What are the Essential Part of DC Machine | Explanation

What are the Essential Part of DC Machine | Explanation

April 6, 2018 pani

Here in this article we explain about essential parts of DC machine with construction details and image. What are the essential parts of the DC Machine? Essential parts are field magnet, armature, commutator, brush and brush gear, bearing and shaft.

 

DC machine (whether a generator or motor) with four poles is shown in Fig. 1. In construction, the dc machine consists of four parts mainly:

1) Field magnets

2) Armature

3) Commutator

4) Brush and brush gear.

 

Figure 1:4-Pole DC Machine

The disassembled dc machine is shown in Fig. 2.

 

Figure 2: DC Machine (Disassembled)

1. Field Magnet System

The object of the field system is to create a uniform magnetic field, within which the armature rotates.

 

Electromagnets are preferred in comparison with permanent magnets on account of their greater magnetic effect and field strength regulation, which can be achieved by controlling the magnetizing current.

 

Field magnet consists of four parts given below:

1.​ Yoke of Frame

2.​ Pole cores

3.​ Pole shoes and

4.​ Magnetizing coils.

 

Cylindrical yoke is usually used which acts as a frame of the machine and carries the magnetic flux produced by the poles. Since the field is stationary, there is no need to use laminated yoke for a normal machine. In small machines, cast iron yokes are used, because of cheapness, but yoke of a large machine is invariably made of fabricated steel due to its high permeability. In the case of small machines the cheapness is the main consideration, and not the weight, but in large machines, weight is the main consideration. Since the permeability of cast steel is about twice of cast iron, the weight of cast steel required will be only halt of the cast iron, if used for the same reluctance. The manufacturing process consists of rolling a steel slab around a cylindrical mandrel and then welding it at the bottom. The lifting eye, feet and the terminal box etc. are welded to the frame afterward. Such yokes possess sufficient mechanical strength and have high permeability. With small machines the yoke may be one piece, but for larger machines it is usual to employ a split yoke, the two halves being bolted together. This promotes ease in handling and convenience of assembly.

 

Pole core is usually of circular section and is used to carry the coils of insulated wires carrying the exciting (or field) current. Pole cores are usually not laminated and made of cast steel. In some machines, pole cores are laminated and composed of electrical grade steel sheets of about 1 mm thickness and insulated with respect to each other.

 

The pole shoe acts as a support to the field coils and spreads out the flux over the armature periphery more uniformly and also being of large cross-section reduces the reluctance of the magnetic path.

 

Figure 3: Cross-Section of Field System of a DC Generator

 

The field poles are usually formed of lamination’s (thin sheets of steel) and are bolted to the frame or yoke to which are also fastened the end bells with their bearings and the brush rigging. In a small machine the poles are cast integral with the yoke from cast iron due to its low cost and less machine required by individual parts. In some machines, the yoke and pole cores are made in signal casting and laminated pole shoes are attached to the pole cores. The pole faces or pole shoes are always laminated to avoid heating and eddy current losses caused by the fluctuations in flux distribution on the pole face due to movement of armature slots and teeth.

 

The objects of the magnetizing or field coils is to provide, under the various conditions of operation, the number of ampere-turns of excitation required to give the proper flux through the armature to induce by the desired potential difference. The magnetic flux produced by the mmf developed by the fields coils pass through the pole pieces, the air gap, the armature core and the yoke or frame. In Fig. 3.  The dotted lines indicate the mean flux path through the complete magnetic circuit. It will be seen that the flux divides through two paths from each pole through the yoke.

 

There are several field constructions adopted according to the type of excitation. In shut field, many turns of fine wire are used, in series field few turns of large cross-sectional area are used and in compound field both shunt and series winding’s are used. Shunt coils are usually wound with double cotton covered wires. The field coils, after proper winding, are dipped in an insulating varnish and baked in an oven, which provides stiffness, mechanical strength and good insulating properties to the winding’s.

 

Figure 4: Laminated Pole Core and Pole Shoe

 

In the design of a generator, the number of poles required by the field structure depends on the speed of the armature and the output for which the machine is designed. In a two-pole machine there are two voltage maximum per revolution of the armature, while in a four-pole machine there are four voltage maximums for one revolution. If the armature speed is kept constant, the number of poles determines the rate at which the individual coils cut the magnetic flux. Hence the output voltage increases with the increases in number of poles for a constant armature speed. In any generator, the field poles are always produced in pairs since a pair is necessary to produce a set of magnetic poles.

