Toroidally wound turns by a sliding carbon brush - which always shorts two or more turns of the winding for the basic reason that Current connection should not get cut from the last turn before catching the next turn. In this process - the Carbon Brush always shorts two or more Turns of Transformer. This contact arrangement produces heat at Carbon junction point of winding on following varying accounts.
1. Shorting of more than one turn through Carbon Brush.( Voltage per turn affect). In windings with thinner wires that have high resistance perturn - this heating is lower in comparison to large heating on thicker wires. In Dimmers of 2 Amps, -this heating is very low in comparison to 200 A coil .A carbon brush of High Resistance will limit this heating on No Load in higher capacity say of 200 A Dimmers - but will get heated up substantially on account of para 3 below. A carbon brush of LOW RESISTANCE will produce high INTER TURN HEAT on Large capacity Dimmers say of 200 A units on NO LOAD but will produce lesser heat on LOAD. To limit this heat- Dimmer manufacturers work out various ways - like selection of grade of carbon, provision of HEAT SINKS on the Brush Holders, providing heat sinks on the winding etc. etc. - but by & large - this heat had been a challenge in the manufacture, design & usage of these Dimmers.
2. Contact resistance (Rc )ofCarbon &varieswith Pressure on carbon brush,grade or Characteristics of the Carbon Brush Material, etc. - Heat Generated=l2XRc Watts.
3. Current Conducting properties of carbon brush - l2R heating on the Carbon X Section carrying current.
4. Self heating of Winding Turn because of self shorting of Copper Turn - due to length of carbon contact on winding - the heating circuit is formed along the length of copper turn in contact with carbon. If length of Carbon to collect current covers say 1/3 length of mean turn of copper as happens in say 200 A Dimmers, the heating will be more if say 1/10 of the winding self shorts the copper length. Longer the Carbon - more is heat & vice versa.
Thus - heat produced at Carbon brushes has been limiting factor & a serious issue in use of variable transformers to the extent thatupto 28A approx.- Air Cooled units could be marginally & safely produced in Single Coil &above this units arerecommended to be used in Oil Cooled condition only - so that heat generated at carbon brush pointsgets dissipated to largeOil content/and even radiators of containers.
In Dimmer Dot Variable Voltage Transformers - TYPE GDCB, this INTER TURN SHORTING CURRENT is BLOCKED by a Special winding Technique & Chokes - so that Current is Collected from two turns by two separate & Isolated current collectors & these two separate currents. These Chokes / reverse biased diodes, block the transfer of current from one turn of the winding to the other thus no shorting of turns takes place. Since the turns do not get shorted, in these units, the carbon brushes are replaced by solid copper / Copper Cadmium or other alloys - that again slide over the winding producing virtually no heat on account of any of the above mentioned causes. Breakage or wear & tear of Carbon issues are totally eliminated- thereby introducing high degree of reliability in this very critical equipment used in all applications requiring varying voltage or control of voltage. Thus the cause & affect of heating at Current Collection points are virtually eliminated - making the units to run cool, saving on Large JUNK of Carbon & Contact Losses, which in conventional variable transformers dominated all other losses.There is VIRTALLY95 % SAVING in these losses&the units run cool - making the Dimmer Dots of even 200 A in Air Cooled Version. Higher AIR COOLED capacities are being worked out &are in pipeline. We have 28/60/100 & 200 A units ready for delivery. THE DESIGN is protected vide Patent No 3095/Del/2011 (applied for). With the break through in technology in this line - we are geared up to produce Variable Transformers upto 5000 A Capacity with sliding copper contacts . This technology is considered farsuperior over Rollar Carbonsetc.for Larger Capacity Variable Transformers.