Rotary variable differential transformer

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A rotary variable differential transformer (RVDT) is a type of electrical transformer used for measuring angular displacement.

Most RVDT are composed of a wound, laminated stator and a salient two-pole rotor. The stator, containing four slots, contains both the primary winding and the two secondary windings. Some secondary windings may also be connected together.

[edit] Operation of RVDT's

The two induced voltages of the secondary windings, $V 1$ and $V 2$ , varies lineary to the mechanical angle of the rotor, θ:

$\theta\ = G \cdot\ \left( \frac{V_1 - V_2}{V_1 + V_2} \right)$

where $G$ is the gain or sensitivity. The second voltage can be reverse determined by:

$V_2 = V_1 \pm \ G \cdot\ \theta\$

The difference $V 1 - V 2$ gives a proportional voltage:

$\Delta\ V = 2 \cdot\ G \cdot\ \theta\$

and the sum of the voltages is a constant:

$C= \sum\ V = 2 \cdot\ V_0$

This constant gives the RVDT great stability of the angular information, independence of the input voltage or frequency, or temperature, and enables it to also detect a malfunction.

Although the RVDT can theoretically operate between ±45°, accuracy decreases quickly after ±35°. Thus, it operational limits lies mostly within ±30°, but some up to ±40°. Certain types can operate up to ±60°.

The advantages of the RVDT are :

low sensitivity to temperature, primary voltage & frequency variations
sturdiness
low cost
simple control electronics
small size

[edit] RVDT varieties

A RVDT can also be designed with two laminations, one containing the primary and the other, the secondaries. These types can operate on larger rotations.

A similar transformer is called the Rotary Variable Transformer and contains only one secondary winding giving only one voltage:

$V = G \cdot\ \theta\$