanced. The unbalance of the phases in the inpuyt supply is passed on to the output proportionately & the Correction of Voltage of all the three phases Collectively follows the Sensing fron one particular phase. These unbalances of supply, of low valurs generally serve the purpose of general purpose Industrial installations, but if the Customer is very particular to see the three phases output balanced & equal these units of type ISD-30 & ISD-33, donot provide satisfaction & in that case, ISD-33 or OSDS33 type of units of the following paragraph should be chosen. Type ISD-30 & OSD-30 units are cheaper than ISD-33 & OSD-33 units for the obvious reasons.



  • Three phase ganged assembly on ONE Shaft of three numbers of single phase toroidally wound double layer or extended winding construction, differential copper gauged, Variable Auto Transformer with limited sectorial movement.
  • One Servo Motor.
  • One Control Unit.


The three phase units of Type ISD-33 & OSD-33 would consist of assemblies comprising three single phase stabilizer units, Star connected either placed in a single cabinet or cubicle, or three separate single phase units each of capacity 1/3rd of the 3 phase. Road, placed side by side at site of installation providing individual correction of each phase ensuring balance & Output voltage, even with unbalanced supply or load.

The motor positioning is affected by the solid state sensor unit which measures the output voltage of the stabilizer, should this sensor energizes the motor to rotate the Variable Transformer brush gear until the correct voltage is restored.

The method of stabilization inherently produces no harmonics and therefore-no distortion in output voltage wave-shape, is introduced. Sensing from the output automatically compensates for any change in load current. Though mechanical, the action of the Servo system is exceptionally fast giving speeds of corrections up to 30- 40 V per second for smaller units & lower for larger units.



  • Three phase toroidally wound double layer or extend winding construction, differential copper gauged, Variable Auto Transformer with limited sectorial movement.
  • Three Servo Motor.
  • Three Control Unit.




  • Toroidally wound three phase Variable Auto Transformer- Ganged Assembly on ONE SHAFT of three single phase units.
  • Three single phase double wound Buck Boost Transformers.
  • One Servo Motor
  • One Control Unit.




  • Three single phase toroidally wound Variable Auto Trasformers
  • Three single phase double wound Buck Boost Trasformers
  • Three Servo Motor.
  • Three Electronic Control Unit.


Features/Operating Characteristics

  • SERVO DOT - Servo Voltage Stabilizers offer an unequalled combination of features providing performance and great versatility & high reliability.
  • All solid state control-No Relays or Remote Control Switches. This allows a great precision and speed than using traditional control systems.
  •  No wave shape distortion introduced.
  • Unaffected by Load Power Factor.
  • High Efficiency - greater than 96%.
  •  High Accuracy - better than +0.5%, Zero to full load. Accuracy screw driver adjustable from +0.5% to +2% to adapt working to a particular application, environment & location.
  • Wide Output voltage adjustment range - one can select by means of a proper ;control, Output Voltage (stabilized) to be equal to nominal voltage or to another value within +5%, the regulating area obviously changing.
  • Front loading or easily accessible, inter-changeable, Plug-in-type control Module makes the unit conveniently serviceable without shut off & down time
  • Built in Voltmeter to read Input or Output Voltage. Expanded scale Voltmeter provided as optional to read Output/Input voltage to accuracy of 1 or 2 volts.
  • Push Button type, Auto-Manual control generally provided, to operate unit in case of Electronic Control Card failure etc.


SD Control System

The outstanding feature of SD stabilizers is the extremely rapid controlled response with minimal overshoot. The permanent magnet rotor incorporated in an A.C. Step Synchronous Motor, provides braking near null position. The slow speed (60 R P M at 240 V-1 Ph A.C. 50 Hz supply) A.C. Motor incorporates 50 poles and is virtually temper proof & non-damage worthy electrically, since the locked rotor current is equal to full load current and is further equal to no load current. Unlike D.C. Motor, which incorporates Carbon brushes & requires constant attention & maintenance of a high level and also reduction gear, of worm gear assembly inside (to reduce the motor virgin speed of 900 to or 3000 RPM at motor commutator shaft to 2-20 RPM at output shaft,) the A.C. Motor is free from Carbon brushes or internal worm-gear assembly which is generally termed as problem area in engineering jargon.

The Sensor/Electronic control Card is a solid state printed circuit built around a highly stable quadcomparator I. C. A highly sensitive error signal is developed from stabilized & temperature compensated avalanche devices chain reference. The low leakage, high PIV, surge & peak voltage pre-treated triacs, protected from transients and reverse biases, function as fast switches to ensure pure unidirectional motor action, with on signals, and are designed to withstand heavy duty working. The sensor is constructed as a compact interchangeable plug in or screwable unit with a view on its maintenance requirements.

The sensitivity control is incorporated allowing adjustment of output accuracy even to LEVELS OF +0.25% for stringent requirements & for general purposes from +0.5% to +2%.

SD Control Cabinet

Where the space of cabinet permits and standard control are called for, a drawable type of control cabinet is provided.

