{"id":2218,"date":"2024-07-20T16:00:48","date_gmt":"2024-07-20T10:30:48","guid":{"rendered":"https:\/\/study.madeeasy.in\/?p=2218"},"modified":"2025-07-22T11:25:31","modified_gmt":"2025-07-22T05:55:31","slug":"dc-machine","status":"publish","type":"post","link":"https:\/\/www.madeeasy.in\/study\/ee\/electrical-machines\/dc-machine","title":{"rendered":"DC Machine Construction"},"content":{"rendered":"<p style=\"text-align: justify;\">A DC machine has two main parts stator and rotor. The stator of the machine does not move, i.e., stationary and normally is the outer part of the machine. The rotor is free to move and normally is the inner part of the machine. Slots are cut on the inner periphery of the stator and outer periphery of the rotor, while conductors are placed in these slots of rotor and stator. The winding in which voltage is induced is called the armature winding and the winding through which a current is passed to produce the main flux is called the field winding.<\/p>\n<p style=\"text-align: justify;\">Stator of a d.c. machine consists of yoke (or frame), field windings, interpoles, compensating winding, brushes and end covers. Rotor consists of armature core, armature winding, commutator and shaft. Stator components are described first.<\/p>\n<p style=\"text-align: justify;\"><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-2219 size-full\" src=\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/dc-machine.jpg\" alt=\"D.C Machine\" width=\"628\" height=\"340\" srcset=\"https:\/\/www.madeeasy.in\/study\/wp-content\/uploads\/2024\/07\/dc-machine.jpg 628w, https:\/\/www.madeeasy.in\/study\/wp-content\/uploads\/2024\/07\/dc-machine-300x162.jpg 300w\" sizes=\"auto, (max-width: 628px) 100vw, 628px\" \/><\/p>\n<h2 style=\"text-align: justify;\"><strong>Yoke :<\/strong><\/h2>\n<p style=\"text-align: justify;\">It has two functions :<\/p>\n<ul style=\"text-align: justify;\">\n<li>it provides path for the pole flux \u03c6 and carries half of it, i.e., \u03c6\/2;<\/li>\n<li>it provides mechanical support to the whole machine. Since the flux carried by <a href=\"https:\/\/study.madeeasy.in\/me\/theory-of-machines\/scotch-yoke\/\" target=\"_blank\" rel=\"noopener\">yoke<\/a> is stationary (i.e., constant), it is not laminated. As stated before, cast iron is used for small d.c. machines and fabricated steel for large d.c. machines. In case d.c. motor is to be operated through a power-electronics converter, the yoke is laminated to reduce the eddy-current losses.<\/li>\n<\/ul>\n<h2 style=\"text-align: justify;\"><strong>Field Poles :<\/strong><\/h2>\n<p style=\"text-align: justify;\">Field pole consists of pole core and pole shoe. The pole core is made from cast steel but the pole shoe is laminated and fixed to the pole core appropriately.<\/p>\n<p style=\"text-align: justify;\">Both pole core and pole shoe are made from thin laminations of sheet steel to reduce the eddy-current losses. The laminated pole is welded or bolted to the yoke.<\/p>\n<div id=\"ez-toc-container\" class=\"ez-toc-v2_0_79_1 ez-toc-wrap-left counter-hierarchy ez-toc-counter ez-toc-light-blue ez-toc-container-direction\">\n<div class=\"ez-toc-title-container\">\n<p class=\"ez-toc-title\" style=\"cursor:inherit\">Table of Contents<\/p>\n<span class=\"ez-toc-title-toggle\"><a href=\"#\" class=\"ez-toc-pull-right ez-toc-btn ez-toc-btn-xs ez-toc-btn-default ez-toc-toggle\" aria-label=\"Toggle Table of Content\"><span class=\"ez-toc-js-icon-con\"><span class=\"\"><span class=\"eztoc-hide\" style=\"display:none;\">Toggle<\/span><span class=\"ez-toc-icon-toggle-span\"><svg style=\"fill: #999;color:#999\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" class=\"list-377408\" width=\"20px\" height=\"20px\" viewBox=\"0 0 24 24\" fill=\"none\"><path d=\"M6 6H4v2h2V6zm14 0H8v2h12V6zM4 11h2v2H4v-2zm16 0H8v2h12v-2zM4 16h2v2H4v-2zm16 0H8v2h12v-2z\" fill=\"currentColor\"><\/path><\/svg><svg style=\"fill: #999;color:#999\" class=\"arrow-unsorted-368013\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" width=\"10px\" height=\"10px\" viewBox=\"0 0 24 24\" version=\"1.2\" baseProfile=\"tiny\"><path d=\"M18.2 9.3l-6.2-6.3-6.2 6.3c-.2.2-.3.4-.3.7s.1.5.3.7c.2.2.4.3.7.3h11c.3 0 .5-.1.7-.3.2-.2.3-.5.3-.7s-.1-.5-.3-.7zM5.8 14.7l6.2 6.3 6.2-6.3c.2-.2.3-.5.3-.7s-.1-.5-.3-.7c-.2-.2-.4-.3-.7-.3h-11c-.3 0-.5.1-.7.3-.2.2-.3.5-.3.7s.1.5.3.7z\"\/><\/svg><\/span><\/span><\/span><\/a><\/span><\/div>\n<nav><ul class='ez-toc-list ez-toc-list-level-1 ' ><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-1\" href=\"https:\/\/www.madeeasy.in\/study\/ee\/electrical-machines\/dc-machine\/#Field-or-Exciting-winding\" >Field (or Exciting) winding :<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-2\" href=\"https:\/\/www.madeeasy.in\/study\/ee\/electrical-machines\/dc-machine\/#Interpoles\" >Interpoles :<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-3\" href=\"https:\/\/www.madeeasy.in\/study\/ee\/electrical-machines\/dc-machine\/#Compensating-windings\" >Compensating windings :<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-4\" href=\"https:\/\/www.madeeasy.in\/study\/ee\/electrical-machines\/dc-machine\/#Brushes\" >Brushes :<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-5\" href=\"https:\/\/www.madeeasy.in\/study\/ee\/electrical-machines\/dc-machine\/#Rotor-components-are-now-described-below\" >Rotor components are now described below :<\/a><ul class='ez-toc-list-level-4' ><li class='ez-toc-heading-level-4'><a class=\"ez-toc-link ez-toc-heading-6\" href=\"https:\/\/www.madeeasy.in\/study\/ee\/electrical-machines\/dc-machine\/#Armature-core\" >Armature core :<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-4'><a class=\"ez-toc-link ez-toc-heading-7\" href=\"https:\/\/www.madeeasy.in\/study\/ee\/electrical-machines\/dc-machine\/#Armature-winding\" >Armature winding :<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-4'><a class=\"ez-toc-link ez-toc-heading-8\" href=\"https:\/\/www.madeeasy.in\/study\/ee\/electrical-machines\/dc-machine\/#Commutator\" >Commutator :<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-4'><a class=\"ez-toc-link ez-toc-heading-9\" href=\"https:\/\/www.madeeasy.in\/study\/ee\/electrical-machines\/dc-machine\/#Shaft\" >Shaft :<\/a><\/li><\/ul><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-10\" href=\"https:\/\/www.madeeasy.in\/study\/ee\/electrical-machines\/dc-machine\/#Electric-Braking-of-DC-Motors\" >Electric Braking of D.C. Motors<\/a><ul class='ez-toc-list-level-4' ><li class='ez-toc-heading-level-4'><a class=\"ez-toc-link ez-toc-heading-11\" href=\"https:\/\/www.madeeasy.in\/study\/ee\/electrical-machines\/dc-machine\/#Disadvantages-of-Mechanical-Braking\" >Disadvantages of Mechanical Braking:<\/a><\/li><\/ul><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-12\" href=\"https:\/\/www.madeeasy.in\/study\/ee\/electrical-machines\/dc-machine\/#Types-of-Electric-Braking\" >Types of Electric Braking<\/a><ul class='ez-toc-list-level-4' ><li class='ez-toc-heading-level-4'><a class=\"ez-toc-link ez-toc-heading-13\" href=\"https:\/\/www.madeeasy.in\/study\/ee\/electrical-machines\/dc-machine\/#Regenerative-Braking\" >Regenerative Braking<\/a><\/li><\/ul><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-14\" href=\"https:\/\/www.madeeasy.in\/study\/ee\/electrical-machines\/dc-machine\/#Regenerative-Braking-in-dc-Shunt-Motors\" >Regenerative Braking in dc Shunt Motors:<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-15\" href=\"https:\/\/www.madeeasy.in\/study\/ee\/electrical-machines\/dc-machine\/#Regenerative-Braking-in-dc-Series-Motors\" >Regenerative Braking in dc Series Motors<\/a><ul class='ez-toc-list-level-4' ><li class='ez-toc-heading-level-4'><a class=\"ez-toc-link ez-toc-heading-16\" href=\"https:\/\/www.madeeasy.in\/study\/ee\/electrical-machines\/dc-machine\/#Dynamic-Braking-or-Rheostatic-Braking\" >Dynamic Braking or Rheostatic