What is a Resolver : Working and Its Applications

A resolver is an electromechanical device like an encoder and the main function of this device is to change the mechanical motion to an electronic signal. But, not like an encoder, it broadcasts an analog signal instead of digital. It is a rotating transformer including three windings namely one primary and two secondary and phased with 90 degrees. The main specifications are its no. of speeds and single speed output. These are used in brushless AC servo motors with a permanent magnet, aerospace, and military applications. This article discusses an overview of a resolver, construction, working, types, and applications.

What is Resolver?

Definition: A rotating electrical transformer that is used to measure degrees of rotation is known as a resolver. It includes digital counterparts like the rotary encoder and the digital resolver. It is used in different velocity and position feedback applications because of their good performance like servo motor feedback, light-duty, heavy-duty and light industrial. These are also called as a motor resolver.


It is an analog device & the electrical outputs of this device are continuous during one whole mechanical rotation. It is a rugged device compared with other feedback devices due to its simple transformer design. It is applicable where the consistent performance is necessary for those vibration, radiation, high shock, high temperature as well as contagion environments. Generally, the selection of this mainly determined by the size of the shaft, transformation ration, and excitation frequency.

Construction of Resolver

It is a special kind of rotating transformer, includes a stator and rotor in a cylindrical shape. These are designed with two sets of windings and multi-slot laminations. Usually, these windings are designed as well as distributed within the slotted lamination through a stable pitch-variable twist otherwise changeable pitch-variable twist model. For a single speed type, the windings will create one whole Sine curve & Cosine curve in one rotation whereas, for a multi-speed type, the windings will create various Sine curves & Cosine curves within one rotation.

Resolver Construction
Resolver Construction

Whenever a single-speed gives complete feedback but multi-speed doesn’t give. The number of speeds obtainable is imperfect with the resolver size. The set of windings is located within the laminations with 90o to each other, known as Sine & Cosine windings. Here, the accuracy can be enhanced once the set of windings within the rotor is shorted internally.

How Does a Resolver Work?

The resolver works on the principle of an electrical transformer. These transformers use copper windings in stator and rotor. Based on the rotor’s angular position, the inductive coupling of the windings will be changed. The resolver energizes by using an AC signal and the output of this can be measured to provide an electrical signal.

Generally, it includes three windings like one primary and two secondaries. These are designed with the help of copper wire on the stator. The primary winding functions like the i/p for an AC signal whereas each of the secondary winding is used as output. In this, the stationary part is designed with iron or steel.

The operation of this can be done by different operating parameters like accuracy, i/p excitation voltage, excitation frequency, maximum current, transformation ration, phase shift, and null voltage.

Types of Resolvers

These are classified into different types which are discussed below.

Receiver Resolver

These are used in the reverse way of transmitter resolvers. The two windings in this are energized and the electrical angle can be represented through the ratio of the sine wave & the cosine wave. In the rotor winding, the system revolves around the rotor to get a zero voltage. At this point, the rotor’s mechanical angle is equal to the applied electrical angle to the stator.

Differential Resolver

These types will merge two diphase main windings in one of the stacks of sheets, as with the receiver, and two diphase secondary windings in the other. The electrical angle relation can be delivered through two secondary windings & remaining angles are mechanical, primary and secondary electrical,

Classical Type

It includes three windings where the primary winding is placed on the rotor whereas the secondary windings placed on the stator.

Variable Reluctance Type

It includes primary and secondary winding at the stator and there is no winding at the rotor

Computing Type

This is used to generate the functions of sine cosine & tangent. By using this, the geometric relationships can be solved.

Synchro Type

It is used in data transmission to perform different functions like transmitting; receiving. It is more precise compare with synchronous.

Difference between Encoder and Resolver

Both the resolver and encoder are used to measure the rotating point of a shaft, changing a mechanical position into an electrical signal. The differences between these two are discussed below.



  It is a solid-state device, used to generate a digital output.It is a rotary transformer, used to gauge degrees of rotation
  It is used in applications that have deceleration rates & high acceleration.It is used in harsh environments including shock loading resistance and high vibration compare with an encoder.
  Less weight & rotational inertia compare with a resolver.It can resist a high temperature because of no solid-state electronics.
Nor durableMore durable
Its accuracy is in a 20 arc seconds range.Accuracy is 3 arc minutes

Advantages & Disadvantages

The advantages of resolver include the following.

  • Accurate
  • Reliable
  • Tolerant to Misalignment
  • Robust
  • Durability

The disadvantages of resolver include the following.

  • Expensive
  • Heavy
  • Requires skillful specification & implementation
  • Bulky

Where is Resolver Used?

The applications of resolver include the following

  • It is used in a harsh environment & extreme applications because of the design
  • These are used in the feedback of the servo motor
  • Surface actuators
  • Used in paper and steel mills for speed & position feedback
  • Control systems of military vehicles
  • Communication position systems
  • Fuel systems of the Jet engine
  • Production of gas and oil
  • It is used in vector resolution to split the vector into different parts
  • Vector angle & component can be determined
  • The amplitude of pluses and pulse resolution can be controlled


1). What is a resolver?

An electro-mechanical device used to convert the mechanical motion to an electronic signal.

2). What are the types of resolvers?

They are classical, variable reluctance, computing, and synchro.

3). What is the main difference between resolver and encoder?

Resolver is used to transmit an analog signal whereas an encoder is used to transmit digital signal

4). How to test a resolver?

An ohmmeter is used to test a resolver for checking the coils for resistance.

5). What are the benefits of using a resolver?

They are robust, reliable, accurate, etc.

Thus, this is all about an overview of resolver which generates a set of waves like sine or cosine. These waves indicate the complete position in an only revolution. These are used in the communication of permanent magnet motor, AC and DC servo motor and speed control. Here is a question for you, what is the input signal of a resolver?

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