 

The number of poles reduces the weight of the core and yoke, overall diameter and length of the machine, length of commutator and cost of copper in the field and the armature. With more number of poles, distortion of field from is not excessive. But with a greater number of poles,

1. The frequency of flux reversals is increased thereby increasing iron losses.

2. Labour charges are increased and

3. The tendency of flash-over between brush arms is increased.

 

The upper limit to the number of poles is imposed by frequency which is of order of 20-30 Hz for large machines and not exceed 50 Hz for small ones. The lower limit is due to the current per brush arm. which should not exceed about 400 An otherwise an excessively long commutator will be required.

 

2. Armature

It is a rotating part of a dc machine and is built up in a cylindrical or drum shape. The purpose of armature is to rotate the conductors in the uniform magnetic field. It consists of coils of insulated wires wound around an iron and so arranged that electric currents are induced in these wires when the armature is rotated in a magnetic field. In addition, its most important function is to provide a path of very low reluctance to magnetic flux. The armature core is made from high permeability silicon-steel stamping’s, each stamping being separated from its neighboring one by thin paper or thin coating of varnish as insulation.

 

A small air gap exists between the pole pieces and the armature so that there will be no rubbing in the machine. However, this gap is kept as small as possible, since the larger the air gap greater the mmf required to create the required flux. The air gap length is about 1.0 mm to 6 mm (say 1 mm for a 1 kW machine, 1.5 to 1.75 mm for medium size machines and 6 mm for an 800 kW machine).

 

Figure 5: Longitudinal View of Armature

 

The use of high grade steel is made (a) to keep hysteresis loss low, which is due to cyclic change of magnetization caused by rotation of the core in the magnetic field and (b) to reduce the eddy currents in the core which are induced by the rotation of the core in the eddy currents is cut into several units. The lamination must be in such a direction that they are perpendicular to the paths of eddy currents and parallel to the flux. Each lamination is about 0.3 to 0.6 mm thick.

 

The slots are either die-cut or punched on the outer periphery of the circular core stamping and the key way is located on the inner diameter, as shown in Fig. 5 and 6.

 

Figure 6: Armature Lamination

 

The slots are normally open type and usually placed parallel to the axis of the armature but are also sometimes skewed at a small angle to the axis to avoid vibration of teeth. The width and depth of the slots are made to accommodate the conductors and the insulation.

 

Figure 7: lot with Insulating

 

Core punching’s upon a diameter of 0.5 m are generally made in one piece as shown in Fig. 6. These core punching’s are usually keyed directly to the shaft and punched with holes near the shaft to give longitudinal ventilating ducts. By the fanning action of the armature, air is drawn in through these ducts, thus producing efficient ventilation.

 

Figure 8: Assembled View of Armature Lamination’s

 

For larger machines having large armature diameter, it is not economical to punch out a complete ring.  Cores of larger diameter are built up of segmental (4 or 6 or even 8) laminations that are attached to the spider by means of a dovetail joint. The joints between segments are staggered in order to preserve the continuity of the magnetic circuit. Radial ventilating ducts through the core are formed by means of spacers placed at intervals of 50 to 100 mm. The width of ventilating ducts varies from 5 to 10 mm.

 

Figure 9: Segmental Lamination

 

The armature is supported at each end by a metal framework called the end bells. The end bells contain the bearings in which the armature rotates. One end bell is left open or made with a cover that can be removed to inspect the brushes. The open end bell also assists in the natural cooling of the machine. In some machines, the brush rigging is mounted to the end bell.

 

3. Commutator

The commutator is a form of rotating switch placed between the armature and the external circuit and so arranged that it will reverse the connections to the external circuit at the instant of each reversal of current in the armature coils.

 

It is a very important part of a dc machine and serves the following purposes:

1.  It provides the electrical connections between the rotating armature coils and the stationary external circuit.

2.  As the armature rotates, it performs a switching action reversing the electrical connections between the external circuit and each armature coil in turn so that the armature coil voltages add together and result in a dc output voltage.

 

3.  It also keeps the rotor or armature mmf stationary in space.

The commutator is essentially of cylindrical structure and is built up of wedge shaped segments of high conductivity hard drawn copper or drop forged copper. These segments are insulated from each other by thin layers of mica (usually of 0.5 to 1 mm thickness). Mica is to be preferred but cannot be used for large commutator because of the difficulty of obtaining large sheets, making the cost of large mica segments prohibitive. On account of cost also migrate is often used for small commutators. The segments are held together by means of two V-shaped rings that fit into the V- grooves cut into the segments.  Nowadays, usually the mica insulation is cut away between the bars to a depth of about 1.5 mm with the help of a special slotting tool. This process is called ‘undercutting the mica’.

