Constructional Features Of DC Machines - Electrical Machines - Electrical

Constructional Features of DC Machines

Overview

A direct current (DC) machine consists of two principal parts: a stationary magnetic structure called the stator (or field system) and a rotating part called the armature (or rotor) carried on a shaft. The stator produces the magnetic field; the armature carries conductors in which the motional or induced electromotive force (emf) appears and which provides the torque when current flows.

Stator (Field) Construction

The stator or field assembly of a DC machine is commonly built around a cylindrical iron body that provides the return path for magnetic flux. Because the field flux is essentially steady (DC), the stator back iron and the pole cores are normally made as solid iron parts rather than laminated stacks.

Field Poles and Pole Shoes

Each pole consists of a pole core and a pole shoe. The pole core supports the pole winding and provides the main magnetic path; the pole shoe spreads the flux in the air gap and smooths the flux distribution at the armature surface. Pole shoes also mechanically hold the field coils in place and are bolted to the stator core.

The pole shoe shape is selected to produce a desired air-gap flux density distribution on the armature surface: in many DC machines the flux density is approximately trapezoidal across the pole arc. The pole shoe therefore affects the air-gap flux wave-shape and the resultant induced emf waveform.

Lamination of Pole Parts

Although the pole core and stator back iron may be solid, the pole shoes are normally laminated. The reason is that the armature surface is slotted and the reluctance seen by the pole shoe varies as armature teeth and slots pass beneath it. This produces an alternating component of flux in the pole shoe at a frequency related to slot number and rotational speed. To limit eddy-current losses caused by this alternating component, pole shoes are laminated.

Field Windings and Excitation

Field windings are placed on the pole cores. Their design (number of turns and conductor size) depends on whether the winding is:

• Separately excited (supplied from an external DC source);
• Shunt (parallel) field (connected across the armature);
• Series field (in series with the armature);
• Compound (combination of shunt and series).

Shunt windings are made of many turns of thin wire to produce the required ampere-turns at relatively low current, while series windings use fewer turns of thicker conductor because they must carry the full armature current.

Armature Construction

The armature carries the conductors in which the emf is induced and through which current flows to produce torque. Constructional features of the armature differ significantly from AC machine rotors because the DC armature must be commutated and therefore uses a commutator and closed winding.

Armature Core and Lamination

The armature core is laminated because the flux linking the armature conductors changes with rotor position and the individual conductors carry alternating currents during commutation; lamination reduces eddy-current losses in the armature body.

Slots, Coils and Double-Layer Winding

The armature surface is slotted to hold conductor sides (coil sides). Most practical DC machines use a double-layer winding, where each slot contains two coil sides (one from the top layer and one from the bottom layer). For a double-layer arrangement, the usual practical convention is:

• The number of coils equals the number of slots (s).
• The number of commutator segments equals the number of coils.
• Each coil has two sides placed in two different slots.

A coil is connected to two commutator segments: one where the coil start terminates and one where the coil finish terminates.

Closed Winding and Commutation Requirement

The armature winding of a DC machine is a closed winding: all coil finishes and starts are connected together in a continuous circuit (no free ends). Because the direction of induced emf in each coil side changes as it moves under alternate poles, a rotating commutator is required to reverse the electrical connections of each coil to the external circuit at the appropriate times so that the external terminals see unidirectional output (in a generator) or so that torque is unidirectional (in a motor).

Lap Winding

In a lap winding the coil connections "lap" back to adjacent commutator segments. Main points for a lap winding:

• If the machine has p poles and s slots, then there are s coils and s commutator segments.
• The number of parallel paths a in a lap winding equals the number of poles, i.e. a = p.
• Consequently a lap-wound machine provides many parallel paths and hence a low terminal voltage but the ability to carry large current (suitable for low-voltage, high-current machines).
• In a p-pole machine, there are s/p commutator segments per pole; coils are grouped so that the total armature winding divides into p equal groups (one per pole).

Wave Winding

In a wave winding all pole pairs are connected in series to form long series paths around the armature. Main points for a wave winding:

• Between two adjacent commutator segments there are p/2 coils (for even p). For example, for p = 4 there are two coils between adjacent commutator segments.
• The number of parallel paths a in a wave winding is always 2, independent of the number of poles: a = 2.
• Wave winding therefore produces a higher terminal voltage and lower current per path compared with lap winding (suitable for high-voltage, low-current machines).
• Opposite brushes are placed approximately s/2 commutator segments apart so that approximately half the coils lie between opposite-polarity brushes.

