Difference Between Rotor and Armature

In electrical machines like motors and generators, students and professionals often get confused between two terms: rotor and armature. While both are crucial components, they are not the same. Understanding the Difference Between Rotor and Armature helps in gaining clarity about how electrical machines work and why their design differs in DC and AC machines.

Table of Contents

What is a Rotor?

The rotor is the rotating part of an electric machine. Its primary role is mechanical—it physically turns to produce torque in a motor or to induce voltage in a generator. Depending on the type of machine, a rotor can have different designs:

Squirrel-cage rotor in induction motors.

Wound rotor with coils in some AC motors.
Rotor with field windings in synchronous machines.

In short, the rotor is defined by its motion—it is the component that rotates.

rotor- rotating part of electric machine

What is an Armature?

The armature is the component of an electrical machine where energy conversion takes place. It carries current and interacts with the magnetic field, either producing mechanical motion (in motors) or generating electrical energy (in generators).

Key features of the armature include:

It is made of coils of wire wound around an iron core.

It can be located on the rotor or the stator, depending on the machine.
In DC machines, the armature is usually on the rotor.
In AC machines, the armature is often on the stator while the rotor carries the field winding.

Unlike the rotor, the armature is defined by its electrical function, not by whether it rotates.

Rotor vs. Armature: Key Differences

Characteristic	Rotor	Armature
Basic Role	The rotating assembly of the machine.	The winding that interacts with the magnetic field for energy conversion.
Definition	Defined by motion (it always rotates).	Defined by function (carries current, produces/receives energy).
Composition	Can be magnets, squirrel-cage bars, or windings.	Coils of insulated copper wire wound on a laminated core.
Location	Always part of the rotating section.	Can be on the rotor (DC machine) or stator (AC machine).
Energy Conversion	Converts energy mechanically by turning.	Converts between electrical and mechanical energy directly.
Example	In induction motors, the rotor is a squirrel cage without armature winding.	In DC motors, the armature winding is placed on the rotor.

Relationship Between Rotor and Armature

In DC machines (motors and generators), the armature winding is placed on the rotor, while the stator holds the field winding.
In AC induction motors, the rotor is not an armature—instead, it is a cage or wound design, and the armature is on the stator.

In synchronous machines (like alternators), the field winding is on the rotor, while the armature winding is on the stator.

Thus, while every machine has a rotor, not every rotor has an armature.

Conclusion

The rotor refers to the rotating part of an electric machine, while the armature refers to the current-carrying part responsible for energy conversion. A rotating armature is a type of rotor, but not all rotors are armatures. In simple terms, the rotor is a mechanical term, whereas the armature is an electrical-functional term. Their roles may overlap in some machines but differ in others, depending on the design of the motor or generator.

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