Linear Variable Differential Transformer

Linear Variable Differential Transformer or LVDT is a displacement transducer, which is mainly used for measurement of linear displacements. LVDT works on the principle of transformer action i.e., mutual induction between primary and secondary coils. 

Construction of LVDT

LVDT essentially consists of a transformer which has one primary coil and two secondary coils with a movable core. Secondary coils are placed symmetrically relative to primary coil. Both secondary coils have equal number of turns and they are identical with each other. Secondary coils are connected in series opposition and when transformer is energized the two voltages induced in two coils have phase difference of 180⁰. Core is placed in between primary and secondary coils and allowed for free movement along its axis as shown in figure given. Core is made up of magnetic material and it facilitates magnetic flux linkage between primary coil and secondary coils. The objects or shafts whose displacements are to be measured are coupled with one of the ends of core. 

Working Principle of LVDT

Like a transformer, LVDT also works on the principle of mutual induction between primary and secondary coils as it's construction is just similar to a transformer but instead of two coils (one primary and one secondary) LVDT's transformer has three coils (one primary coil and two secondary coils). Further, two secondary coils are connected in series opposition in such a way that when transformer is energized the two voltages induced in two coils have phase difference of 180⁰ or simply we can say that two voltages induced in two secondary coils have reversed polarities so that addition of emfs induced in both coils because of linear displacements of core becomes net output of LVDT. When core moves up or down from central position about its axis, LVDT gives some output because magnetic flux linking each secondary coil changes and are not same which in turn produces voltages of different magnitudes and of reversed polarities in coils.

Working of LVDT

The object whose displacement need to be measured is attached to the core by suitable means. With the displacements of objects, core is moved either up or down. When core is moved towards down-side from central position, magnetic flux linking with secondary coil 2 (SC2) increases whereas magnetic flux linking with secondary coil 1 (SC1) decreases. Thus, the voltage induced in SC2 increases and in SC1 decreases. As discussed above, voltage induced in SC1 & SC2 are 180⁰ out of phase that is these voltages have reversed polarities than each other. Also these secondary coils are connected in series, therefore, addition of voltages in two coils is the net output voltage of LVDT. For example, say at any instance voltage induced in coils SC1 is -2 volts and in coil SC2 is 5 volts, so net output voltage will be 3 volts. This voltage will have same phase as SC2. Similarly, when core is moved up-side from central position, voltage induced in SC1 increases and in SC2 decreases. Thus net output will be the addition of two voltages. On the other hand, when core is at central position, magnetic flux linking with SC1 & SC2 are same and voltages induced in both coils are also same but with polarities reversed than each other. Thus, net output voltage of LVDT will be zero.


Because of residual voltages due to stay magnetic and capacitance effects, the net output voltage at central position may not zero at all the times. LVDTs are available in wide ranges. LVDTs have linear output over a wide range of input displacements.

Advantages of LVDT Instruments

• Minimum input force costs in sensing by LVDT because of negligible friction in movement of force.
• High sensitivity.
• Linear Output.