Commutator and Brushes

The commutator is a cylindrical assembly mounted on the armature shaft. It consists of a number of copper segments, each insulated from the next by a thin layer of insulating material (commonly mica). The number of commutator segments equals the number of coils. Each coil connects to two commutator segments (start and finish).

Construction Details

• Each commutator segment has a raiser (a raised portion) to which the armature conductor is welded or attached; the flat outer surface of the segments forms the sliding contact for brushes.
• Brushes are usually made of carbon/graphite and are spring-loaded to maintain contact with the commutator surface. Graphite provides lubrication and reduces wear.
• Good contact and low sparking require careful surface finish of copper segments and correct brush material and spring pressure.

Interpoles and Compensating Windings

When a DC machine carries heavy armature current the armature reaction distorts the main field flux. This distortion affects the magnetic field distribution in the commutation region (the neutral or pole-tip zone), producing a net emf in coils undergoing commutation and causing sparking and poor commutation.

Interpoles

An interpole (also called a commutating pole) is a small auxiliary pole placed in the interpole region between each main pole. Interpoles are wound with a few turns of conductor and are connected in series with the armature so that their polarity and mmf change with armature current. The interpole mmf is set to neutralise the armature reaction in the commutation zone and thus to assist spark-free commutation.

Compensating Windings

A compensating winding is placed in slots in the pole face (embedded in the pole tips) and is connected in series with the armature. Its aim is to cancel the cross-magnetising component of armature reaction under the main poles so that the overall field distribution remains close to the intended shape. Compensating windings reduce overall distortion and improve load performance and commutation.

Side View and Assembly Notes

A typical side view of a DC machine (shaft, bearings, field poles, armature, commutator and brushes) shows how the parts are assembled:

• The armature is mounted on the shaft and supported by bearings at both ends.
• The field poles and pole shoes are bolted to the stator core; field windings are placed on the pole cores.
• The commutator is mounted concentric with the armature and rotates with it; brushes are mounted in holders and pressed onto the commutator surface by springs.
• Interpoles and compensating windings are provided where required for improved commutation and reduced distortion.

Summary of Primary Constructional Components

• Stator core: provides magnetic return path; usually solid where DC flux predominates.
• Field poles and pole shoes: carry field winding; pole shoes shape the air-gap flux; pole shoes laminated to reduce eddy losses from slotting harmonics.
• Field winding: shunt/series/separately excited arrangements provide flux; turn count and conductor size depend on excitation type.
• Armature core and winding: laminated core with slotted surface; double-layer closed windings (lap or wave) provide induced emf and current paths.
• Commutator: copper segments insulated by mica; reverses conductor connections for unidirectional terminal voltage or torque.
• Brushes: carbon/graphite brushes provide sliding contact to commutator and are spring-loaded.
• Interpoles: small poles between main poles, series-connected with armature, used to improve commutation.
• Compensating windings: embedded in pole faces and series-connected with armature to neutralise armature reaction under main poles.

Key Practical Relationships and Remarks

• Number of commutator segments = number of coils (for the usual double-layer armature winding).
• Lap winding: number of parallel paths a = p (suitable for low voltage, high current applications).
• Wave winding: number of parallel paths a = 2 (suitable for high voltage, low current applications).
• Pole shoes are laminated to reduce eddy losses resulting from the slotting of the armature surface and the resultant alternating component of flux seen by the pole shoe.
• Interpoles and compensating windings are connected in series with the armature so that their mmf varies with armature current and counteracts the effects of armature reaction.

Concluding Remarks

The constructional details of a DC machine - from choice of lamination, shape of pole shoe and field winding design, to armature winding type, commutator construction and use of interpoles/compensating windings - directly affect performance, efficiency and the quality of commutation. Proper selection and design of these elements are essential for reliable operation across the machine's intended voltage and current range.

In further study one derives expressions for induced emf and torque in DC machines and calculates ampere-turns required for interpoles and compensating windings to achieve satisfactory commutation under load.