Rotary Variable Differential Transformer

RVDT is an inductive type passive transducer.

• It measures force in terms of displacement of ferromagnetic core of a transformer.
• It converts rotary or angular displacement into electrical voltage.
• It is also known as Rotary Variable Differential Transducer.

Principle

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

Construction

• RVDT consist of rotating iron core and one primary excitation coil and two secondary output coils.
• A fixed alternating current excitation is applied to the primary stator coil that is electromagnetically coupled to the secondary coils.
• This coupling is proportional to the angle of the input shaft.
• The output pair is structured so that one coil is in-phase with the excitation coil, and the second is 180 degrees out-of-phase with the excitation coil.

• 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 angular motion of the iron core.
• RVDT has the accuracy of ±1% for rotation upto ±40° and ±4% for rotation upto ±60°
• RVDT is not advisable to use If rotation is greater than ±60°, RVDT is not advisable to use.

Advantages

• Low sensitivity to temperature, primary voltages & frequency variations
• Low cost
• Small size
• Simple control

Dis-advantages

• 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.

Applications

• It is used where rotational or angular displacements are to be measured.
• In throttle mechanism of aeroplane.