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Home Electrical and Electronics Electrical To measure the displacement and to determine the characteristics of LVDT (Linear Variable Differential Transformer).
Electrical Lab Experiments

To measure the displacement and to determine the characteristics of LVDT (Linear Variable Differential Transformer).



Aim

To measure the displacement and to determine the characteristics of LVDT (Linear Variable Differential Transformer).

Apparatus Required:

LVDT, Digital displacement indicator, Calibration jig (with micrometre).

Theory: LVDT (Linear Variable Differential Transformer)

The most widely used inductive transducer to translate the linear motion into electrical signals is the linear variable differential transformer (LVDT). The basic construction of LVDT is shown in Figure 1.

Figure 1. Linear Variable Differential Transformer

The transformer consists of a single primary P and two secondary windings S1 and S2 wound on a cylindrical former. The secondary windings have equal number of turns and are identically placed on either side. A moveable soft iron core is placed inside the transformer. The displacement to be measured is applied to the arm attached to the soft iron core. In practice the arm is made of highly permeability, nickel iron which is hydrogen annealed. This gives low harmonics low null voltage and high sensitivity. This is slotted longitudinally to reduce eddy current losses. The assembly is placed in stainless steel housing and the end leads provides electrostatic and electromagnetic shielding. The frequency of AC applied to primary windings may be between 50 Hz to 20 kHz. Since the primary winding is excited by an alternating source, it produces an alternating magnetic field which in turn induces alternating current voltage in the two secondary windings. Figure 2 depicts a cross-sectional view of an LVDT. The core causes the magnetic field generated by the primary winding to be coupled to the secondary. When the core is centred perfectly between both secondary and the primary as shown, the voltage induced in each secondary is equal in amplitude and 180 degree out of phase. Thus the LVDT output (for the series-opposed connection shown in this case) is zero because the voltage cancels each other. E0 = Es1-Es2 = 0.

Figure 2. View of LVDT Core and Windings

Displacing the core to the left causes the first secondary to be more strongly coupled to the primary than the second secondary. The resulting higher voltage of the first secondary in relation to the second secondary causes an output voltage that is in phase with the primary 94 voltage. Likewise, displacing the core to the right causes the second secondary to be more strongly coupled to the primary than the first secondary. The greater voltage of the second secondary causes an output voltage to be out of phase with the primary voltage.

Procedure:

  1. Plug power chord to AC mains 230 V, 50 Hz and switch on the instrument.
  2. Place the READ/CAL switch at READ position.
  3. Balance the amplifier with the help of zero knob so that display should read zero without connecting the LVDT to instrument.
  4. Replace the READ/CAL switch at CAL position.
  5. Adjust the calibration point by rotating CAL knob so display should read 10.00 i.e., maximum calibration range.
  6. Again keep the READ/CAL switch at READ position and connect the LVDT cable to instrument.
  7. Make mechanical zero by rotating the micrometre. Display will read (00.00) this is null balancing.
  8. Give displacement with micrometre and observe the digital readings.
  9. Plot the graph of micrometre reading v/s digital reading.
Model Graph

Tabulations:

Micrometer Displacement (mm) Core Displacement (mm) Secondary Output Voltage(V)
1
2
3
4
5
6
7

Graph

Result

Post Lab Questions

  1. What are the three principles of Inductive transducers?
  2. What are the limitations of LVDT?
  3. Where LVDT is used?
  4. What are the different types of transducers used for displacement measurement?
  5. What is the difference between variable resistance & variable inductance displacement transducer?











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