What is Linear Variable Differential Transformers(LVDT)

As the core moves, these mutual inductances change, causing the voltages induced in the secondaries to change. The coils are connected in reverse series, so that the output voltage is the difference (therefore, “differential”) between the two secondary voltages. When the core is in its central position, equidistant between the two secondaries, equal but opposite voltages are induced in these two coils, so the output voltage is zero. When the core moves in one direction, the tension in one coil increases as others decrease, which causes the output voltage to increase from zero to a maximum. This voltage is in phase with the primary voltage. When the core moves in the other direction, the output voltage also increases from zero to a maximum, but its phase is opposite to that of the primary.The magnitude of the output voltage is proportional to the distance moved by the core (up to its travel limit), so the device is described as “linear”. The voltage phase indicates the direction of travel. Because the sliding core does not touch the inside of the tube, it can move without friction, so the LVDT is a highly reliable device. The absence of sliding or rotating contacts allows the LVDT to be completely sealed against the environment.