RVDT- Construction, Working, Application, Advantages and Disadvantages

Construction and working of RVDT


Introduction of RVDT:

RVDT full form stands for a Rotary variable differential transformer. A rotary variable differential transformer (RVDT) is an electro-mechanical transducer that provides a variable AC output voltage that is proportional to the angular displacement of its input shaft. As RVDT is an AC-controlled device, there is no electronic component inside it. Also, the electrical output of RVDT is obtained by the difference in secondary voltages of the transformer, so it is also called a Differential Transformer.


As in our previous article, we have already discussed the complete theory of LVDT. As we know RVDT is also an Inductive Transducer, not a Transformer. Its design and working are similar to LVDT. So for a better understanding of the concepts of RVDT, please follow our previous article titled as Construction and working of LVDT.

What is RVDT?

RVDT is an electro-mechanical inductive transducer that converts angular displacement into the corresponding electrical signal. It is the most widely used inductive sensor due to its high accuracy level. Since the coil of RVDT is designed to measure an angular position, it is also known as an angular position sensor. Unlike LVDT, RVDT is also a passive differential transducer.

RVDT Construction

The design and construction of RVDT is similar to LVDT. The only difference is the shape of the core in transformer windings. LVDT uses the soft iron core to measure the linear displacement whereas RVDT uses the Cam-shaped core (Rotating core) for measuring the angular displacement.

To understand the construction of RVDT in detail, please follow our previous article about LVDT construction.

RVDT Theory:

If we denote both the secondary voltages by Es1 and Es2  (see in below fig.) and also the sensitivity of RVDT is G. Then the angular displacement of the shaft will vary as:

The secondary voltage is determined with the help of the equation given below:


The differential output Es1 – Es2 will be determined as:

A total sum of voltages will be calculated as a constant C.

RVDT working principle

The working principles of RVDT and LVDT both are the same and based on the mutual induction principle. When AC excitation of (5-15) Volt at a frequency of 50-400 Hz is applied to the primary windings of RVDT a magnetic field is produced inside the core. This magnetic field induces a mutual current in secondary windings. Then due to transformer action, the induced voltages in secondary windings (S1 and S2) are Es1 and Es2  respectively. Hence the net output voltage will be the difference between both the induced secondary voltages.

Hence Output will be  E0 = Es1 – Es2.

RVDT construction and working

Now according to the position of the core, there are three cases that arise. So Let’s discuss these three cases one by one in detail.

  • Case 1: When the core is at the Null position: When the core is at the null position then the flux linkage with both the secondary windings will be the same. So the induced emf (Es1 and Es2 ) in both the windings will be the same. Hence the Net differential output voltage E0 = Es1 – Es2 will be zero (E0 = Es1 – Es2  = 0). It shows that no displacement of the core.
  • Case 2: When the core rotates in the clockwise direction: When the core of RVDT rotates in the clockwise direction. Then, in this case, the flux linkage with S1 will be more as compared to S2. This means the emf induced in S1 will be more than the induced emf in S2. Hence Es1 > Es2  and Net differential output voltage E0 = Es1 – Es2 will be positive. This means the output voltage E0 will be in phase with the primary voltage.
  • Case 3: When the core rotates in the anti-clockwise direction: When the core of RVDT rotates in the anti-clockwise direction. Then, in this case, the flux linkage with S2 will be more as compared to S1. It means the emf induced in S2 will be more than the induced emf in S1. Hence Es1 < Es2  and Net differential output voltage E0 = Es1 – Es2  will be negative. This means the output voltage E0 will be in phase opposition (180 degrees out of phase) with the primary voltage. 

Advantages of RVDT

The following are the main advantages of RVDT:

  • High Accuracy.
  • Compact and strong construction.
  • The consistency of RVDT is high.
  • Long life span.
  • Very high Resolution.
  • Low cost.
  • High durability.
  • Linearity is excellent.
  • The performance is repeatable.
  • Easy to handle.

Disadvantages of RVDT

The disadvantages of RVDT mainly include the following.

  • Since the output of RVDT is linear ( about +40 degrees or -40 degrees), it restricts its usability.
  • The contact among the measuring exteriors as well as the nozzle is not possible for all time.

Applications of RVDT

RVDT is most commonly used as a sensor nowadays, also it doesn’t experience any functional problems due to its contactless structure. Hence the main applications of RVDT include the following.

  • Actuators for controlling flight as well as engine.
  • Fuel valve as well as hydraulics.
  • Brake with a cable system.
  • Modern machine tools.
  • Nose wheel steering systems.
  • Weapon and Torpedo system.
  • Engine fuel control system.
  • Aircraft and avionics.
  • Engines bleed air systems.
  • Robotics

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Hey, I'm Satish Gupta an Engineer by profession and blogger by passion. I am writer and founder of this blog, Here I publish contents related to Electrical and Electronics Engineering..

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