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In an LVDT, the output voltage is zero when

  • A.The core is fully inserted into the primary coil
  • B.The core is at the null position
  • C.The core is displaced towards one secondary coil
  • D.The excitation voltage is zero

Answer: B

The output voltage is zero when the core is at the null position.

An LVDT has one primary coil and two secondary coils wound in series opposition. With the core centred, equal voltages are induced in both secondaries and they cancel, so the output is zero. Moving the core one way raises one secondary and lowers the other, and the output grows in proportion to the displacement, with its phase showing the direction.

A core moved towards one secondary gives a non-zero output, and 'fully inserted into the primary' is not the balanced position. Zero excitation also gives zero output, but only because the device is switched off, not because of the core's position.

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