Braking<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-4'><a class=\"ez-toc-link ez-toc-heading-17\" href=\"https:\/\/www.madeeasy.in\/study\/ee\/electrical-machines\/dc-machine\/#Plugging-or-Reverse-Current-Braking\" >Plugging or Reverse Current Braking<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-4'><a class=\"ez-toc-link ez-toc-heading-18\" href=\"https:\/\/www.madeeasy.in\/study\/ee\/electrical-machines\/dc-machine\/#Present%E2%80%93Day-Uses-of-DC-Machines\" >Present\u2013Day Uses of D.C. Machines<\/a><\/li><\/ul><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-19\" href=\"https:\/\/www.madeeasy.in\/study\/ee\/electrical-machines\/dc-machine\/#Previous-Year-Question-for-GATE-DC-Machines\" >Previous Year Question for GATE | DC Machines<\/a><\/li><\/ul><\/nav><\/div>\n<h3 style=\"text-align: justify;\"><span class=\"ez-toc-section\" id=\"Field-or-Exciting-winding\"><\/span><strong>Field (or Exciting) winding :<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p style=\"text-align: justify;\">The pole is excited by a winding wound around the pole core. This winding, called field or exciting winding, is prepared from copper. The number of turns and cross-section of field winding depend upon the type of d.c. machine as under :<\/p>\n<ul style=\"text-align: justify;\">\n<li>For d.c. shunt machine large number of turns of small cross-section are used.<\/li>\n<li>For d.c. series machine, small number of turns of large cross-section are used.<\/li>\n<li>For d.c. compound machine, both shunt (thin wire) and series (thick wire) field windings are used.<\/li>\n<\/ul>\n<h3 style=\"text-align: justify;\"><span class=\"ez-toc-section\" id=\"Interpoles\"><\/span><strong>Interpoles :<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p style=\"text-align: justify;\">These are fixed to the yoke in between the main poles of a d.c. machine. These are usually tapered with sufficient sectional area at the root to avoid <a href=\"https:\/\/study.madeeasy.in\/ee\/types-of-laws\/magnetic-circuits\/\" target=\"_blank\" rel=\"noopener\">magnetic<\/a> saturation. The interpole winding, consisting of a few turns of thick wire, is connected in series with the armature so that its magnetomotive force is proportional to armature current.<\/p>\n<h3 style=\"text-align: justify;\"><span class=\"ez-toc-section\" id=\"Compensating-windings\"><\/span><strong>Compensating windings :<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p style=\"text-align: justify;\">These windings are placed in the slots cut in the pole faces of a d.c. machine. Compensating winding is also connected in series with the armature circuit.<\/p>\n<h3 style=\"text-align: justify;\"><span class=\"ez-toc-section\" id=\"Brushes\"><\/span><strong>Brushes :<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p style=\"text-align: justify;\">Brushes are housed in box-type brush holders attached to the stator end cover or the stator yoke. A small spring keeps the brushes pressed on to the commutator surface. The brush pressure on the commutator surface must be carefully adjusted. Too small a brush pressure may lead to excessive arcing between the brush-commutator contact. If the brush pressure is too high, it may cause excessive wear of the commutator surface and the brushes.<\/p>\n<p style=\"text-align: justify;\">Brushes are made of carbon for small d.c. machines, electrographite for all d.c. machines and copper graphite for low-voltage high-current d.c. machines<\/p>\n<h3 style=\"text-align: justify;\"><span class=\"ez-toc-section\" id=\"Rotor-components-are-now-described-below\"><\/span><strong>Rotor components are now described below :<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h3>\n<h4 style=\"text-align: justify;\"><span class=\"ez-toc-section\" id=\"Armature-core\"><\/span><strong>Armature core :<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h4>\n<p style=\"text-align: justify;\">It serves the twin purpose of :<\/p>\n<ul style=\"text-align: justify;\">\n<li>Housing the armature coils in the slots and,<\/li>\n<li>Providing the low-reluctance path to the magnetic flux \u03c6\/2. It is made from 0.35 to 0.50 mm thick laminations of silicon steel to keep down the iron losses.