The cabinet houses all the controls & indications in adrawable type of stabdardized cabinet, interchangeable with the units of our make & of different capacities. The unit further incorporates a plug in type. Front accessible, on line

Workable electronic control card and largely helps maintenance at remote places & by people even not so skilled in the electronic fields. Typical layout of the controls & indicators on the panel is as follows:-

Optional features

Servo Dots can be built with any of the following features to meet requirements of various customers

  • Miniature Circuit Breaker, to Switch ON-OFF the unit & also to cut off the unit in case of Continued overload or short circuit on output mains.
  • Digital Voltmeter with 0.5" Character, LCD Display , in place of pointer type Voltmeter.
  • Fuses-Rewirable type or HRC type, either on Input or on Output or both on Input & Output.
  • Low & High Voltage Cut Off facilities in case Voltage on Input goes lower than or higher than the specified input range . This is achieved through microswitch contacts placed at terminal limits of the variable transformer & through these contacts, the relays or contactors for cutting in or cutting off the supply are operated. By this arrangement supply ;would only be cut off or cut in, if the input voltage falls below or exceeds, the rated range of the voltage stabilizer.
  • Low voltage Cut Off or High Voltage Cut Off or both Low & High voltage Cut Off in case Voltage on Output goes lower than or higher than the specified safe limits of output voltage, on account of any reason. Here in, the electronic control circuit itself operates a relay or contactor or relay & contactor both. It is possible by this arrangement to cut off the supply to any equipment, if the same exceeds specified safe limits. For example, a Voltage Stabilizer of +/- 1% output voltage, can be provided with automatic cut in & cut off at + 5% output voltage or + 10% output voltage. To safeguard against instantaneous trips, time delay elements of the para. `f' can be built along with. This is much better arrangement than that of para. `d’
  • Time delay facility for Cut Off, either in low Cut Off or in high Cut Off, to avoid instantaneous or repeated trip off, against short duration, low or high transients or peak Voltage or Currents and to provide delays for switching in for certain applications.
  • Buzzer or alarm facility for unsafe low or high voltages.
  • Remote sensing & remote control facilities.
  • Arrangement for manual operation provided by putting knob on the extended motor shaft projecting outside the main cubicle or by use of a screw driver operated on a slot on motor shaft in side the housing, accessible through a hole in the chassis.
  • Voltage Stabilizer Bye-Pass or Through-Pass Change Over Switches-consisting essentially of a set of two Change Over switches or Contactors, wired up in a way that in one ON-position of the two Change Over Switches, Voltage Stabilizer is in Circuit & in other position, the Voltage Stabilizer is isolated & byepassed, with direct flow of supply to the installation. The arrangement is highly useful especially for large installations where in bye-passing, provides uninterrupted supply to the installation, and also provides rest period & breathing to the unit Sometimes this also helps to take care of over-loads, for shorter time and subsequent shutting off/bye-passing provides cooling of the unit to keep heat-time rating well utilized. The bye-passing also helps to bring the units into routine & preventive maintenance for the purposes of examining & changing the Carbon brushes or other wise without bringing break in supply to the installation and without also the need to remove the heavy load connecting leads from the units & preserve the same at power work stations. During the period of safe voltages, the Voltage Stabilizer, can be kept byepassed so that wear & tear of the Carbon brushes & usage wear, is substantially saved & ageing of electronic/electrical components etc. is further conserved.
  • Metering Panel fitted with Ammeters & Volt meters, with Selector Switches for 3 Phases System both for Input & Output side or individual Volt meters or Ammeters for each of the phases etc.


Inbuilt Robustness of Mechanical System Motorisation & Dimmer Constructions.

With the total in house manufacturing facilities being available, special care in design & manufacture has been adopted for the Variable Transformer for Servo Transformer for Servo Stabilizers. The Variable Transformers for Air natural cooled models, incorporate mono-block integral assemblies to mount the Variable Transformer coils on one side & motor on other side with special on-mount arrangement of mount micro-switches through fine adjustment micro-switch mounting plates, and on-built special metal insert moulded nylon gears & nylon cam to soft operate the micro-switches. The Carbon Brush bearing face of the coil is held in as a wall configuration so that dust does not settle on the same while in storage or in operation. The track of the coil on which the carbon brush moves is specially treated with a special cement & ground smooth finished by optical grinding process, The carbon brush system is internally spring loaded with front carbon brush loading facility & is of self-aligning & full wiping type. The improved track provide comparatively a very reliable & long lived variable transformer with carbon brush life 2-3 times of the normal units of other makes. Special units also have improved (3 to 5 times) Carbon Brush Area to the Current ration with obvious advantages.

The winding of the Toroidally wound transformers are circumferentially gripped to bear rough handing of coils & save the winding tracks from minor accidental damages. This gripping also improves short circuit with standing capacity & ensures continued contacts on ensures continued contacts on overloads. Units are strengthened internally for their holing system by better rough transportations & handlings. Larger units are oil natural cooled and are placed on frames in tanks with special care so that minimal stresses are passes are passes on to motorisation portion of the assemblies, during usages or transportation.

Compactness & Electrical Efficiency of ISD Models

ISD models incorporate unique techniques in a way that dual layer or extended winding construction is adopted on the Variable Transformer so that the need for a Series Transformer is eliminated. Such units thus bring saving in one major component as of the Series Transformer as to be found in conventional servo Stabilizers and thus are more compact, light weight, simpler and save on load losses & iron losses by about 20 to 30% overall,

Corresponding to the portion of the series transformer eliminated from such designs.

Design innovations, superior technologies and advanced manufacturing practices-hold the secret of fast growth of this organization.

After Sales Service

The units are built with design features so that minimum service would be needed. Carbon brushes of the Variable Transformers are the only components which wear out and may require change from periods varying from 2 to 10 years depending on use. Some components of Electronic card may fail buy ageing, lightening surges or power line faults. For this maintenance Electronic Card is also plug in type & can be replaced without much difficulty. Special designs of Carbon Brush Holders & Special treatment of the winding tracks result in the minimum wear, but still a set of spare carbon brushes are provided with every unit of the Variable Transformer.

The company has a wide network of distributes/ dealers for its products through out the country and serve the customer with it’s services.


All the equipments are guaranteed for a period of one year against any manufacturing defects as per our standard warranty clause.