Linear Variable Diffrential Transformer , LVDT

  Linear Variable Diffrential   Transformer:

Linear Variable Differential Transformer (LVDT) The LVDT converts th displacement into an electrical signal.  LVDT consists of one primary winding (P) and two secondary windings (51 and S2).with equal number of turns wound on a cylindrical former. 

The two secondary windings are connected in series opposition and are placed identically on either side of primary winding to which an AC excitation voltage is connected.

 A movable soft iron core is placed within the cylindrical former. When the displacement to be measured is applied to the arm of the core, the LVDT converts this displacement into an electrical signal. 

The construction of LVDT is,in figure 5.1

fig 5.2 

The operating principle of LVDT:-

The operating principle of LVDT depends on mutual inductance.

In the LVDT when the primary winding is supplied with A.C. supply voltage, it generates alternating magnetic field. Due to this magnetic field an alternating voltage will be induced in the two secondary windings. In the figure (B) es1 is the output voltage of secondary winding S1 and es2 is the output voltage of secondary winding S2 In order to get single differential output voltage two secondary windings are connected in series opposition. Thus the differential output voltage is given by,

e0=es1-es2

When the core is placed symmetrically with respect to two secondary windings an equal amount of voltage will be induced in both windings. 

Therefore esl - es2 and the output voltage is '0'. Hence, this position is known as null position. Now if the core is moved towards up from null position, more magnetic field links with secondary winding S1, and small field links with secondary winding S2. Therefore more voltage will be induced in S1 and less in S2 i . e . , e s 1

will be larger than e s 2 . Hence the differential output voltage is e0 = es1-es2 and is in phase with primary voltage.

But when the core is moved towards down from null position more magnetic field links with secondary winding S2 and small field links with secondary winding S1. 

Therefore more voltage will be induced in S2 and less in S1, i.e.,es2 will be larger than es1. Hence, the differential output voltage is e0 = es2 – es1 and is 180° out of phase with primary voltage. 

Thus the output voltage e0 position of the core and hence the displacement applied to the arm of the core.

   Merits of  LVDT :-

1. LVDT has good linearity i.e.. it produces linear output voltages.

2. It can measure displacements of very high range (usually from 1.25 mm to 250 mm.)

3. LVDT has high sensitivity.

4. Since it produces high output, it does not require amplifier devices.

5. LVDT has low hysteresis.

6. It consume less power (about < 1w)

Demerits:-

1. It is sensitive to stray magnetic fields.

2. Performance of LVDT is affected by variations in temperature.

3. It has limited dynamic response.

4. To provide high differential output, it requires large displacements.