Demystify DC Machine Anatomy: Constructional Features Explored

Two basic parts: Stator (which standstill) and Rotor (which rotates)
Stator has salient poles that are excited by one or more field windings
The armature winding is located on the rotor with current flowing through it by carbon brushes making contact with copper commutator segments
Both the main poles and armature core are made up of laminated materials to reduce core losses.
Big dc machines also have commutating poles between the main poles of the stator.
Each commutating pole has its own winding known as commutating winding (interpoles or compoles)

The field windings are located around the pole cores and are connected in series and/or in shunt (i.e., in parallel) with the armature circuit.
The shunt winding is made up of many turns of relatively thin wires, whereas the series winding has only a few turns and is made up of thicker wires
The series and shunt windings are located on the d-axis (called the field axis), or direct axis, because the air-gap flux distribution due to the field windings is symmetric at the center line of the field poles
Both the compensating and commutating winding brushes are located on the q-axis (called the quadrature axis) because it is 90 electrical degrees from the d-axis and represents the neutral zone

The complete assembly of various parts in a scattered form of a DC machine and the essential parts of a DC machine are shown

The outer cylindrical frame to which main poles and inter poles are fixed is called yoke.
It also helps to fix the machine on the foundation.
It serves two purposes:
1. It provides mechanical protection to the inner parts of the machine.
2. It provides a low reluctance path for the magnetic flux.
Yoke is made of
- smaller machines: cast iron
- larger machines : cast steel or fabricated rolled steel
- since these materials have better magnetic properties as compared to cast iron.

Fixed to the magnetic frame or yoke by bolts.
They serve the following purposes:
1. Support the field or exciting coils.
2. Spread out the magnetic flux over the armature periphery more uniformly.
3. Since pole shoes have larger X-section, the reluctance of magnetic path is reduced.
Usually made of thin cast steel or wrought iron laminations which are riveted together under hydraulic pressure.

Enamelled copper wire is used for the construction
The coils are wound on the former and then placed around the pole core.
When DC is passed through the field winding, it magnetises the poles which produce the required flux.
The field coils of all the poles are connected in series in such a way that when current flows through them, the adjacent poles attain opposite polarity

It is cylindrical is shape and keyed to the rotating shaft.
At the outer periphery slots are cut,which accommodate the armature winding.
The armature core serves the following purposes:
1. It houses the conductors in the slots.
2. It provides an easy path for magnetic flux.

Since armature is a rotating part of the machine, reversal of flux takes place in the core, hence hysteresis losses are produced.
To minimise these losses silicon steel material is used for its construction.
When it rotates, it cuts the magnetic field and an emf is induced in it.
This emf circulates eddy currents which results in eddy current loss in it.
To reduce these losses, armature core is laminated, in other words we can say that about 0.3 to 0.5 mm thick stampings are used for its construction.
Each lamination or stamping is insulated from the other by varnish layer.

The insulated conductors housed in the armature slots are suitably connected known as armature winding.
The armature winding acts as the heart of a DC machine.
It is a place where one form of power is converted to the other form i.e., in case of generator, mechanical power is converted into electrical power and in case of motor, electrical power is converted into mechanical power.
On the basis of connections, there are two types of armature windings
- Lap winding
- Wave winding

Connections are such that the number of parallel paths is equal to number of poles.
Thus, if machine has P poles and Z armature conductors, then there will be P parallel, paths, each path will have Z/P conductors in series.
In this case, the number of brushes is equal to the number parallel paths.
Out of which half the brushes are positive and the remaining (half) are negative.

Connections are such that the numbers of parallel paths are only two irrespective of the number of poles.
Thus, if machine has Z armature conductors, there will be only two parallel paths each having Z/2 conductors in series.
In this case, the number of brushes is equal to two i.e., number of parallel paths.

It is an important part of a DC machine and serves the following purposes:

It connects the rotating armature conductors to the stationary external circuit through brushes.
It converts AC induced in the armature conductors into unidirectional current in the external load circuit in generator action, whereas, it converts the alternating torque into unidirectional (continuous) torque produced in the armature in motor action.

The commutator is of cylindrical shape and is made up of wedge-shaped hard drawn copper segments.
The segments are insulated from each other by a thin sheet of mica.
The segments are held together by means of two V-shaped rings that fit into the V-grooves cut into the segments.
Each armature coil is connected to the commutator segment through riser.

Brushes are pressed upon the commutator and form the connecting link between the armature winding and the external circuit.
They are usually made of high grade carbon because carbon is conducting material and at the same time in powdered form provides lubricating effect on the commutator surface.
Brushes are held in particular position around the commutator by brush holders and rocker.

It holds the spindles of the brush holders.
It is fitted on to the stationary frame of the machine with nut and bolts.
By adjusting its position, the position of the brushes over the commutator can be adjusted to minimise the sparking at the brushes.

Attached to the ends of the main frame and support bearings.
The front housing supports the bearing and the brush assemblies whereas the rear housing usually supports the bearing only

Bearings may be ball or roller bearings these are fitted in the end housings.
Their function is to reduce friction between the rotating and stationary parts of the machine.
Mostly high carbon steel is used for the construction of bearings as it is very hard material.

Made of mild steel with a maximum breaking strength.
Used to transfer mechanical power from or to the machine.
The rotating parts like armature core, commutator, cooling fan etc. are keyed to the shaft.