Construction of dc motor | yoke field armature commutator brushes
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In this article we will discuss about the construction of DC motor. Direct current (DC) motors were widely used in various industries. Once 3 Phase A.C generation and distribution became popular, use of dc motors get redued slightly. But there are certain applications where dc motor offers distinct advantages over 3 phase induction or synchronous motor. DC motor finds its use in rolling mills, overhead cranes, electric traction, drives for process industry, machine tools etc. So it is obvious that we should know the construction of dc motor. Like any other motor, dc motor also converts electrical energy to mechanical one. D.C motors are generally placed in open locations which is exposed to dust, dirt, moisture and mechanical damage. So the motor requires some special protective covering. In the next section we will discuss about the details construction of dc motor.
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Constructional parts of DC motor
A DC motor is constructed with several individual sections and each of them serves their individual duty.DC motor most commonly comprises of four main parts. These are :- Field /excitation system
- Armature
- Commutator
- Brush gear and bearings
The details description about these constructional parts are described as follows.
Field /excitation system:
The field system refers to that portion of motor from where the necessary magnetic fields are developed. This is the stationary part where copper coils are wound on the salient poles of the motor. The field provides necessary magnetization . This stationary part is called the stator of dc motor. The stator consists of a frame or yoke, poles and interpoles and field windings.
- Frame or yoke : The outer frame of a DC motor
is termed as yoke. It is a hollow cylinder which is usually made up of cast steel or rolled steel. The frame or yoke is not only a part of the magnetic circuit but it also provides mechanical support to the poles of the motor. Since field winding is stationary so there is no need to yoke lamination. In early ages, cast iron yokes were used which was later replaced by cast steel yokes . Now forged steel yokes are normally used. Such yokes possess sufficient mechanical strength and have high permeability. For smaller size dc motor we can use a single yoke , but a split yoke is generally used in case of large motor.
- Main field pole: The magnetic field of a DC motor consists of pole core and pole shoe. An even number of pole cores are bolted with the yoke. Each pole core has a pole shoe which is a curved structure . The pole shoe serves twin purpose :
i. They increases cross sectional area of the magnetic circuit and reduces its reluctance.
ii. They also helps in supporting the field coils.
The pole core is made up of cast iron or cast steel, but pole shoes are provided with laminations to reduce eddy current loss.
- Interpoles : Interopoles are placed in between two poles. Interpoles may be laminated or solid and are built of wrought iron or mild steel. They are fixed on yoke by means of screws. Interpoles are provided in order to achieve superior commutation.
Field windings : The pole cores are wound by copper windings
which is termed as field windings. The field coils are usually made up of copper wire or strip. Generally, strong insulation is provided with each coils to ensure the safety. The field winding is wound with both shunt and series coils. The field coils are connected in such a way that when current flows through it, alternate north (N) and south (S) poles are produced in the direction of rotation of the rotor.
which is termed as field windings. The field coils are usually made up of copper wire or strip. Generally, strong insulation is provided with each coils to ensure the safety. The field winding is wound with both shunt and series coils. The field coils are connected in such a way that when current flows through it, alternate north (N) and south (S) poles are produced in the direction of rotation of the rotor.Armature
The rotating part of dc motor is called the armature. A laminated cylindrical structure, named armature core is mounted upon the shaft of the armature. This armature is therotory paotion of a dc motor which can move. The entire rotor part comprises of armature core, armature windings and commutator.
- Armature core: Armature core is a laminated cylindrical structure
and it has several slots or grooves on its outer surface . Laminations are provided to reduce the effects of eddy currents and each grooves are insulated from each other. The insulation is very mush necessary because these slots contains insulated conductors . The armature core serves dual purpose of housing the armature coils where emf is induced and also provides a low reluctance path for magnetic flux . Armature core cannot be made up of solid iron piece and complete laminations must be provided . Stampings of armature core are about 0.35 to 0.5 mm thick and insulated with a thin coating of varnish. For large motors having large armature diameter, one single lamination is not cut but instead it is made up of four , six or eight segmental laminations. Armature core in case of large size motors is divided into a number of packets by radial ventilation spacers to have better cooling .
- Armature winding : Armature winding is an arrangement of conductors to develop desired emfs by relative motion in a magnetic field. These windings are first wound in the form of flat rectangular coils and are pulled into proper shape in a coil puller. The conductors are placed in the armature slots which are lined with a tough insulating material called Latheroide paper. Here normally two layer winding with diamond shaped coils are used.
According to the degree of closure produced by the winding , armature windings are of two types :
a) Open coil type and b) closed coil type . The closed coil windings are again of two types: Gramme -ring winding and Drum windings .
a) Open coil type and b) closed coil type . The closed coil windings are again of two types: Gramme -ring winding and Drum windings .
Armature coils can be connected to the commutator to form either lap or wave windings.
Lap winding : The ends of each armature coil are connected to adjacent commutator segment . Total number of parallel paths(A) is equal to the number of poles (P) i.e A = P.
Wave winding : The ends of armature coil are connected to commutator segments some distance apart. In case of wave winding A = 2 .
Commutator :
The commutator is a cylindrical structure 
and is built up of wedge shaped segments of high conductivity hard drawn copper . These segments are insulated from each other by thin layers of mica usually 0.5 to 1 mm thickness. The commutator is a form of rotating switch placed between the armature and the external circuit . Following purposes are served by the commutator :
and is built up of wedge shaped segments of high conductivity hard drawn copper . These segments are insulated from each other by thin layers of mica usually 0.5 to 1 mm thickness. The commutator is a form of rotating switch placed between the armature and the external circuit . Following purposes are served by the commutator :- It provides electrical connections between rotating armature coils and stationary external circuit.
- It collects current from armature conductors. It rectifies alternating current induced in the armature conductors into unidirectional current for external load circuit.
Brushes :
The function of brushes is to collect
current from the armature conductors and supply it to the external load circuit . The brushes are rectangular in shape and rests on commutator. Brushes are manufactured in a variety of compositions. They may be classified as carbon, carbon graphite and copper . Most of the motors have brushes radially placed, ie their centre line is radial to the commutator. The brushes are housed in a box type structure called brush holders. The brush yoke, brush holders and brushes form the brush gear. The brushes are held under pressure over the commutation by a combination of brush holders and spring whose tension may be adjusted.
current from the armature conductors and supply it to the external load circuit . The brushes are rectangular in shape and rests on commutator. Brushes are manufactured in a variety of compositions. They may be classified as carbon, carbon graphite and copper . Most of the motors have brushes radially placed, ie their centre line is radial to the commutator. The brushes are housed in a box type structure called brush holders. The brush yoke, brush holders and brushes form the brush gear. The brushes are held under pressure over the commutation by a combination of brush holders and spring whose tension may be adjusted.
Bearings : In small motors ball bearings are used at both ends. For larger motor, roller bearings are used at driving end and ball bearing at commutator end. Where end thrust is more thrust bearings are used. Large motors use pedestal bearings.
Specifications of dc motor
A metallic plate is provided with every DC motor which is placed in the outer covering of dc motor. All technical specifications are provided into this plate. The main specifications of a dc motor are given below :
- Rated output in H.P
- Voltage in volts
- Speed in rpm
- Type: series,shunt or compound
- Excitation voltage in volts
- Insulation class
- Temperature rise in degree centigrade
- Loading type
- full load current in Ampere
- Efficiency in percentage
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