<\/li>\n<\/ul>\n<h4 style=\"text-align: justify;\"><span class=\"ez-toc-section\" id=\"Armature-winding\"><\/span><strong>Armature winding :<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h4>\n<p style=\"text-align: justify;\">The armature winding is made from copper. It consists of large number of insulated coils, each coil having one or more turns. The coils are usually former wound. These are placed in slots and appropriately connected in series and parallel depending upon the type of winding required. There are basically two winding types :<\/p>\n<p style=\"text-align: justify;\">(i) Lap winding.<br \/>\n(ii) Wave winding.<\/p>\n<h4 style=\"text-align: justify;\"><span class=\"ez-toc-section\" id=\"Commutator\"><\/span><strong>Commutator :<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h4>\n<p style=\"text-align: justify;\">It is of cylindrical structure. It is built up of wedge-shaped segments of high conductivity hard-drawn copper to reduce its wear and tear. Segments are insulated from each other by 0.8 mm thick mica sheets. The segments are tapered as shown in fig. so that their assembly results in circular shape. Hub H and ring R are insulated from commutator segments by mica sheet M and V-shaped so as to prevent the segments from flying out due to centrifugal forces. Each commutator segment, has a riser where conductors from the armature winding are connected.<\/p>\n<p style=\"text-align: justify;\"><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-2222 size-full\" src=\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/one-commutator-segment.jpg\" alt=\"One commutator segment\" width=\"254\" height=\"213\" \/><\/p>\n<h4 style=\"text-align: justify;\"><span class=\"ez-toc-section\" id=\"Shaft\"><\/span><strong>Shaft :<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h4>\n<p style=\"text-align: justify;\">On mature shaft are mounted<\/p>\n<p style=\"text-align: justify;\">(i) hub H of commutator<br \/>\n(ii) spider in big machines or armature core in small machines and<br \/>\n(iii) bearings.<\/p>\n<p style=\"text-align: justify;\">End covers are connected to the yoke on one side and to the bearings and shaft on the other side<\/p>\n<h3 style=\"text-align: justify;\"><span class=\"ez-toc-section\" id=\"Electric-Braking-of-DC-Motors\"><\/span>Electric Braking of D.C. Motors<span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p style=\"text-align: justify;\">Electric braking is usually employed in applications to stop a unit driven by motors in an exact position or to have the speed of the driven unit suitably controlled during its deacceleration. In applications requiring frequent, quick, accurate or rapid emergency stops, electric braking is used. For example, in suburban <a href=\"https:\/\/study.madeeasy.in\/ee\/electrical-electronic-measurements\/piezo-electric-transducers\/\" target=\"_blank\" rel=\"noopener\">electric<\/a> trains quick stops are required. Electric braking allows smooth stops with out any inconvenience to passengers.<br \/>\nWhen a loaded hoist is lowered, electric braking keeps the speed within safe limits, otherwise, the drive speed will reach dangerous values.<br \/>\nWhen a train goes down a steep gradient, electric braking is employed to hold the train speed within safe limits. Similarly, in applications involving other active loads, electric braking is very commonly used.<br \/>\nThe braking force can also be obtained by using mechanical brakes.<\/p>\n<h4 style=\"text-align: justify;\"><span class=\"ez-toc-section\" id=\"Disadvantages-of-Mechanical-Braking\"><\/span>Disadvantages of Mechanical Braking:<span class=\"ez-toc-section-end\"><\/span><\/h4>\n<p style=\"text-align: justify;\">The following are the main disadvantages of mechanical braking:<\/p>\n<ul style=\"text-align: justify;\">\n<li>It requires frequent maintenance and replacement of brake shoes.