What is RVDT (Rotary Variable Differential Transformer)? Working Principle, Construction & Applications

RVDT is used to sense the angular displacement. It is an inductive transducer, which converts angular displacement into an electrical signal. Here,

R = Rotary motion / Angular displacement

V = Variable inductance

D = Differential, which means output is difference of two secondary outputs

T = Transformer, because it functions as the former of primary and secondary windings.

Working principle of RVDT

Rotation of iron core (magnetic core/armature) is responsible for varying the differential voltage of two secondary windings of a transformer, which is a measure of angular displacement.

Construction of RVDT

RVDT consists of two secondary windings having equal number of  turns, but wound in opposite direction of primary winding connected across single phase A.C. supply. A magnetic core or armature is placed in the air gap between primary and secondary windings. It is free to move in either direction depending on angular displacement of shaft.

Fig. 1: Illustrative View of RVDT

Working of RVDT

When single phase A.C. supply is given to the circuit, it produces magnetic flux, which completes its path through S₁ and S₂. While completing the path, the flux produced by primary winding is linked to number of conductors of secondary windings. Therefore, according to faraday’s law of emf, an emf is produced in secondary winding. As shown in Fig. 2, the output voltage of S₁ is E_S1 and that of S₂ is E_S2. To convert the output of S₁ and S₂ into single voltage signal, S₁ and S₂ are connected in series as shown in Fig. 2. Thus, the output of transducer will be the difference of two voltages i.e. differential output voltage. i.e. E_o = E_S1 – E_S2.

Fig. 2: RVDT

Various positions of Core and Corresponding Displacement Measurements:

When the core is at central position, the magnetic flux linking to both Sl and Sz is equal and hence, equal emfs are induced in S₁ and S₂. Thus, E_S1 = E_S2. Therefore,

E_o = E_S1 – E_S2  = 0

This position of core is called as normal (null) or zero position. It concludes that, output voltage is zero at null position.

When the core rotates anti-clockwise, then the flux linking to coil S₁ increases and at the same time, the flux linking to coil Sa decreases. Hence, emf induced in coil S₁ (E_S1) is greater than that of S₂ (E_S2). As a result of which, output measured between secondary terminals is in phase with primary voltage.

When the core rotates clockwise, then the flux inking to coil S₂ Increases and at the same time, the flux linking to coil S₁ decreases. Hence, emf induced in coil S₂ (E_S2) is greater than that of S₁ (E_S1). As a result of which, output measured between secondary terminals is out of phase With primary voltage. Hence, output (E_S1 – E_S2) measured between two secondary terminals of RVDT is directly proportional to angular displacement of magnetic core.

Advantages of RVDT

1. Infinite resolution and linear operation.
2. Ability to measure clockwise as well as anti-clockwise angular displacement.

Disadvantages of RVDT

1. Range of RVDT is limited up to ±40° only, even though, it is a 360° device.
2. Better linearity is obtained only, if small angular displacements with ± 5° are measured.

Applications of RVDT

1. In Industrial applications.
2. For steering, braking operations in automotive applications.
3. Defense applications, where low noise is expected to arise due to friction.
4. Flight control applications.

CHARACTERISTICS/ SPECIFICATIONS OF RVDT

1. RVDT is a continuous rotational device (360).
2. High resolution to very small fracton of degree can be achieved in practice by RVDT.
3. The range of most linear operations for a typical RVDT is only about 40.
4. The range of a typical RVDT is only 40 as compared to theoretically described 360 rotation phenomenon. However, if RVDT is used to measure angular displacement within ±5, we get an Improved accuracy.
5. The foremost advantage of RVDT is the ack of physical contact between rotor and stator.
6. RVDT shows linearity over smaller angular displacement.
7. The resolution of RVDT is theoretically definite. Resolution of small fractions of degree is generally achieved in practice.