Armature: Definition, Function And Parts (Electric Motor & Generator)

What is an Armature?

An armature is the winding in which the operating emf is induced in a motor or generator. It carries load alternating or direct current. Its physical location depends on the machine. In a conventional brushed Direct Current machine, the armature winding is usually on the rotor and connects through a commutator. In a large AC generator, the armature is usually the stationary stator winding. Electronic commutation controls phase current in a brushless DC motor, but that machine has no mechanical commutator.

The armature winding occupies slots in a laminated core and interacts with the machine’s magnetic field across the air gap. The armature can be on the rotor or the stator. A stator is always stationary, and a rotor always rotates.

The word armature came from an older term for the iron piece associated with a magnet. Modern machine documentation should identify the winding and its location instead of assuming every armature is a rotating DC assembly.

How Does an Armature Work in an Electric Motor?

A motor develops electromagnetic torque when a current-carrying conductor interacts with a magnetic field. The force direction can be found with Fleming’s left-hand rule. Induction creates rotor current in an induction motor, while DC and synchronous machines can receive armature current directly from their supply.

In a brushed DC motor, stationary field poles establish the air-gap flux. Brushes feed the rotating armature through the commutator. As each coil passes the magnetic neutral region, the commutator reverses its connection so the developed torque continues in the required direction. The stationary field does not need to rotate.

For any motor, torque results from the interaction between armature current and field flux. The shaft transfers converted mechanical power to the load. Copper loss, magnetic loss, brush drop, windage, friction and stray-load loss reduce the shaft output below the electrical input.

How Does an Armature Work in an Electric Generator?

A generator converts mechanical power to electrical power. Relative motion between conductors and magnetic flux induces emf according to Faraday’s law. A connected load then draws current from the armature winding.

Machine construction determines which part moves. A conventional DC generator has a rotating armature and stationary field. Most synchronous alternators place the three-phase armature winding on the stator and rotate a DC-excited or permanent-magnet field. The stationary high-voltage winding is easier to insulate and connect to the external circuit.

Emf induced in each rotating DC armature coil reverses as the coil passes successive poles. The commutator mechanically rectifies those coil connections, producing a unidirectional electric current at the brushes. An alternator has no armature commutator and supplies AC directly from its stator terminals.

Armature Parts & Diagram

The diagram and table describe a conventional rotating armature for a brushed DC machine. Its core, winding, commutator and shaft form one assembly. A stationary AC armature has a laminated stator core and winding but does not rotate on the shaft or use an armature commutator.

Part	Description
Core	A hollow cylindrical structure made of laminated silicon steel plates to reduce eddy current and hysteresis losses. It has slots on its outer surface to accommodate the armature winding.
Winding	A set of coils made of copper or aluminum wires that are insulated from each other and from the core. It can be a lap wound or a wave wound, depending on the number of current paths and voltage levels required.
Commutator	A cylindrical structure made of copper segments that are separated by insulating materials such as mica or plastic. It is pressed onto the shaft and aligned with the slots of the core. It connects each coil of the armature winding to a pair of brushes that slide on its surface.
Shaft	A rigid rod that supports and rotates the armature core and commutator. It transfers mechanical power from or to the prime mover or load device.

Armature Losses

Armature-related losses depend on whether the winding and core rotate or remain stationary. The three terms shown below cover winding copper and two core-loss components, but they do not represent every loss in a complete machine.

• Copper loss: Winding resistance produces I²R loss. Resistance must be referred to the winding operating temperature. Conductor area, length, parallel paths, cooling and allowable temperature rise determine the practical design. The displayed relation is:

Here Pc is armature-winding copper loss, Ia is the winding current under the stated circuit convention, and Ra is the armature resistance at the applicable temperature.

• Eddy-current loss: Time-varying flux induces circulating currents in conductive core material. Thin insulated laminations, suitable electrical steel and controlled flux density reduce this loss. Increasing the machine air gap is not a general treatment because it also changes excitation and performance. The displayed expression is a simplified proportional relation:

Here Pe is eddy-current loss, ke is an empirical coefficient for the material, waveform and chosen units, Bm is peak flux density, f is reversal frequency, t is lamination thickness and V is core volume. Measured loss data are normally used for final design.