<\/li>\n<li>Braking power is wasted as heat.In spite of the disadvantages, mechanical braking is used along with electric braking to ensure reliable operation of the drive. Mechanical brakes are also used to hold the drive at standstill because many braking methods do not produce torque at standstill.<\/li>\n<\/ul>\n<h3 style=\"text-align: justify;\"><span class=\"ez-toc-section\" id=\"Types-of-Electric-Braking\"><\/span>Types of Electric Braking<span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p style=\"text-align: justify;\"><strong>There are three types of braking a dc motor:<\/strong><\/p>\n<p style=\"text-align: justify;\">1. Regenerative braking<br \/>\n2. Dynamic braking or rheostatic braking<br \/>\n3. Plugging or reverse current braking<\/p>\n<h4 style=\"text-align: justify;\"><span class=\"ez-toc-section\" id=\"Regenerative-Braking\"><\/span>Regenerative Braking<span class=\"ez-toc-section-end\"><\/span><\/h4>\n<p style=\"text-align: justify;\">It is a form of braking in which the kinetic energy of the motor and its driven machinery is returned to the <a href=\"https:\/\/study.madeeasy.in\/subjects\/what-is-power-systems\/\" target=\"_blank\" rel=\"noopener\">power supply system.<\/a> This type of braking is possible when the driven load forces the motor to run at a speed higher than its no-load speed with a constant excitation. Under this condition, the motor back emf E<sub>b<\/sub> is greater than the supply voltage V, which reverse the direction of motor armature current. The machine now begins to operate as a generator and the energy generated is supplied to the source.<\/p>\n<p style=\"text-align: justify;\">Regenerative braking can also be carried out upto very low speeds if the motor is connected as a separately excited generator and excitation is increased as the speed is reduced, so that<\/p>\n<p style=\"text-align: justify;\"><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-2223 aligncenter\" src=\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/regenerative-braking.jpg\" alt=\"Regenerative Braking\" width=\"245\" height=\"38\" \/><\/p>\n<p style=\"text-align: justify;\">are satisfied and the motor does not enter into saturation on increasing excitation.<\/p>\n<p style=\"text-align: justify;\">Regeneration is possible with a shunt and separately excited motors and with compound motors with weak series compounding. Regenerative braking is used specially where more frequent braking or slowing of drives is required. It is most useful in holding a descending load of high potential energy at a constant speed. For example, regenerative braking is used to control the speed of motors driving loads such as electric locomotives, elevators, cranes and hoists. While descending, the load in this operation acts as the prime mover by virtue of its potential energy. The motor acts as a generator. The generated power is thus returned to the supply. The returned power is available for other devices operating from the same source of supply. Regenerative braking cannot be used for stopping the motor. It is used for controlling the speed above the no-load speed of the motor driving the descending loads or overhauling loads.<\/p>\n<p style=\"text-align: justify;\">The necessary condition for regeneration is that the back emf E<sub>b<\/sub> should be greater than the supply voltage so that the armature current is reversed and the mode of operation changes from motoring to generating.