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ELECTRIC MACHINE & DRIVES

ELECTRIC MACHINE & DRIVES 9
EEK 467 ELECTRIC MACHINE DRIVES Dr. Ir. SYAFRUDIN MASRI 1DC Motor • The direct current (dc) machine can be used as a motor or as a generator. • DC Machine is most often used for a motor. • The major advantages of dc machines are the easy speed and torque regulation. • However, their application is limited to mills, mines and trains. As examples, trolleys and underground subway cars may use dc motors. • In the past, automobiles were equipped with dc dynamos to charge their batteries. 2DC Motor • Even today the starter is a series dc motor • However, the recent development of power electronics has reduced the use of dc motors and generators. • The electronically controlled ac drives are gradually replacing the dc motor drives in factories. • Nevertheless, a large number of dc motors are still used by industry and several thousand are sold annually. 3Construction 4DC Machine Construction Figure 8.1 General arrangement of a dc machine 5DC Machines • The stator of the dc motor has poles, which are excited by dc current to produce magnetic fields. • In the neutral zone, in the middle between the poles, commutating poles are placed to reduce sparking of the commutator. The commutating poles are supplied by dc current. • Compensating windings are mounted on the main poles. These shortcircuited windings damp rotor oscillations. . 6DC Machines • The poles are mounted on an iron core that provides a closed magnetic circuit. • The motor housing supports the iron core, the brushes and the bearings. • The rotor has a ringshaped laminated iron core with slots. • Coils with several turns are placed in the slots. The distance between the two legs of the coil is about 180 electric degrees. 7DC Machines • The coils are connected in series through the commutator segments. • The ends of each coil are connected to a commutator segment. • The commutator consists of insulated copper segments mounted on an insulated tube. • Two brushes are pressed to the commutator to permit current flow. • The brushes are placed in the neutral zone, where the magnetic field is close to zero, to reduce arcing. 8DC Machines • The rotor has a ringshaped laminated iron core with slots. • The commutator consists of insulated copper segments mounted on an insulated tube. • Two brushes are pressed to the commutator to permit current flow. • The brushes are placed in the neutral zone, where the magnetic field is close to zero, to reduce arcing. 9DC Machines • The commutator switches the current from one rotor coil to the adjacent coil, • The switching requires the interruption of the coil current. • The sudden interruption of an inductive current generates high voltages . • The high voltage produces flashover and arcing between the commutator segment and the brush. 10DC Machine Construction Rotation I /2 I /2 rdc I rdc rdc Pole Brush winding Shaft 1 2 8 3 7 NS 6 4 5 Insulation Copper segment Rotor I rdc Winding Figure 8.2 Commutator with the rotor coils connections. 11DC Motor Operation 12DC Motor Operation • In a dc motor, the stator Rotation poles are supplied by dc I /2 I /2 rdc rdc I rdc Pole excitation current, which Brush winding Shaft produces a dc magnetic field. 1 2 8 • The rotor is supplied by 3 7 NS dc current through the 6 4 5 brushes, commutator and coils. Insulation Copper • The interaction of the segment Rotor I rdc Winding magnetic field and rotor current generates a force that drives the motor 132 1 DC Motor Operation v B • Before reaching the neutral zone, a the current enters in segment 1 and S N exits from segment 2, 30 V dc • Therefore, current enters the coil b end at slot a and exits from slot b v during this stage. I rdc • After passing the neutral zone, the (a) Rotor current flow from segment 1 to 2 (slot a to b) current enters segment 2 and exits from segment 1, B • This reverses the current direction a through the rotor coil, when the coil N S 30 V vv dc passes the neutral zone. b • The result of this current reversal is the maintenance of the rotation. I rdc (b) Rotor current flow from segment 2 to 1 (slot b to a) 14 1 2DC Generator Operation 152 1 DC Generator Operation v B • The NS poles produce a a dc magnetic field and the S N 30 V dc rotor coil turns in this field. b v • A turbine or other I rdc machine drives the rotor. (a) Rotor current flow from segment 1 to 2 (slot a to b) • The conductors in the slots cut the magnetic flux B a lines, which induce S N voltage in the rotor coils. 