Linear Variable Differential Transformer (LVDT)

• LVDT is an inductive type passive transducer.
• It measures force in terms of displacement of ferromagnetic core of a transformer.
• It converts translational or linear displacement into electrical voltage.
• It is also known as Linear Variable Differential Transducer.

Principle

It is based on the principle of electro-magnetic induction.

Construction

• LVDT consist of cylindrical transformer where it is surrounded by one primary winding in the centre of the former and two secondary windings at the sides.
• The number of turns in both the secondary windings are equal, but they are opposite to each other.
• The primary winding is connected to the ac source.
• A movable soft iron core slides within hollow former and therefore affects magnetic coupling between primary and two secondary.

• When the iron core lies at the centre of both secondary, the output differential voltage remains unaffected and have zero magnitude.
• When the core moves towards secondary-1, it induces more emf across it and less emf across secondary-2. Let’s assume that it is positive displacement. Due to more flux links with the secondary-1 than secondary-2.
• When the core moves towards secondary-2, it induces more emf across it and less emf across secondary-1. Lt’s assume that it is negative displacement. Due to more flux links with the secondary-2 than secondary-1.
• The output differential voltage is proportional to the displacement of the iron core.

• High range (1.25 mm to 250 mm)
• No frictional losses
• High input and high sensitivity
• Low hysteresis
• Low power consumption
• Direct conversion to electrical signals

• LVDT is sensitive to stray magnetic fields so they always require a setup to protect them from stray magnetic fields.
• They are affected by vibrations and temperature.

• It is used where displacements ranging from fraction of mm to few cm are to be measured. It act as primary transducer.
• They can also act as secondary transducer. E.g. the bourdon tube which acts as primary transducer and convert pressure into linear displacement then LVDT converts it into electrical signal.