DC machine-types of dc motor
/ Construction of Dc machine
- A d.c machine consist of main two parts
- Stationary part- It is designed mainly for producing a magnetic flux.
- Rotating part- It is called the armature, where mechanical energy is converted into electrical such as electrical generator.
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| dc machine |
The stationary and rotating parts are separated from each other by an air gap
- The stationary part of dc machine consist of main poles, designed to create the magnetic flux, commutating poles interposed between the main poles and designed to ensure sparkless operation of the brushes at the commutator in very small machines with a lack of space commutating poles are not used and frame/yoke.
- The armature is a cylindrical body rotating in the space between the poles and comprising a slotted armature core, a winding inserted in the armature core slots, a commutator, and brush gear.
The frame is the stationary part of a machine to which are fixed the main and commentating poles and by means of which the machine is bolted its bed plate. The right shape portion which serves as the part for the main and commutating pole fluxes is called yoke.
//** Cast iron used to be the material for the frame/yoke in early machines but now it has been replaced by cast steel. This is because cast iron is saturated by an flux density of about 0.8 wb/m square while saturation which cast steel is at about 1.5 wb/m square. Thus the cross-section of a cast iron frame is about twice that of a cast steel frame for the same value of magnetic flux. Hence, if it is necessary to reduce the weight of machine, cast steel is used. Another disadvantage with the use of cast iron is that its mechanical and magnetic properties are uncertain due to the present of below holes in the casting. Lately, rolled steel yokes have been developed with the improvements in the welding techniques. The advantage of fabricated yokes are that three are no pattern changes and magnetic and mechanical properties of the frame are absolutely consistent.
//** Cast iron used to be the material for the frame/yoke in early machines but now it has been replaced by cast steel. This is because cast iron is saturated by an flux density of about 0.8 wb/m square while saturation which cast steel is at about 1.5 wb/m square. Thus the cross-section of a cast iron frame is about twice that of a cast steel frame for the same value of magnetic flux. Hence, if it is necessary to reduce the weight of machine, cast steel is used. Another disadvantage with the use of cast iron is that its mechanical and magnetic properties are uncertain due to the present of below holes in the casting. Lately, rolled steel yokes have been developed with the improvements in the welding techniques. The advantage of fabricated yokes are that three are no pattern changes and magnetic and mechanical properties of the frame are absolutely consistent.
- //It may be advantageous to use cast iron for frames but for medium and large sizes usually rolled steel is used.
*The end shield bearings, and sometimes the pedestal bearings, are of ball or rolled type.However, more frequently plane pedestal bearing are used.In machines with large diameter armatures a brush-holder yoke is frequently fixed to the frame.
/Field poles
**** Formerly the poles were cast integral with the yoke. This practice is still being followed for small machines. But in present day machines it is usual to use either a completely laminated pole,. or solid steel poles with laminated pole shoes.
*** Laminated construction is necessary because of the pulsations of field strength that result when the notched armature rotor magnetic structure passes the pole shoe. Variations in field strength result in internal eddy currents being generated in a magnetic structure.. they eddy current cause losses, they may be largely prevented by having laminated magnetic structures. Laminated structure allow magnetic flux to pass along the length of the lamination, but do not allow electric eddy currents to pass across the structure from one lamination to another. The under end of the laminated pole is curved to fit very closely into the main frame..
*** Different method are used for attaching poles to the yoke.In case of big size,smaller size the back of poles is drilled and trapped to receive pole bolt.In larger sizes, a circular or a rectangular pole bar is fitted to the pole. This pole bar is drilled and tapped and the bolts passing through laminations screw the tapped bar.
*** Laminated construction is necessary because of the pulsations of field strength that result when the notched armature rotor magnetic structure passes the pole shoe. Variations in field strength result in internal eddy currents being generated in a magnetic structure.. they eddy current cause losses, they may be largely prevented by having laminated magnetic structures. Laminated structure allow magnetic flux to pass along the length of the lamination, but do not allow electric eddy currents to pass across the structure from one lamination to another. The under end of the laminated pole is curved to fit very closely into the main frame..
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| dc machine yoke/body part |
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