Figure. RVDT |

**Rotary Variable Differential Transformer (RVDT)**is a variation of LVDT and used to sense angular displacement. It is a passive transducer.

**RVDT**provides a variable alternating current (AC) output voltage that is linearly proportional to the angular displacement of its input shaft. Unlike LVDT, the input of this transducer is differential value of rotary variable i.e. angular rotation to generate voltage output.

## Working Principle of RVDT

The reluctance seen by the primary mmf changes with the rotation of core by shaft. This results in change in the magnetic flux with rotation of the shaft. Due to this change in magnetic flux with rotation of core, the flux linkage of secondary winding also changes. Therefore, as per the transformer action, an emf is induced in secondary winding. The magnitude of induced emf will depend on the rate of change of rotation. The more the rate of change of rotation, the more will be the rate of change of flux and hence more emf will be induced.

As can be seen from the figure 1, the two secondary winding are connected in series but in phase opposition. This is done to get a single output voltage from the transducer. If E_{s1}, E_{s2} and E_{0} be the emf induced in the two secondary winding S_{1} & S_{2} and output voltage respectively then

E_{0} = E_{s1} – E_{s2}

Under normal condition of RVDT, the flux linkage of both the secondary winding are same due to their symmetrical placing w.r.t. primary and core. Therefore, the induced emf E_{s1} and E_{s2} are equal and hence output voltage E_{0} of the transducer in such condition is zero. Therefore, normal position of RVDT is called **NULL position**.

Clockwise rotation of core causes an increasing voltage E_{s2} in one of the one secondary winding while counter clockwise rotation leads to increase in voltage E_{s1} of another secondary winding. Thus the direction as well as magnitude of angular rotation can be ascertained from the magnitude and phase of transducer output voltage. Phase of transducer output voltage means whether (E_{s1} – E_{s2}) is positive or negative.

In case of **anti-clockwise rotation of core**, the value of E_{s1} will be more than that of E_{s2} and hence (E_{s1} – E_{s2}) will be **positive**. In this case we say that output voltage E_{0} is in phase with the primary voltage. With the same logic, when **core is rotated in clockwise direction**, the output voltage will be **negative** i.e. out of phase with primary voltage.

**Summary:** The secondary windings are connected in series opposition. At the null position, output voltages of secondary windings S_{1} and S_{2} are equal. Hence the net output of RVDT is zero. When there is a clockwise rotation, it produces an increasing voltage of a secondary winding of one phase. When there is an anti-clockwise rotation, it produces an increasing voltage of a secondary winding of opposite phase.

## Applications of RVDT

**Excercise**

Q. Describe the working of RVDT with the help of neat circuit diagram.