The document discusses the linear variable differential transformer (LVDT), which measures displacement by converting non-electrical displacement into an electrical output signal. An LVDT consists of one primary winding in the center surrounded by two secondary windings, and uses mutual induction to produce a voltage proportional to the position of a movable iron core within the coils. The differential output voltage provides a linear measurement of the core's displacement. LVDTs have advantages like linearity, infinite resolution, and ruggedness, but are sensitive to stray fields and temperature changes. They are commonly used for measuring displacement, thickness, and hydraulic cylinder position.

1. C_16_AEI_305_2.2
Prepared by
K. Lakshmana Rao, Lecturer in AEIE
Under the guidance of P.RamaKrishna, Head of AEIE
at A.A.N.M &V.V.R.S.R Polytechnic, Gudlavalleru
State Board of Technical Education & Training
Andhra Pradesh
1
e – Lesson Module for C-16 Curriculum
Year/Semester : III Semester
Branch : Applied Electronics & Instrumentation
Subject : AEI-305, Process Instrumentation
Topic : Displacement and position measurements
Sub Topic : Linear variable differential transformer
Duration : 50 min

2.  To understand principle and operation of LVDT
 To know about Advantages and Disadvantages
 To know about Applications and Specifications
Main objectives of this Lesson
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3. C_16_AEI_305_2.2 3
Recap
In last class we have learnt about the
 Displacement
 Linear potentiometer and its advantages and
Disadvantages.

4.  LVDT works under the principle of mutual induction.
Principle
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 LVDT measures displacement which is a nonelectrical
energy is converted into an electrical energy.
Click here
To view the video on mutual induction
 LVDT produces an electrical output which is proportional
to the displacement of the moveable core

5. LVDT consists of a cylindrical transformer where it is
surrounded by one primary winding (P1) in the centre and
two secondary windings (S1and S2 ) at the sides.
Construction
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The number of turns in both secondary windings are
equal, but they are opposite to each other.
The primary winding is connected to an AC source.
 A soft iron core moves within the hollow former and
affects the magnetic coupling between the primary and
two secondary windings.

6. C_16_AEI_305_2.2 6
Fig: LVDT

7. 7C_16_AEI_305_2.2
 The displacement to be measured is
applied to an arm attached to the soft
iron core .
Working
 The magnitude of the voltages
depends upon the position of the iron
core with respect to the center of the
coil.
 The differential output of LVDT is given by
E0 = Es1- Es2
Where
Es1 is induced voltage in the secondary winding s1
Es2 is induced voltage in the secondary winding s2.

8. Case 1: When the core is in its null position.
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 When the core is in its null position equal voltages in
secondary windings.
 At null position E0=0, because the flux linking with both
secondary windings is equal, hence equal emf are
induced in them. (Es1 = Es2 )

9. Case 2: When the core is moved to the left its null position.
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 When the core is moved to the left of null position more
flux links with winding S1 and less with winding S2.
 Hence, output voltage is Es1 is greater than Es2. The
output voltage is positive and in phase with input signal.

10. C_16_AEI_305_2.2 10
Case 3: When the core is moved to the right its null position.
 When the core is moved to the right of null position more
flux links with winding S2 and less with winding S1.
 Hence, output voltage Es2 is greater than Es1.The output
voltage is negative and 180o out of phase with input
signal.

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 The voltage change in either secondary winding is
proportional to the movement of the core.
 The output voltage is measured to determine the
displacement.
 The output voltage of an LVDT is a linear function of the
core displacement within a limited range of motion.
Click here
To view the video on LVDT 1
Click here
To view the video on LVDT 2

12. Advantages
 Linearity
 Infinite resolution
 High output
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 Ruggedness
 High sensitivity
 Low power consumption

13. 13
Disadvantages
 Large displacement are required for appreciable
differential output.
 They are sensitive to stray magnetic fields.
 It must be selected to operate on AC signals.
C_16_AEI_305_2.2
 Temperature sensitive.

14. C_16_AEI_305_2.2 14
 As basic element in Extensometers
 In Thickness measuring units
 In Electronic comparators
 Hydraulic cylinder displacement

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In this class we have discussed about
• Principle of operation of Linear variable differential
transformer
• Advantages and Disadvantages
• Applications of LVDT
Summary

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1. Linear variable differential transformer has
(a) Two primary coils connected in phase and a
secondary coil.
(b) Two primary coils connected in opposition and a
secondary coil.
(c) One primary coil and two secondary coils.
Connected in phase
(d) One primary coil and two secondary coils
connected in opposition.
Quiz

17. C_16_AEI_305_2.2 17
2. LVDT converts
(a) Electrical energy into mechanical energy
(b) Mechanical displacement into electrical energy
(c) Mechanical energy into electrical energy
(d) None of the above

18. C_16_AEI_305_2.2 18
3. Very small displacements are effectively
measured using
(a) LVDT
(b) Strain gauge
(c) Thermistor
(d) Tachogenerator

19. Assignment
1. State the principle of Linear variable differential
transformer.
2. Explain the operation of Linear variable differential
transformer with neat diagram.
19C_16_AEI_305_2.2

20. End of Lesson 2
In
Chapter 2 of AEI-305
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