Linear Variable Differential Transformer

           The construction of LVDT is explained here. The differential transformer is a passive inductive transformer. It is also known as a Linear Variable Differential Transformer. The transformer consists of a single primary winding P1 and two secondary windings S1 and S2 wound on a hollow cylindrical former. The secondary windings have an equal no. of turns and are identically placed on either side of the primary windings.

(Source: Circuits today)

                A movable soft iron core slides within the hollow former and therefore affects the magnetic coupling between the primary and two secondaries.The displacement to be measured is applied to an arm attached to the soft iron core.

When the core is in its normal position, equal voltages are inducted in the two secondary windings. The frequency of the ac applied to the primary winding ranges from 50hz to 20 khz.

                The output voltage of the secondary windings S1 is Es1 and that of S2 is Es2. In order to convert the output from S1 to S2 into a single voltage signal, the two secondaries S1 and S2 are connected in series opposition as shown:

                   Hence the output voltage of the transducer is the difference of the two voltages. Therefore the differential output voltage Eo=Es1-Es2. When the core is at its normal position the flux linking with both secondary windings is equal, and hence equal emfs are induced in them. Hence at null position Es1=Es2. Since the output voltage of the transducer is the difference of the two voltages, the output voltage Eo is zero at null position.

                  Now, if the core is moved to the left of the null position more flux links with winding S1 and less with winding S2. Hence the output voltage Es1 of the secondary windings S1 is greater than Es2. The magnitude of the output voltage of the secondary is Es1-Es2, in phase with Es1.

                 Similarly, if the core is moved to the right of the null position more flux links with winding S2 and less with winding S1. Hence the output voltage Es2 of the secondary windings S2 is greater than Es1. The magnitude of the output voltage of the secondary is Es2-Es1, in phase with Es2.The amount of voltage change in either secondary winding is proportional to the amount of movement of the core. Hence, we have an indication of the amount of linear motion. By noting which output is increasing or decreasing, the direction of motion can be determined.

LVDTs have a variety of some applications like Crankshaft Balancer, Pill making machine etc..I'll be posting the working of those applications too.