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8.1 DC Motor

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8.1 DC Motor IV B. Tech I semester (JNTUH-R15) Prepared By Ms. Lekha Chandran, Assistant Professor Unit-1 ELECTRIC DRIVE ELECTRICAL ENERGY It is flexible Easilyavailable Can be converted to other forms of energy. Can be easily transported to required location Economical Mature technology Saves manual labor in industry and domestic applications UTILISATION Electrical energy is used in variousapplications: 1. Electric Drives; DC and ACmotors 2. Electric heating and welding 3. Illumination 4. Electric Traction 5. Electric Vehicles DC MOTOR • The direct current (dc) machine can be used asa motor or as agenerator. • DC Machine is most often used for amotor. • The major advantages of dc machines are theeasy speed and torqueregulation. • However, their application is limited to mills, mines and trains. As examples, trolleys and underground subway cars may use dcmotors. • In the past, automobiles were equipped withdc dynamos to charge theirbatteries. DC MOTOR • Eventoday the starter is a series dc motor • However, the recent development of power electronics has reduced the use of dc motorsand generators. • The electronically controlled ac drives aregradually replacing the dc motor drives infactories. • Nevertheless, a large number of dc motors arestill used by industry and several thousand are sold annually. CONSTRUCTI ON DC MACHINE CONSTRUCTION General arrangement of a dc machine DC MACHINES • Thestator of the dc motor has poles, which are excited by dc current to produce magnetic fields. • Inthe neutral zone, in the middle between the poles,commutating poles are placed to reduce sparking of the commutator. The commutating poles are supplied by dccurrent. • Compensating windings are mounted on the main poles. These short-circuitedwindings damp rotor oscillations. DC MACHINES • The poles are mounted onan iron core that provides a closed magneticcircuit. • The motor housing supports the iron core, the brushesand thebearings. • The rotor has a ring-shaped laminated iron core withslots. • Coils with several turns are placed in the slots. The distance between the twolegs of the coil is about 180 electric degrees. DC MACHINES • The coils are connected inseries through the commutator segments. • The ends of each coil are connected to acommutator segment. • The commutator consists of insulated copper segments mounted on an insulatedtube. • Twobrushesare pressedto the commutator to permit current flow. • The brushes are placed in the neutral zone, where themagnetic field is close to zero, to reduce arcing. DC MACHINES • The rotor has a ring-shaped laminated iron core withslots. • The commutator consists of insulated copper segments mounted onan insulated tube. • Two brushes are pressedto the commutator to permit currentflow. • The brushes are placed in the neutral zone, where the magneticfield is close to zero, to reducearcing. DC MACHINES • The commutator switches the current from one rotor coil to the adjacentcoil, • The switching requiresthe interruption of the coil current. • The sudden interruption ofan inductive current generates high voltages. • The high voltage produces flashover and arcingbetween the commutator segmentand thebrush. DC MACHINE CONSTRUCTION Rotation I /2 Ir_dc/2 I r_dc Brush r_dc Pole winding Shaft | 1 2 8 N 7 3 S 6 4 5 Insulation Copper Rotor segment Ir_dc Winding Fig:Commutator with the rotor coils connections. DC MACHINE CONSTRUCTION Fig: Details of the Commutator ofa dc motor. DC MACHINE CONSTRUCTION Fig:DC motor stator with polesvisible. DC MACHINE CONSTRUCTION Fig:Rotor of adc motor. DC MACHINE CONSTRUCTION Fig: Cutaway view of a dc motor. DC MOTOR OPERATION DC MOTOR OPERATION • In a dc motor, the stator poles are supplied by dc Rotation I I /2 Ir_dc/2 r_dc r_dc excitation current,which Brush Pole winding produces a dc magnetic Shaft field. 1 2 • The rotor is supplied by 8 dc current through the N 7 3 S 6 4 brushes, commutatorand 5 coils. • The interaction of the Insulation Copper Rotor I segment magnetic field and rotor Winding r_dc current generates aforce that drives themotor DC MOTOROPERATION v B • The magnetic field linesenter a into the rotor from the north S 30 N pole (N) and exit toward the Vdc south pole(S). b • The poles generate a v magnetic field that is Ir_dc perpendicular to thecurrent (a) Rotor current flow from segment 1 to 2 (slot a to b) carryingconductors. • The interaction betweenthe B field and the current a produces a Lorentzforce, S N v 30 v V • The force is perpendicular to dc both the magnetic field and b conductor Ir_dc (b) Rotor current flow from segment 2 to 1 (slot b to a) DC MOTOROPERATION v B • The generated force turns therotor a until the coil reaches the neutral S 30 N point between thepoles. Vdc • At this point, the