Variac- variable autotransformer
Radha Gobinda
21-12-13
What is variac?
Variac is the trademark name of a variable autotransformer.
Variac provides a voltage-adjustable source of alternating current (AC)
electricity.
A variable autotransformer is a single-coil transformer in which two portions
of the same coil are used as the primary and the secondary.
They also differ from regular autotransformers in that they are equipped with
automatic switchgear in order to automatically step a voltage up or down,
depending on preset conditions.
http://www.nmr.mgh.harvard.edu/~reese/VariacPage/
http://en.wikipedia.org/wiki/Autotransformer
How variacs work
Variable autotransformers consist of a regular autotransformer that has been
scratched in order to expose part of the coil, a sliding brush, and any automatic
switches that a particular Variac requires.
In the autotransformer, a primary voltage is connected from the bottom to a
specific winding known as a “tap,” and a secondary voltage is connected from
the bottom to a separate winding.
Each winding or tap represents a different voltage.
As voltages travel through the coil from both the primary and the secondary,
the overall voltage is increased or decreased, depending on the placement of
the primary and the secondary voltages and the number of taps in between
them.
In a variable autotransformer, the secondary voltage is applied to the exposed
portion of the autotransformer via a conductive sliding brush.
Applications
Variable autotransformers automatically regulate voltages for industrial
equipment, simulate power line conditions, and control switches that are
dependent on external events.
Variable autotransformers are also used as automatic alternatives to regular
autotransformers, which can be used in any situation that a regular transformer
can be.
Used in both Synchronous motors and induction motors.
Used in electrical apparatus testing labs since the voltage can be smoothly and
continuously varied.
They find application as boosters in AC feeders to increase the voltage levels.
Two types are tyically seen; bench models and open frame types.
The bench model contains the winding
in a ventilated metal shield, with the
control knob on top. The bench model
typically has an on/off switch, a fuse,
one or more AC outlets, a line cord
and sometimes a current or voltage
meter.
Open frame types, are intended for
panel mounting in equipment racks or
as a part of some larger piece of
equipment. Electrical connections and
the inductor are exposed, but the
operating principles are the same as
the bench model.
Advantages and Disadvantages
Variable autotransformers require less windings and a smaller core than a
regular transformer, allowing them to be used in smaller applications.
Its efficiency is more when compared with the conventional one.
Voltage regulation of autotransformer is much better.
Low requirements of excitation current.
Less copper is used in its design and construction
Variable autotransformers can be automated, allowing them to quickly change
between voltage ratios.
They are smaller, lighter, less expensive, and handle lower voltages, generally
ranging between 110 V and 240 V.
In conventional transformer the voltage step up or step down value is fixed
while in autotransformer, we can vary the output voltage as per out
requirements and can smoothly increase or decrease its value as per our
requirement.
Variable autotransformers do not provide the electrical insulation between their
windings like regular transformers do.
This presents a safety hazard as it possibly allows a high primary’s full input
voltage to pass directly to a low secondary’s output.
Thank you