<\/p>\n<h3 style=\"text-align: justify;\"><span class=\"ez-toc-section\" id=\"Regenerative-Braking-in-dc-Shunt-Motors\"><\/span><strong>Regenerative Braking in dc Shunt Motors:<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p style=\"text-align: justify;\">Under normal operating conditions the armature current is given by<\/p>\n<p style=\"text-align: justify;\">\u2013 I<sub>a<\/sub> = V-E<sub>b<\/sub>\/R<sub>a<\/sub><\/p>\n<p style=\"text-align: justify;\">When the load (such as lowering of load by a crane, hoist or lift) causes the motor speed to be greater than the no-load speed, the back emf E<sub>b<\/sub> becomes greater than the supply voltage V. Consequently, armature current I<sub>a<\/sub>\u00a0 becomes negative. The machine now begins to operate as a generator.<\/p>\n<h3 style=\"text-align: justify;\"><span class=\"ez-toc-section\" id=\"Regenerative-Braking-in-dc-Series-Motors\"><\/span><strong> Regenerative Braking in dc Series Motors<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p style=\"text-align: justify;\">In case of a dc series motor an increase in speed is followed by a decrease in the armature current and field flux. The back emf E<sub>b<\/sub> cannot be greater than the supply voltage. Regeneration is not possible in a plain dc series motor since the field current cannot be made greater than armature current. However, in applications such as traction, elevators, hoists etc., where dc series motors are used extensively, regeneration may be required. For example, in an electro-locomotive moving down a gradient, a constant speed may be necessary and in hoist drives the speed is to be limited whenever it becomes dangerously high. One commonly used method of regenerative braking of the dc series motor is to connect it as a shunt motor. Since the resistance of the field winding is low, a series resistance is connected in the field circuit to limit the current within the safe value.<\/p>\n<h4 style=\"text-align: justify;\"><span class=\"ez-toc-section\" id=\"Dynamic-Braking-or-Rheostatic-Braking\"><\/span>Dynamic Braking or Rheostatic Braking<span class=\"ez-toc-section-end\"><\/span><\/h4>\n<p style=\"text-align: justify;\">In dynamic braking, the dc motor is disconnected from the supply and a braking resistor R<sub>b<\/sub> is immediately connected across the armature. The motor now works as a generator, producing the braking torque.<\/p>\n<p style=\"text-align: justify;\">For the braking operation, the separately excited (or shunt) motor can be connected either as a separately excited generator, where the flux is kept constant, or it can be connected as a self-excited shunt generator, with the field winding in parallel with the armature. Fig. (b) shows the dynamic braking of separately excited dc motor. Fig. (c) shows the dynamic braking of a dc shunt motor.<\/p>\n<p style=\"text-align: justify;\"><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-2224 size-full\" src=\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/motoring.jpg\" alt=\"Motoring\" width=\"608\" height=\"229\" srcset=\"https:\/\/www.madeeasy.in\/study\/wp-content\/uploads\/2024\/07\/motoring.jpg 608w, https:\/\/www.madeeasy.in\/study\/wp-content\/uploads\/2024\/07\/motoring-300x113.jpg 300w\" sizes=\"auto, (max-width: 608px) 100vw, 608px\" \/><\/p>\n<p style=\"text-align: justify;\"><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-2225 size-full\" src=\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/braking-with-self-excitation.jpg\" alt=\"Braking with self excitation\" width=\"279\" height=\"194\" srcset=\"https:\/\/www.madeeasy.in\/study\/wp-content\/uploads\/2024\/07\/braking-with-self-excitation.jpg 279w, https:\/\/www.madeeasy.in\/study\/wp-content\/uploads\/2024\/07\/braking-with-self-excitation-130x90.jpg 130w\" sizes=\"auto, (max-width: 279px) 100vw, 279px\" \/><\/p>\n<p style=\"text-align: justify;\">This method is also called rheostatic braking because an external braking resistance R<sub>b<\/sub> is connected across the armature terminals for electric braking. During electric braking when the motor works as a generator, the kinetic energy stored in the rotating parts of the motor and connected load is converted into electric energy. It is dissipated as heat in the braking resistance R<sub>b<\/sub> and armature circuit resistance R<sub>a<\/sub><\/p>\n<p style=\"text-align: justify;\">For dynamic braking, the series motor is disconnected from the supply, the field connections are reversed and the motor is connected in series with a variable resistance R<sub>b<\/sub> as shown below in figure (b).