30 V v v dc • The coil has two sides: b one is placed in slot a, the other in slot b. I rdc (b) Rotor current flow from segment 2 to 1 (slot b to a) 16 1 22 1 DC Generator Operation v B • In Figure 8.11A, the a conductors in slot a are S N 30 cutting the field lines V dc entering into the rotor b from the north pole, v • The conductors in slot b I rdc are cutting the field lines (a) Rotor current flow from segment 1 to 2 (slot a to b) exiting from the rotor to the south pole. B a • The cutting of the field lines generates voltage in S N 30 V v v dc the conductors. • The voltages generated in b the two sides of the coil I rdc are added. (b) Rotor current flow from segment 2 to 1 (slot b to a) 17 1 22 1 DC Generator Operation v B • The induced voltage is a connected to the generator S N 30 terminals through the V dc commutator and brushes. b • In Figure 8.11A, the induced v voltage in b is positive, and in I rdc a is negative. (a) Rotor current flow from segment 1 to 2 (slot a to b) • The positive terminal is connected to commutator B a segment 2 and to the conductors in slot b. S N 30 V v v dc • The negative terminal is connected to segment 1 and b to the conductors in slot a. I rdc (b) Rotor current flow from segment 2 to 1 (slot b to a) 18 1 22 1 DC Generator Operation v B • When the coil passes the a neutral zone: S N 30 V – Conductors in slot a are dc then moving toward the b south pole and cut flux lines exiting from the rotor v I – Conductors in slot b cut the rdc flux lines entering the in (a) Rotor current flow from segment 1 to 2 (slot a to b) slot b. • This changes the polarity B a of the induced voltage in the coil. S N 30 V v v dc • The voltage induced in a b is now positive, and in b is negative. I rdc (b) Rotor current flow from segment 2 to 1 (slot b to a) 19 1 22 1 DC Generator Operation v B • The simultaneously the a commutator reverses its S N 30 V dc terminals, which assures that the output voltage b v (V ) polarity is dc I unchanged. rdc (a) Rotor current flow from segment 1 to 2 (slot a to b) • In Figure 8.11B – the positive terminal is B connected to commutator a segment 1 and to the S N 30 V conductors in slot a. v v dc – The negative terminal is b connected to segment 2 and to the conductors in slot b. I rdc (b) Rotor current flow from segment 2 to 1 (slot b to a) 20 1 2Generator 21DC Generator Equivalent circuit • 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 voltage source. • The stator coil has resistance, which is connected in series. • The pole flux is produced by the DC excitation/field current, which is magnetically coupled to the rotor • The field circuit has resistance and a source • The voltage drop on the brushes represented by a battery 22DC Generator Equivalent circuit V brush R a R Load f Φ max I ag I f V V f dc E ag Electrical Mechanical power out power in • Figure 8.12Equivalent circuit of a separately excited dc generator. 23DC Generator Equivalent circuit • The magnetic field produced by the stator poles induces a voltage in the rotor (or armature) coils when the generator is rotated. • The dc field current of the poles generates a magnetic flux • The flux is proportional with the field current if the iron core is not saturated: Φ= K I ag 1 f 24DC Generator Equivalent circuit • The rotor conductors cut the field lines that generate voltage in the coils. E= 2N Bl v ag r g • The motor speed and flux equations are : D g Φ= Bl D v=ω ag g g 2 25DC Generator Equivalent circuit • The combination of the three equation results the induced voltage equation: D ⎛⎞ g ⎜⎟ E= 2N Bl v= 2N Blω= N() Bl Dω= NΦω ag r g r g r g g r ag ⎜⎟ 2 ⎝⎠ • The equation is simplified. E= NΦω= N K Iω= K Iω ag r ag r 1 f m f 26DC Generator Equivalent circuit • When the generator is loaded, the load current produces a voltage drop on the rotor winding resistance. • In addition, there is a more or less constant 1–3 V voltage drop on the brushes. • These two voltage drops reduce the terminal voltage of the generator. The terminal voltage is; E=V+ I R+V ag dc ag a brush 27Motor 28DC Motor Equivalent circuit V Electrical brush R power in R a f Φ max DC Power I am V V I dc supply f f E am Mechanical power out • Figure 8.13 Equivalent circuit of a separately excited dc motor • Equivalent circuit is similar to the generator only the current directions are different 29DC Motor Equivalent circuit • The operation equations are: • Armature voltage equation V= E+I R+V dc am am a brush The induced voltage and motor speed vs angular frequency E= K Iω am m f ω= 2π n m 30DC Motor Equivalent circuit • The operation equations are: • The combination of the equations results in K Iω= E=V−I R m f am dc am m The current is calculated from this equation. The output power and torque are: P out P= E I T== K I I m am f out am am ω 31