What is LVDT ?


It is a type of inductive transducer which is used to measure the displacement. The inductive transducer means they are changing the inductance of the coil due to the change in inductance the non-electrical quantity is converted into electrical quantity. The LVDT(linear variable differential transformer) is used measure the displacement and this displacement is measured by measuring the output voltage. The displacement is non-electrical quantity and displacement is measured by changing the inductance of the coil and this displacement will be measured with the help of the output voltage.

The LVDT is called a transformer, because in this LVDT we will have two types of windings primary windings and secondary windings just like winding present in transformer, so here also we have two types of windings that is primary and secondary winding that’s why its called transformer. Differential transformer because the output of the transformer it is the difference between voltages across the secondary windings, so the output voltage it is the difference of the voltage that’s why its called differential transformer and linear variable because it is measuring the linear displacement, so linear motion is converted into a voltage.



In this diagram consists of two secondary windings (s1 and s2) and one primary winding (p) which is being wounded across the cylindrical former. These two windings have equal number of turns and are identically placed on either side of primary. The primary winding is connected to an alternating current source and the soft iron core is placed inside the former. The displacement to be measured is applied to the arm attached to the soft iron core.The soft iron core is made up of high permiability nickel iron which is hydrogen annealed. When it is hydrogen annealed this gives low harmonics low null voltage and high sensitivity to the LVDT. The soft iron core is slotted longitudinally as to reduce the eddy current and the whole assembly of LVDT it is placed in a stainless steel housing and the end lids provide electrostatic and electromagnetic shielding.


The primary winding is excited by an alternating current source and the frequency of the current source is between 50hz-20khz. Due to the alternating current flow through or excitation of the primary winding electromagnetic field is produced, this electromagnetic field is induces the alternating currents in the two secondary windings s1 and s2.

The output voltage is difference of the voltages in the two windings,

E0 = Es1 – Es2

when the soft iron core is at null position,(exactly at middle position

Es1 = Es

E0 = 0

core os moved to the left of null position, (the output voltage in phase with the input voltage)

Es1 > Es2

E0 = Es1 – Es2

core is moved to the right of null position,(the output voltage is 180° out of phase with the primary voltage)

Es1 < Es2

E0 = Es2 – Es1

The amount of voltage change in either secondary winding is proportional to the amount of movement of the core.

The amount of output voltage may be measured to determine the displacement and the output signal may also be applied to a recorder or to a controller that can restore the moving system to this normal position. The output voltage of LVDT is a linear function of the core displacement within a limited range of motion say 5mm from the null position.


OC = residual voltage, OB = linear range & Ф = 180°, OA of Ф = 60°,

y-axis = output voltage, x-axis = displacement.


 Residual voltage idially at null positioin, the output voltage should be zero.

 There is a small amount of voltage which is present at the null position called the resudual voltage.

 This residual voltage can be present due to pressence of harmonics in the supply voltage. due to the harmonics produced in output voltage on account of use of iron core.

 Either an incomplete magnetuic or electric unbalance.


1. High range (range of displacement can be measure is 1.25mm to 250mm) and dynamic response is low.

2. No friction and electrical isolation (no physical contact between core and coil, there is no wear and tear due to friction)

3. Immunity from external effects (seperation betwwwn core and coil permits the isolation of pressurized, corrosive or caustic fluids).

4. High input and high sensitivity.

5. Ruggedness(can tolerate high shock and vibrations).

6. Low power consumption.


 Primary transducer(displacement is converted into voltage).

 Wide use applications of applications in automation, power turbines, hydraulics,aircraft etc.

Also Read – What is Transducers?

Also Read – Main parts of DC machine

Also Read – Losses of 3- phase Induction Motor

Also Read – Construction of DC machines

Also Read – Definition and types of alternator

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