<\/p>\n<p style=\"text-align: justify;\"><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-2226 size-full\" src=\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/braking-with-separate-excitation.jpg\" alt=\" Braking with separate excitation\" width=\"622\" height=\"458\" srcset=\"https:\/\/www.madeeasy.in\/study\/wp-content\/uploads\/2024\/07\/braking-with-separate-excitation.jpg 622w, https:\/\/www.madeeasy.in\/study\/wp-content\/uploads\/2024\/07\/braking-with-separate-excitation-300x221.jpg 300w\" sizes=\"auto, (max-width: 622px) 100vw, 622px\" \/><\/p>\n<p style=\"text-align: justify;\">The field connections are reversed to make sure that the current through the field winding flows in the same direction as before (that is from S<sub>1<\/sub> to S<sub>2<\/sub>) in order that the back emf E<sub>b<\/sub> aids the residual flux. The machine now works as a self-excited series generator.<\/p>\n<p style=\"text-align: justify;\">The braking operation is slow with self excitation. When quick braking is required, the machine is connected for the separate excitation, and a suitable resistance is connected in series with the field to limit the current to a safe value.<\/p>\n<p style=\"text-align: justify;\">Dynamic braking is an inefficient method of braking, because all the generated energy is dissipated as heat in\u00a0 resistances.<\/p>\n<p style=\"text-align: justify;\"><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-2228 size-full\" src=\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/motoring-1.jpg\" alt=\"Motoring\" width=\"534\" height=\"215\" srcset=\"https:\/\/www.madeeasy.in\/study\/wp-content\/uploads\/2024\/07\/motoring-1.jpg 534w, https:\/\/www.madeeasy.in\/study\/wp-content\/uploads\/2024\/07\/motoring-1-300x121.jpg 300w\" sizes=\"auto, (max-width: 534px) 100vw, 534px\" \/><\/p>\n<h4 style=\"text-align: justify;\"><span class=\"ez-toc-section\" id=\"Plugging-or-Reverse-Current-Braking\"><\/span>Plugging or Reverse Current Braking<span class=\"ez-toc-section-end\"><\/span><\/h4>\n<p style=\"text-align: justify;\">In this method the armature terminals (or supply polarity) of a separately excited (or shunt) motor when running are reversed. Therefore, the supply voltage V and the induced voltage Eb (back emf) will act in the same direction. Thus during braking the effective voltage across the armature will be (V + E<sub>b<\/sub>) which is almost twice the supply voltage. The armature current is reversed and a high braking torque is produced. In order to limit the armature current to a safe value, an external current-limiting resistor R<sub>c<\/sub> is connected in series with the armature.<\/p>\n<p style=\"text-align: justify;\">I = -(V+E<sub>b<\/sub>)\/R<sub>a+<\/sub>R<sub>c<\/sub><\/p>\n<p style=\"text-align: justify;\">For braking a series motor either the armature terminals or field terminals (but not both) are reversed. Reversing of both given only normal working operation.<\/p>\n<p>The braking torque is not zero at zero speed. When used for stopping a load, the motor must be disconnected from the supply at or near zero speed, otherwise, it will speed up in the reverse direction. Centrifugal switches are used to disconnect the supply.<\/p>\n<p>Plugging is a highly inefficient method of braking because in addition to the power supplied by the load, power supplied by the source is wasted in resistances.<\/p>\n<p>Plugging is commonly used in controlling elevators, rolling mills, printing presses and machine tools etc.