Construction of DC machine
Today I give a brief information about the construction of DC machine. What are the essential part of it and how it made?
DC machine is actually an alternation current machine but furnished with a special device, called the Commutator which under certain conditions converts ac into dc and vice-versa.
DC motors are often used in applications requiring a wide range of motor speeds or precise control of motor output - rolling mills, overhead cranes, and traction; drives for process industry, battery driven vehicles, machine tools requiring precise speed control, in a miniature range of tape recorders, cameras etc. Small dc motors are widely employed in control applications. Small dc generators are used for power supply in ships, air-crafts, automobiles and other vehicles isolated from inland ac network system.
DC machine is actually an alternation current machine but furnished with a special device, called the Commutator which under certain conditions converts ac into dc and vice-versa.
DC motors are often used in applications requiring a wide range of motor speeds or precise control of motor output - rolling mills, overhead cranes, and traction; drives for process industry, battery driven vehicles, machine tools requiring precise speed control, in a miniature range of tape recorders, cameras etc. Small dc motors are widely employed in control applications. Small dc generators are used for power supply in ships, air-crafts, automobiles and other vehicles isolated from inland ac network system.
Main part of a DC machine
In DC machine there are many parts but here we consider only about field magnets, armature, commutator, brush and brush gear. In below fig. all part are shown.
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| DC Machine |
- Field system
The purpose of field system is to provide uniform magnetic field, in which armature can rotate. Electromagnets are more suitable in comparison with permanent magnets on account of their greater magnetic effects and field strength regulation, which can be achieved by controlling the magnetizing current. Field magnet consists of four parts given below: i) Yoke ii) Pole cores iii) Pole shoes and iv) Magnetizing coils.
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| Field System yoke |
yoke
The cylindrical yoke is usually used which acts as a frame of the machine and carries the magnetic flux produced by the poles. There is no need of lamination because magnetic field in the yoke is stationary. Due to cost consideration for small machine cast iron is used as yoke material. And for large machine fabricated steel is used. For a small machine, the yoke may be one piece but for larger machines, it is usual to employ a split yoke. In small machines, the cheapness is the main consideration, and not the weight but in large machines, weight is the main consideration. So the permeability of cast steel is about twice of cast iron, the weight of cast steel required will be only half of the cast iron if used for the same reluctance.
Pole core
Pole core is usually of circular section and is used to carry the coil of insulated wires carrying the exciting current. They are made from cast steel. In some machine, pole core is made of the laminated core with a thickness of 1mm.
Pole shoe
The pole shoe gives support to the field coils and uniformly spreads the flux over the armature periphery. The pole shoe is the essential part of field poles. The field poles are usually formed of laminations and are bolted to the frame or yoke to which are also fastened end bells with their bearings and the brush rigging. In some machines, the poles are made integral with the yoke of cast iron due to its low cost and less machining required by individual parts. in some machine, the yoke and pole core are made in single 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 the flux distribution on the pole face due to movement of armature slots and teeth.
more numbers of poles reduce 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 numbers of poles distortion of a field from is not excessive. but with more number of pole (1)frequency of flux reversals is increased thereby increasing iron losses (2)labor charges are increased and (3) tendency of flash-over between brush arms is increased.
there are several field constructions adopted according to the type of excitation. in shunt field, many turns of five wire are used, in a series field few turns of a large cross sectional area are used and in a compound filed both shunt and series winding are used. shunt coils are usually wound with double cotton covered wire. the field coils, after proper winding, are dipped in an insulating varnish and baked in an Owen, which provides stiffness, mechanical strength and good insulating properties to the windings.
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 maximums 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 increase in a 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.
more numbers of poles reduce 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 numbers of poles distortion of a field from is not excessive. but with more number of pole (1)frequency of flux reversals is increased thereby increasing iron losses (2)labor charges are increased and (3) tendency of flash-over between brush arms is increased.
there are several field constructions adopted according to the type of excitation. in shunt field, many turns of five wire are used, in a series field few turns of a large cross sectional area are used and in a compound filed both shunt and series winding are used. shunt coils are usually wound with double cotton covered wire. the field coils, after proper winding, are dipped in an insulating varnish and baked in an Owen, which provides stiffness, mechanical strength and good insulating properties to the windings.