 

Figure 10: Commutator

 

The copper is insulated from the Vee-rings and the hub by micanite carefully moulded to the exact shape required. Frequently the hub is not insulated, but sufficient clearance is left to avoid the need for this insulation.

 

If the armature and commutator diameters do not differ much, the winding ends are directly soldered to the commutator bars. Otherwise they are soldered with copper lugs or risers. The rises have air spaces between them so that air is drawn across the commutator thereby keeping the commutator cool.

 

Figure 11: Section view of Commutator Segment

 

The commutator is pressed on the armature shaft, and the outer periphery is then machined to provide a smooth surface with which a stationary carbon (or graphite or copper) brush can maintain continuous contact as the armature and commutator rotate. Great care is taken in building the commutator because even slight eccentricity will cause the brushes to bounce, causing undue sparking.

 

4. Brushes

The function of brushes is to collect current from the commutator and supply it to the external load circuit (the armature of the machine being connected to the external load circuit via the commutator and brushes). The brushes are rectangular in a variety of compositions and degrees of hardness to suit the commutations and degrees of hardness to suit the commutation requirements. They may be classified roughly as carbon, carbon graphite, metal graphite and copper. The allowable current density at the brush contact varies from 5 A per square cm in the case of carbon to 23 A per square cm in the case of copper.

 

Figure 12: Brushes

 

Copper brushes are employed only for machines designed for large currents at low voltages. Unless very carefully lubricated, they cut the commutator very quickly and, in any case, the wear is rapid. Graphite and carbon graphite brushes are self lubricating and are, therefore, widely used. Even with the softest brushes, however, there is a gradual wearing away of the commutator, and if the mica between the commutator segments does not wear down so rapidly as the segments do, the high mica will cause the brushes to make poor contact with the segments, and sparking will result, with the mica is frequently “undercut” to a level below the commutator surface by means of a narrow milling cutter.

 

Figure 13: Brush Holder

 

Sooner or later, commutators generally wear out of true and must be turned down in a lathe, but interpole machines with soft brushes and undercut mica will run for a long time without any maintenance on the commutator. No lubrication is applied to the surface of a commutator with undercut mica, because the lubricant will carbon or graphite dust from the brushes and hold it in the grooves above the mica and thus provides leakage paths for the current from segment to segment, which may ultimately develop into a short-circuit or “flash-over” on the commutator. All modern brushes, except the copper ones, contain enough graphite to provide adequate lubrication.

 

The brushes are housed in brush-holders (usually of the box type) which are mounted on the brush-holder studs or brackets. In turn, the brush-holder studs are mounted on a brush yoke or rocker arm. The brush-rocker can be rotated so as to change the position of the whole brush system in relation to the machine poles. The brush-holder studs are insulated from the brush yoke by means of insulating sleeves and discs. The brush yoke, brush holders and brushes make the brush gear.

 

Most of the motors have brushes radially placed, that is, their center line is radial to the commutator. This permits operation in both directions. Ever in non-reversing machines, in both directions the brushes are usually radial. However, in some machines the brushes are set in inclined position to avoid vibration which may result in sparking at the brushes.

 

The brushes are held under pressure over the commutator by a combination of brush holders and springs whole tension may be adjusted. It is important that the desired pressure be applied to the brushes (say 1.5-2.5 N/) to ensure satisfactory commutation-large pressure will cause heating of the commutator and brushes owing to friction whereas the low pressure may cause sparking owing to imperfect contact between the brushes and the commutator.

 

Figure 14: Staggering of Brushes

 

Brushes are staggered as illustrated in Fig. 14 in order to prevent ridge formation on the surface of the commutator. Each set of position brushes being staggered with relation to the previous set of positive brushes, and the negative brushes similarly arranged. This way each track on the commutator is covered by equal number of positive and negative brushes.

 

5. Bearing

With small machines, ball bearings may be used at both ends. For larger machines, roller bearings are used at the driving end, and ball bearings may be used at the non-driving end, i.e. at the commutator end. Thrust bearings are used where excessive end thrust is anticipated. Sleeve bearings, with ring lubrication are used for motors when very silent running is required. For large machines, pedestal bearings are generally used.

 

6. Shift

The shaft is made of mild steel with a maximum breaking strength. The shaft is used to transfer mechanical power from or to the machine. The rotating parts such as armature core, commutator, cooling fan etc. are keyed to the shaft.

 

Figure 15: Sectional View of Rotor Assembly of a DC Machine

 

Sectional view of dc rotor consisting of armature shaft, armature core, armature winding and commutator is illustrated in Fig. 15.