What is an armature?
An armature is made up of a number of main components; the core, the commutator, the winding and the shaft.
About the core
The core of an armature is made up of lots of thin metal plates called laminations, which are usually about 0.5mm thick. The thickness of laminations depend upon the frequency at which the armature is designed to work.
Laminations are stamped out on a press. They are circular with a hole stamped out of the centre, through which the shaft is pressed, and stamped slots around the edge where the coils will eventually sit. Laminations are aligned and stacked together to produce the core.
Rather than using a solid piece of steel, the core is constructed from stacked laminations to reduce the amount of energy lost as heat in the core. The energy losses, known as iron losses, are caused by eddy currents which are small rotating magnetic fields forming in the metal due to the iteration of rotating magnetic fields which are established when the unit is running. If laminations are used the eddy currents can only form in one plane and this considerably reduces the losses.
The core is then pressed onto a shaft, usually held in place by having a coarse knurl on the shaft. Some older armatures have threads turned onto the shaft and are bolted together.
About the commutator
The commutator is pressed onto the shaft and is held on with a coarse knurl, in the same way as the core.
The commutator is constructed from copper bars, separated from one another by an insulating material. This insulating material is normally a thermoset plastic but sheet mica was used in older armatures.
The commutator has to be precisely aligned with the slots of the core when pressed onto the shaft as the wires from each of the coils will come out of the slots and connect to the commutator bars. For the magnetic circuit to work efficiently it is important that the coils in the armature have the correct angular displacement from the commutator bar to which it is connected.
About the windings
Before the winding process begins the slots in the core are insulated to stop the copper wire in the slots coming into contact with the laminated core.
Coils are inserted into the armature slots and connected to the commutator in turn. This can be done in many different ways depending on the design of armature.
There are normally two types of armatures, these are referred to as Lap Wound and Wave Wound.
In a lap winding the finishing end of one coil is connected to a commutator segment and to the starting end of the adjacent coil. In a wave winding the two ends of each coil are connected to commutator segments separated by the distance between the poles. This allows the series addition of the voltages in all the windings between brushes. This type of winding only requires one pair of brushes. In a lap wound armature the number of paths matches the number of brushes and poles.
In some armature designs there may be two or three separate coils in the same slot, connected to adjacent commutator segments. This is done if the voltage required across that coil is deemed to be too high. By spreading the voltage over three individual coils and segments all be it that the three coils are in the same slot, the field strength in the slot can be as high as if there were just the one coil, but it will reduce arcing on the commutator, and make the machine more efficient.
In many armatures the slots are also skewed, this is achieved by each lamination being slightly out of line. This is done to reduce cogging, and give a smoother rotation from one pole to the next.
About the shaft
The shaft is a stiff rod of concentrically machined steel mounted between two bearings which define the axis for components mounted onto it. It must be thick enough to transmit the torque required by the machine and stiff enough to control any out of balance forces. Its length, bearing points and speed are chosen to minimise harmonic distortion.
How does an armature operate?
The rotation of the armature is caused by the interaction of two magnetic fields. One magnetic field is produced by the field winding (in some machines, like wiper motors, the field winding is substituted with a permanent magnet). The second field is produced by the armature when a voltage is applied to the brushes contacting the commutator.
When a current passes through the armature winding, it establishes a magnetic field. This magnetic field is out of line with the field established by the field coil. This causes a force of attraction to one pole and repulsion from the other. As the field coil is fixed in place this force causes the armature to move. As the commutator is attached to the shaft it also moves by the same degree and switches the pole as it does so. The armature continues to try and chase the magnetic pole, causing it to spin.
If a voltage is not applied to the brushes but rather the field is excited and the armature driven mechanically, the armature (which has a complete circuit through all its coils by its connections on the commutator) will, by the action of the wires cutting the lines of magnetic flux produced by the field winding, generate a voltage in the armature windings. This voltage will be AC because it approaches passes and then moves away from the pole. But the commutator being attached to the shaft continually switches the polarity as it rotates, such that the actual output seen across the brushes is DC. This is how the armature works as a dynamo.