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 maximums 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 increase in a 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.
2. Armature number of pole
It is a rotating part of a dc machine and is built up in a cylindrical or drum shape. The purpose of the armature is to rotate the conductors in the uniform magnetic field. When the armature rotates in a magnetic field such arrangement is done that current is induced in the coil wound on an iron core drum. The main function of the armature is to provide a low reluctance path for magnetic flux. The material used in the construction of armature is high permeability silicon-steel stampings, each stamping, being separated from its neighboring one by thin paper or a thin coating of varnish as insulation.
To prevent rubbing in the armature small air gap exists between the pole pieces and the armature.
But this air gap length is about 1mm to 6mm small because larger the air gap greater is the MMF required the create the required flux.
There is two benefit to using high-grade steel (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) the eddy currents in the core which are induced by the rotation of the core in the magnetic field. These losses are reduced by using laminations and stampings to cut the path of eddy current in several units. The size of each lamination is about 0.3 to 0.6mm thick.
The slots are the normally open type and usually placed parallel to the axis of the armature but are also sometimes skewed at a smack angle to the axis to avoid vibration of teeth. the width and depth of the slots are obtained to accommodate the conductors and the insulation.
core punchings up to a diameter of 0.5m are generally made in one piece. these core punchings 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.
for larger machines having large armature diameter, it is not economical to punch out a complete ring. a core of larger diameter is built up of segmental laminations that are attached to the spindle 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 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.
the armature is supported at each end by a metal frame work called the end bells. the end balls contains 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.
core punchings up to a diameter of 0.5m are generally made in one piece. these core punchings 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.
for larger machines having large armature diameter, it is not economical to punch out a complete ring. a core of larger diameter is built up of segmental laminations that are attached to the spindle 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 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.
the armature is supported at each end by a metal frame work called the end bells. the end balls contains 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
This part is placed between armature and external circuit. The function of the commutator is reverse the connections to the external circuit at the instant of each reversal of current in the armature coils. It acts like a rotating switch.
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| Commutator |
There are three purposes of commutator
i) It provides the electrical connection between the rotating armature coils and the stationary external circuit.
ii) When the armature is rotated, commutator acts like a switch and perform action reversing the electrical connections between the external circuit and each armature coil in turn so that the armature coil voltage adds together and result in a dc output voltage.
iii) It also keeps the rotor MMF stationary in space.
the commutator is essential 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 1mm 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 segment prohibitive. on account of cost also mica nite 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 the 'under cutting mica'.
The copper is insulated from the Vee-rings and the hub by mica nite carefully molded 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 diameter do not differ much, the winding ends are directly soldered to the commutator bars. otherwise, they are soldered with copper lugs or risers. the risers have air spaces between them so that air is drawn across the commutator thereby keeping the commutator cool.
the commutator is pressed on to the armature shaft, and the outer periphery is then machined to provide a smooth surface with which a stationary carbon brush can maintain continuous contact with 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 use of brushes is to collect current from the commutator and supply it to the external load circuit. The shape of brushes does not rectangular and place at on the commutator. They are made from carbon, carbon graphite, graphite, metal graphite, and copper. The rating of carbon brush is 5 A per square cm and for copper brush 23 A per square cm.
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| 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 consequent damage to the commutator. to prevent this, the mica is frequently "undercut " to a level below the commutator surface by means of a narrow milling cutter.
sooner or later, commutators generally wear out of true and must be turned down on a lathe, but Interpol 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 collect carbon or graphite dust from the brushes and hold it in the grooves above the mica and thus provide leakage paths for the current from segment to segment, which way 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 which are mounted on the brush-holder studs or brackets. in turn, the brush-holders studs are mounted on a brush yoke or rocker arm. the brush-holders can be rotated so as to change the position of the whole brush system in relation to the machine poles. the brush-holders 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.
5. Bearing
In small machine ball bearing is used at both ends. For the large machine, roller bearing is used at the driving end, and ball bearing at the non-driving end. Sometime thrust bearings are used to prevent excessive thrust.
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