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Variacs or variable auto transformer is a Continuously variable voltage transformer, having a single layer winding on a toroidal core of high grade Steel.

In an autotransformer portions of the same winding act as both the primary andsecondary winding. the winding has at least three taps where electrical connections are made. One end of the winding is usually connected in common to both the voltage source and the electrical load. The other end of the source and load are connected to taps along the winding. Different taps on the winding correspond to different voltages, measured from the common end.

On one face of the copped winding, along the periphery, a special durable contact surface is made. A specially designed carbon brush slides smoothly on the surface. A contact arm holding the contact brush is fitted to the central shaft and as the control knob is rotated, the carbon brush slides over the winding and taps off a portion of voltage across the winding.

Any desired voltage within the maximum range can be obtained by adjusting the position of the brush. The calibrated dial reads approximately the output voltage at no-load, when the input is at its rated value. Rotation of the brush arm by manual of motor drive delivers an output voltage from zero to or above line voltage.

Variable Auto Transformer, manufactured by Purevolt is the most useful and effective device for step less, break less and continuous control of A.C voltage and therefore, for control of various other parameters, dependent on A.C voltage. It is ideal controlling device for numerous applications in laboratories and in industrial & commercial fields.

The basic Auto Transformer is meant for operation off a nominal voltage of 240 V A.C & can give output voltage anywhere between 0 – 240 V or upto 290 V, by a simple transformer action. Three such Variable Auto transformer when connected electrically in star & mechanically in tandem, became suitable for operations of 415 V 3 Phase A.C. supply & to give output of 0 – 415 V or upto 470 V.

TECHNICAL SPECIFICATIONS VARIABLE AUTO TRANSFORMER / TOROIDAL TRANSFORMER:

Rated for 240V A.C. 50/60Hz. single phase supply and three phase model for 415V A.C. 50/60Hz three phase 4 wire supply.

The output voltage of these Toroidal or variable transformer can be varied smoothly over two ranges:
(i) Zero to full supply voltage (ii) Zero to approx. 12% higher than the supply voltage

These auto transformers or dimmers are Suitable for indoor use on maximum ambient temperature of 45oC

Variable Auto Transformers are rated 240 volts Input and 0-270Volts output. Similarly, three 240-volt units are ganged in a star connection to result in a 470-volt line-to-line three-phase assembly. For 480-volt open delta applications, two 240-volt units are used in series in each of the two legs of the open delta assembly. In any of the above voltage applications, higher current requirements are met by paralleling two or more units in the ganged assembly.

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Salient Features for Dimmerstat / Variable Auto Transformers :

Continuously Variable Auto Transformers have many operating advantages and conveniencesover other manually operated voltage control devices for alternating current use. Some of the features are mentioned below :

Smooth Control:

The Variac / dimmerstat can be set to any output voltage within its range. Output voltage is continuously adjustable from zero to maximum. As voltage between turns is small and as the bush arm is in constant contact with more than one turn, harmful sparkling is avoided and output can be set very easily to a fraction of a volt from zero onwards by rotation of the knob.

Low Operating Torque :

A glass smooth commutator, special sintered bushings for the shaft and perfect assembling of the coils, result in low operating torque for our auto-transformers.

Small Size:

The variable Auto Transformers is much smaller than any other type of control of equal rating

Better Efficiency:
Very low losses under all loud conditions. Toroidal Variable Auto Transformer have low-loss grain oriented strip wound cores. Careful copper core selection and design assures the user of low watt loss as compared to other types of controllers.

No wave form distortion:
Our variacs provide an output voltage wave from which is a faithful and distortion less replica of the input voltage wave form.

Better Regulation:
Output voltage is substantially independent of loud. The drop in voltage from ‘no load’ to ‘full load’ is minimal in our variacs.

Linear Output Volts:
Output Voltage varies linearly with angle of rotation of control knob.

Temperature rise:
Less than 50 deg.C for continuous duty.

Advanced Design:
Excellent electrical specification is matched by rugged, compact construction. Brush
replacement is simple and convenient. Two, three or more units can be ganged on one shaft for series, parallel or three-phase operation.

Long Life:
When used correctly the Variacs has a very long operating life. The life of a Variacs is
essentially the same as that of any standard fixed ratio power transformer.

Rugged Construction:
Our variacs are designed for heavy duty trouble free operations. All components are designed to give maximum life to the unit under extreme operating conditions.

Low Magnetic Current:
By using an adequate section of the high quality grain oriented silicon steel, the iron losses are controlled to the minimum. The no load current is invariably less than 1% of the rated current.

Negligible Maintenance nad Trouble Free Long Life:

Variacs have been designed to guarantee performance. Therefore no special care needs to be taken in normal working conditions. Timely replacement of carbon tips and cleaning of the commutator surface of foreign particles and accumulated dust will ensure a considerably long, maintenance free and interrupted life to the unit.

Variacs fitted with voltmeter, ammeter, fuse, switch, pilot lamp etc. can be made in Enclosed type models with extra cost. For the control of the contact arm for a remote point, or automatic control of the output voltage, Variac can be made with a motor drive. The motor is coupled to the central shaft of the Variac and as motor rotates, the contact arm moves over the plated surface. The motors are noiseless in operation and work on 240 – 50 Hz single phase AC. The rotation of the motor can be reversed by interchanging the motor connection by means of a SPDT switch.

Precise design assembly of the brush (at a constant pressure to a smoothly finished and securely bonded commutator surface provides excellent mechanical performance, long life, and low-driving torque. Brush replacement is seldom needed, but it is easily performed. High safety margins of voltage, current-carrying capacity and dielectric strength are why you can expect longer life

Simple Instalation and Connection:

Mounting of units is convenient and easy. Most Series units are designed with an adjustable shaft to accommodate either bench or panel mount (adjustable to accommodate varying panel thickness). Terminals are easily accessible; screw, lug or solder design. Connections deliver increasing output voltage with either clockwise or counter-clockwise knob rotation. Manually operated units have standard dials graduated 0-100% or 0-270V (output voltage).