Variable Voltage Auto Transformers

Wherever electricity is used VARIAK has helped make it more versatile by providing an easy and efficient source feeding power at variable voltage. If you are not yet familiar with control system, you are missing some important benefits of using electric power.

VARIAK is a trade mark of their continuous variable voltage auto transformers .They usually form part of system in which other components such as transformers, rectifier, stacks etc. are used as power components and error sensing devices and servo drive motors used for automatic operation. We engineers are well experienced in such applications and are always available for consultation to prospective users.

VARIAK is an autotransformer using a round strip wound core, instead of the usual E & I laminations. The entire winding is in a single layer, with a small gap left between start and end of the winding. The proportion between iron section and copper turns is such that at the maximum applied AC voltage of 50 Hz, it does not get saturated and is designed to take only a small magnetizing current. The applied voltage is uniformly distributed over the winding. Thus, if 240 Volts are given to 240 turns, every turn shares one volt. Voltage between say 10 consecutive turns is thus, 10 volts. By carefully barring the insulation of the wire over one face, a sliding brush is made to contact any turn of the winding. The output is taken between one end and the brush arm, The brush, while moving contacts the next turn before it breaks away from the previous. Thus, there is no interruption in output voltage, While brush slides along the windings contact surface. The “Shorting” that the brush produces between the two turns, does produce a current that circulates between them, but it is quite small to produce any undesirable effects like heating or sparking. This is because of the voltage between two consecutive turns is very small. Being a true transformer the maximum voltage available is made about 12% higher than the input voltage. The maximum current that can be drawn depends on the size of the wire used. The important specification of VARIAK model is the maximum current it can give out to the load. Models from 0.7 amp to 200 amps are made.

How about VARIAK for 3 phases supply ?

They are made by connecting three single phase. VARIAK together. The brush arms are moved by a common shaft. The three winding are connected in star and the star point is connected to the neutral of the system. Thus, a 3 phase voltage variable from 0 to 440 volts is obtained. VARIAK are available with motor drive. The type of motor used for driving is specially made for this application. It is 240 volts AC motor operating with speed of 60 RPM. Synchronous at 50 Hz. It is instantly reversible and can be stopped without overrun. Gears between motor and VARIAK shaft are used to get lower speed. The time taken for the full sweep is the specification used. E.g. 15 seconds, 30 seconds etc. That is the time taken for the brush to move from zero voltage position to the maximum voltage position