magnetic field b becomes practically zerotogether v with theforce. Ir_dc • However, inertia drives the motor (a) Rotor current flow from segment 1 to 2 (slot a to b) beyond theneutral zone where the direction of the magnetic field B reverses. a • To avoid the reversal of the force S N 30 V direction, the commutatorchanges v v dc the current direction, which b maintains the counterclockwise rotation. Ir_dc (b) Rotor current flow from segment 2 to 1 (slot b to a) DC MOTOROPERATION v B • Before reaching the neutral zone, a the current enters insegment 1 and exits from segment2, S 30 N Vdc • Therefore, current enters the coil end at slot a and exits from slot b b during thisstage. v I • After passing the neutral zone, the r_dc current enters segment 2 and exits (a) Rotor current flow from segment 1 to 2 (slot a to b) from segment1, B • This reverses the currentdirection a through the rotor coil, when the S 30 N coil passes the neutral zone. v v Vdc • The result of this currentreversal is b the maintenance of therotation. Ir_dc (b) Rotor current flow from segment 2 to 1 (slot bto a) DC GENERATO R OPERATIO N DC GENERATOROPERATION v B • The N-S poles produce adc a magnetic field and the S N rotorcoil turns in thisfield. 30 Vdc • Aturbine or othermachine b drives therotor. v Ir_dc • The conductors in the (a) Rotor current flow from segment 1 to 2 (slot ato slots cut the magnetic flux b) lines, which inducevoltage B in the rotorcoils. a S N • The coil has two sides: 30 v v Vdc one is placed in slot a, the other in slotb. b Ir_dc (b) Rotor current flow from segment 2 to 1 (slot b to a) DC GENERATOROPERATION • In Figure 8.11A, the v B conductorsin slot aare a cutting the field lines S N entering into the rotor 30 Vdc from the northpole, b • The conductors in slot b v are cutting the fieldlines Ir_dc exiting from the rotor to (a) Rotor current flow from segment 1 to 2 (slot ato the south pole. b) • The cutting of the field B lines generates voltagein a theconductors. S 30 N • The voltages generatedin v v Vdc the two sides of the coil areadded. b Ir_dc (b) Rotor current flow from segment 2 to 1 (slot b to a) DC GENERATOROPERATION • The induced voltage is v B connected to thegenerator a S N terminals through the 30 V commutator andbrushes. dc • In Figure 8.11A, the induced b voltage in b is positive, and in v Ir_dc a isnegative. (a) Rotor current flow from segment 1 to 2 (slot ato • The positive terminal is b) connected tocommutator B segment 2 and to the a conductors in slotb. S N v 30 V • The negative terminal is v dc connected to segment 1and b to the conductorsin slota. Ir_dc (b) Rotor current flow from segment 2 to 1 (slot b to a) DC GENERATOROPERATION • When the coil passesthe v B neutralzone: a S N 30 – Conductors in slot a arethen Vdc moving toward the south pole and cut flux lines b exiting from therotor v – Conductors in slot b cut the Ir_dc flux lines entering the inslot (a) Rotor current flow from segment 1 to 2 (slot ato b. b) • This changes the polarity B of the induced voltage in a thecoil. S 30 N • The voltage induced in ais v v Vdc now positive, and in b is b negative. Ir_dc (b) Rotor current flow from segment 2 to 1 (slot b to a) DC GENERATOROPERATION v B • The simultaneously the a S N 30 commutator reverses its Vdc terminals, which assures b that the output voltage v I (Vdc) polarity is unchanged. r_dc (a) Rotor current flow from segment 1 to 2 (slot ato • In Figure 8.11B b) – the positive terminal is B connected to commutator a segment 1 and to the S N 30 V conductors in slota. v v dc – The negative terminal is b connected to segment 2 and to the conductors in slotb. Ir_dc (b) Rotor current flow from segment 2 to 1 (slot b to a) DC MACHINE EQUIVALENT CIRCUIT GENERATOR DC GENERATOR EQUIVALENTCIRCUIT • The magnetic field produced by the stator poles induces a voltage in the rotor (or armature) coils when the generator is rotated. • This induced voltage is represented by a voltagesource. • Thestator coil has resistance, which is connected inseries. • Thepole flux is produced by the DC excitation/field current, which is magnetically coupled to therotor • Thefield circuit has resistanceand asource • Thevoltagedrop on the brushes represented by abattery DC GENERATOR EQUIVALENTCIRCUIT V brush R Rf a Load max I If ag Vf Vdc Eag Mechanical Electrical power in power out Equivalent circuitof a separatelyexcited dc generator. DC GENERATOR EQUIVALENTCIRCUIT • The magnetic field produced by the stator poles induces a voltage in the rotor (or armature) coils when the generator isrotated. • The dc field current of the poles generatesa magneticflux • Theflux is proportional with the field current if the iron core is notsaturated: ag K1 I f DC GENERATOR EQUIVALENTCIRCUIT • The rotor conductors cut the field linesthat generate voltage in thecoils.
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