<\/p>\n<h4 style=\"text-align: justify;\"><span class=\"ez-toc-section\" id=\"Present%E2%80%93Day-Uses-of-DC-Machines\"><\/span>Present\u2013Day Uses of D.C. Machines<span class=\"ez-toc-section-end\"><\/span><\/h4>\n<p style=\"text-align: justify;\">At present time bulk of electric energy is generated in the form of alternating current. Hence the use of d.c. generators is very limited. They are mainly used in supplying excitation of small and medium range alternators. For industrial applications of d.c. like electrolytic processes, welding processes and variable speed motor drives, the present trend is to generate a.c. and then to convert a.c. into d.c. by rectifiers. Thus, dc generators have generally been superseded by rectified as supplies for many applications.<\/p>\n<p>Direct current <a href=\"https:\/\/study.madeeasy.in\/ee\/two-phase-servomotor\/\" target=\"_blank\" rel=\"noopener\">motors<\/a> are very commonly used as variable-speed drives and in application s where severe torque variations occur<\/p>\n<p style=\"text-align: justify;\"><strong>The main applications of the three types of d.c. motors are given below:<br \/>\n<\/strong><\/p>\n<ul>\n<li style=\"text-align: justify;\"><strong>Series Motors:<\/strong> These motors are used where high starting torque is required and speed can vary, for example, traction, cranes, etc.<\/li>\n<li style=\"text-align: justify;\"><strong>Shunt Motors:<\/strong> These motors are used where constant speed is required and starting conditions are not severe, for example, lathes, centrifugal pumps, fans, blowers, conveyors, lifts etc.<\/li>\n<li style=\"text-align: justify;\"><strong>Compound Motors:<\/strong> These motors are used where high starting torque and fairly constant speed is required, for example, presses, shears, conveyors, elevators, rolling mills, heavy planers etc.<br \/>\nSmall d.c. machines (in fractional kilowatt rating) are used primarily as control devices such as techogenerators for speed sensing and servomotors for positioning and tracking.<\/li>\n<\/ul>\n<h3><span class=\"ez-toc-section\" id=\"Previous-Year-Question-for-GATE-DC-Machines\"><\/span>Previous Year Question for GATE | DC Machines<span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p><iframe loading=\"lazy\" title=\"Previous Year Question for GATE | DC Machines | Electrical Machines | MADE EASY\" width=\"800\" height=\"450\" src=\"https:\/\/www.youtube.com\/embed\/Xpio5nKUFUk?feature=oembed\" frameborder=\"0\" allow=\"accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share\" referrerpolicy=\"strict-origin-when-cross-origin\" allowfullscreen><\/iframe><\/p>\n","protected":false},"excerpt":{"rendered":"<p>A DC machine has two main parts stator and rotor. The stator of the machine does not move, i.e., stationary<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[630,5],"tags":[636,635,638,639,637,640],"class_list":["post-2218","post","type-post","status-publish","format-standard","hentry","category-electrical-machines","category-ee","tag-disadvantages-of-mechanical-braking","tag-electric-braking","tag-motoring","tag-reverse-current-braking","tag-types-of-electric-braking","tag-uses-of-d-c-machines"],"_links":{"self":[{"href":"https:\/\/www.madeeasy.in\/study\/wp-json\/wp\/v2\/posts\/2218","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.madeeasy.in\/study\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.madeeasy.in\/study\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.madeeasy.in\/study\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.madeeasy.in\/study\/wp-json\/wp\/v2\/comments?post=2218"}],"version-history":[{"count":0,"href":"https:\/\/www.madeeasy.in\/study\/wp-json\/wp\/v2\/posts\/2218\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.madeeasy.in\/study\/wp-json\/wp\/v2\/media?parent=2218"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.madeeasy.in\/study\/wp-json\/wp\/v2\/categories?post=2218"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.madeeasy.in\/study\/wp-json\/wp\/v2\/tags?post=2218"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}