• Hysteresis loss: Repeated magnetic-domain reversal dissipates energy in the core. Low-loss electrical steel and suitable flux density reduce it. The displayed Steinmetz-type relation is an approximation whose coefficient and flux-density exponent depend on material, frequency range, waveform and units:

Here Ph is the modelled hysteresis loss, kh is the empirical material coefficient, Bm is peak flux density, f is reversal frequency and V is the active core volume. Manufacturer core-loss curves or validated finite-element loss models provide better design data.

The displayed sum combines only these three armature terms. Complete-machine loss can also include brush contact, stray-load, field-winding, bearing, seal, fan and windage losses, depending on construction:

The next ratio is a generic input-output efficiency expression. Standards determine efficiency for the complete machine at a stated load, temperature and test method rather than assigning a universal standalone armature efficiency:

In this generic ratio, ηa denotes the defined efficiency, Po is output power and Pi is input power at the same operating point. State the boundary and included losses before using the value.

Armature Design

Armature design balances electromagnetic loading, thermal limits, mechanical stress, insulation life, commutation and manufacturing constraints. Important variables include:

• The number of slots: Slot and pole combinations affect winding reactance, harmonics, torque ripple, tooth flux density and manufacturability. More slots can improve waveform distribution but increase insulation space, tooling and winding complexity. No slot count is best for every machine.
• The shape of slots: Open, semi-closed and closed slots trade winding access against tooth-tip leakage, permeance variation, retention and cooling. The suitable shape depends on winding construction, voltage, speed and mechanical forces.
• The type of winding: In a commutator machine, Lap winding provides more parallel paths and suits lower-voltage, higher-current designs. Wave winding provides fewer parallel paths and suits higher voltages at lower current. Equalisers, commutation and current sharing still need design checks.
• The conductor size: Allowable current density follows from copper loss, cooling, duty, insulation temperature and slot fill. Raising current density can shrink the conductor but raises loss and temperature; lowering it uses more copper and slot area.
• The air-gap length: The gap between stator and rotor affects magnetising requirement, flux distribution, noise, unbalanced magnetic pull and mechanical clearance. A smaller gap can reduce excitation but demands tighter tolerances. A larger gap improves clearance while increasing magnetising demand.

Armature Design (continued)

The following equations are useful for preliminary checks when their machine and winding assumptions apply:

• EMF equation: For a conventional DC armature, generated emf depends on flux per pole, total active conductors, speed, pole count and parallel paths. Use the actual lap or wave winding connection and consistent units.

Here Ea is induced emf in volts, ϕ is useful flux per pole in webers, Z is the total active armature conductors, N is speed in revolutions per minute, P is pole count and A is the number of parallel armature paths.

• MMF equation: Armature magnetomotive force depends on ampere-conductors and winding distribution. The displayed DC-machine relation uses a stated parallel-path convention; designers then resolve its direct- and quadrature-axis effects as armature reaction.

Here Fa is the defined armature MMF in ampere-turns, Ia is armature current, Z is total active conductors and A is the parallel-path count. Confirm whether the equation gives total, per-pole or axis MMF before applying it.

• Torque equation: Mechanical power and angular speed give torque at the same operating point. Use converted electromagnetic power for developed torque or shaft output power for shaft torque, and account for losses between those boundaries.

Here T is torque in newton metres, P is the corresponding power in watts, and ω is angular speed in radians per second. The relation is undefined at zero speed and does not replace a starting-torque calculation.

Conclusion

The armature is the load-current winding of a rotating electric machine. A brushed DC machine normally has a rotating armature and commutator, while a large synchronous generator normally has a stationary armature and rotating field. Motor action produces torque; generator action induces emf.

Reliable design needs machine-specific electromagnetic, thermal, mechanical and insulation analysis. Standards determine whole-machine loss and efficiency by defined tests. Resistance, continuity and insulation checks provide useful evidence during maintenance, but a complete diagnosis can also require commutator inspection, surge or winding tests, vibration analysis, thermal checks and a controlled running test.