what is an LVDT ( Linear variable differential transformer) or Differential transformer? Working, advantages and disadvantages|

Linear variable differential transformer (LVDT) -

LVDT is a passive inductive transducer and is commonly used employed to measure force( or weight,
pressure and acceleration etc. Which depend on  force) in terms of the amount and direction of displacement of an object.

 Construction -

Refer fig.b

  • It consists of one primary winding and two secondary winding (S1 and S2) which are placed on either side of the primary mounted on the same magnet core. The magnetic core is free to move axially inside the coil assembly and the motion being measured is mechanically coupled to it.
  • the two secondaries S1 and S2 have equal number of turns are bat are connected in series opposition  so that e.m.f ( E1 and E2) induced in them are 180 digree out of phase with each other and hence , cancel each other out. Fig.C

Working principle -

  • When the core is n the center ( called reference position) the induced voltages E1 and E2  are equal and opposite. Hence they cancel out and the output voltage Vo is Zero.
  • When the external applied force moves the core towards coil S2, E2 is increased but E1 is decreased in magnitude though they are still anti phase with each other . the net voltage available is ( E1 - E2) and is in phase with E2.
similarly, when the magnetic core moves towards coil S1, E1 > E2 and Vo = E1 - E2 and is in phase with E1.
thus, from above discussion, we find that the magnitude of Vo is a function of the distance moved by the core and its polarity or phase indicates as to in which direction it has moved .
          If core is attached to a moving object, the magnitude of Vo gives the position of that object. 


  • Inductive transducer.
  • The secondary winding have equal to primary winding.
  • Soft iron code made of high permeability nickel iron.
  • Frequency of A.C applied to primary winding may be between 50 Hz.


  • High range.
  • LVDT is a friction less device.
  • Infinite mechanical life.
  • High sensitivity.
  • Low hysteresis loss.

The disadvantages of the LVDT are shown below in details.
  1. Large displacement is required for getting the considerable differential output.
  2. The LVDT transformer is very sensitive to the stray magnetic field.
  3. The performance of the transducer is affected by the vibrations.
  4. The dynamic response is controlled mechanically by the mass of the core and electrically by the frequency of the current.
  5. The performance of the LVDT is affected by the temperature.



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