What is a Linear Variable Differential Transformer

A linear variable differential transformer is an inductive transducer that is used to convert linear displacement in to electrical signals for the purpose of measurement and control. It consists of a primary transformer winding and two identical secondary windings with equal number of turns. The secondary windings are placed on either side of the primary, both an equal distance apart from it. A movable soft iron core with a high permeability is present between the primary and the secondary windings. This core is made up of hydrogen annealed nickel iron. The linear displacement to be measured is applied on the movable arm of this core. The secondary windings are connected to each other in series in the outer circuit in such a way that the voltages induced in them oppose each other in direction i.e. the net voltage in the outer circuit is the difference between the voltages induced in the two secondary windings.

The working of an LVDT is very simple to understand. Initially when there is no displacement, the core is in a null position at the centre. Now when the primary is excited by an A.C voltage, two equal voltages are induced in both the secondary windings due to equal flux linkage with the primary. The net output voltage in this case in the outer circuit of the secondary windings is zero. Now let a displacement is applied on the movable arm of the core which will move it closer to one secondary and farther from the other. The linkage between the primary and the closer secondary will increase while the linkage between primary and the farther secondary will decrease. This change in linkage will result in a change in magnitude of the voltages induced in the two secondary windings.

Thus an output voltage will be obtained in the outer circuit which will give a measure of the linear displacement by which the core was moved from its null position. The direction of this displacement can also be known by analyzing the phase of the output voltage. It will be in phase with the primary voltage when the core will move towards the left from the null position and will be in 180 degrees out of phase with the primary voltage when the core will move towards the right.

Usually an LVDT can measure linear displacements varying from 1.25mm to 250mm while also having advantages like electrical isolation between the windings and mechanical isolation between the core and the windings. One of its main drawbacks is its lack of sensitivity as an appreciable amount of displacement is required to obtain sufficient voltage changes in the output circuitry which can be measured and analyzed.

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