DC Machines. Introduction - commutation. Commutation. I. Introduction Electrical Energy Technology

Transcription

1 UNVERSTY OF TECHNOLOGY, SYDNEY FACULTY OF ENGNEERNG Electricl Enery Technoloy DC Mchines Topics to cover: 1. ntroduction 2. EMF & Torque 3. Equivlent Circuit 4. Perornce. ntroduction Principle o DC chines A DC otor is device or convertin DC electricl enery into rotry (or liner) echnicl enery. This process cn be reversed, s in DC enertor, to convert echnicl to electricl power. The workin principle o the DC (nd AC s well) enertor is Frdy s Lw, which sttes tht e nd electric current i the circuit is closed, is produced when conductor cuts throuh netic orce lines. The opposite o the lw pplies or the DC (nd AC) otor. Motion is produced when current crryin wire is put in netic ield. ntroduction - couttion n DC chines the current in ech wire o the rture is ctully lterntin, nd hence device is required to convert the lterntin current enerted in the DC enertor by electronetic induction into direct current, or t the rture o DC otor to convert the input direct current into lterntin current t pproprite ties, s illustrted in Fi.1. Fi.1(): DC enertor: induced AC e is converted to DC volte; (b) DC otor: input direct current is converted to lterntin current in the rture t pproprite ties to produce unidirectionl torque. Couttion Fi.1 () DC enertor

2 Couttion DC Mchine Construction Fi.1 (b) DC otor Fi.2 DC chine construction. DC Mchine: Mjor Coponents The sttor o the chine hs poles, which re excited by current to produce netic ields. n the neutrl zone, in the iddle between the poles, couttin poles re plced to reduce sprkin o the couttor. The couttin poles re supplied by current. Copenstin windins re ounted on the in poles. These short-circuited windins dp rotor oscilltions. DC Mchine: Mjor Coponents The poles re ounted on n iron core tht provides closed netic circuit. The otor housin supports the iron core, the brushes nd the berins. The rotor hs rin-shped linted iron core with slots. Coils with severl turns re plced in the slots. The distnce between the two les o the coil is bout 180 electric derees. The coils re connected in series throuh the couttor seents.

3 DC Mchine: Mjor Coponents The ends o ech coil re connected to couttor seent. The couttor consists o insulted copper seents ounted on n insulted tube. Two brushes re pressed to the couttor to perit current low. The brushes re plced in the neutrl zone, where the netic ield is close to zero, to reduce rcin. DC Mchine: Couttor The rotor hs rin-shped linted iron core with slots. The couttor consists o insulted copper seents ounted on n insulted tube. Two brushes re pressed to the couttor to perit current low. The brushes re plced in the neutrl zone, where the netic ield is close to zero, to reduce rcin. Fi.3 Couttor DC Mchine: Couttor The couttor switches the current ro one rotor coil to the djcent coil. The switchin requires the interruption o the coil current. The sudden interruption o n inductive current enertes hih voltes. The hih volte produces lshover nd rcin between the couttor seent nd the brush. DC Mchine: Couttor N r_ /2 Sht nsultion Rotor Windin Brush r_ Rottion 1 5 r_ r_ /2 Copper seent Pole windin S Fi.3 Couttor with the rotor coils connections.

4 DC Mchine Construction DC Mchine Construction Fi.4 DC otor sttor with poles visible (4 pole chine). Fi.5 Rotor o DC otor. DC Mchine Construction Fi.6 Cutwy view o DC otor.. E & Torque The netic ield produced by the sttor poles induces electronetic orce (e) in the rotor (or rture) coils when the chine rottes or is rotted. The pole lux is produced by the DC excittion/ield current, which is neticlly coupled to the rotor. The lux is proportionl to the ield current i the iron core is not sturted: Φ = K 1 The rotor conductors cut the ield lines tht enerte e in the coils E = 2 N r where N r is the nuber o coils in series, B the vere lux density, l eective xil lenth, v the velocity o the conductor. B l v

5 E & Torque The otor speed nd lux equtions re : D v = ω Φ = B l πd / p 2 where ω is the rotor echnicl nulr speed, D rotor dieter, p the nuber o poles, nd φ lux per pole. The cobintion o the three equtions ives the induced e: D p E = 2 N r B l v = 2 N r B l ω = N r l 2 π p p C = N r Φ ω = Φ ω = kφ ω π π 2 ( B πd ) ω T E = ω = k Φ E & Torque where k is rture constnt, C is the totl nuber o conductors, is the nuber o the prllel pths o the rture windin, p is the nuber o netic poles, =2 or wve windins, nd =p or lp windins.. Equivlent Circuit.(1) A seprtely excited enertor V R Φ x E R V Lod V.(2) A seprtely excited otor R Φ x E R Electricl power in V DC Power supply Mechnicl power in Electricl power out Mechnicl power out V = E R V = R T = T sht Tloss V + = E R V = R T = T lod + Tloss

6 .(3) Shunt otor.(4) Series otor R R E Φ x R DC Power supply E R DC Power supply V Φ x V P out P out V + V = V = R = E R For shunt enertor V E R V = V = R = = + = + V = E + ( R + R ) For series enertor V = E ( R + R ) = = = = V = E + ( R + R ) E P out For series enertor.(5) Copound otor V ) s R Φ x = + = + R s R p p p s p DC Power supply V V = V = R = E ( R + R s = s = + p V p = V = R p p p p p V = V V. Perornce (1) Seprtely excited enertor Open circuit chrcteristic t = 0 ω = ω = E V ( i. ev. ) = E = k R rted Externl chrcteristic: (V t vs. ) ω = ω rted = rted The reltion o E nd is the netistion curve.

7 Build up o v t or sel-excittion: V. Perornce (2) Shunt enertor Conditions or buildin up v t : Residul netis nitil netistion usin btteries, etc. Field windin bein connected correctly to rture windin such tht the enerted lux ids the residul netis swp the terinls. The slop o the resistnce line (Re+R) ust be sller thn o the irp line reduce Re. Externl chrcteristic: V. Perornce (2) Shunt enertor cont. = V t = E = L = E L + VL + R + R R R ( L e VL + R + R e ) V. Perornce (3) Seprtely excited otor V. Perornce (3) Seprtely excited otor cont. Method or speed control: Vryin terinl volte or speed below the bse speed, suitble or constnt torque lod; Field wekenin or speed control bove the bse speed, suitble or constnt power lod.

8 V. Perornce (4) Shunt otor V. Perornce (5) Series otor V. Perornce (6) Eiciency V. Perornce (6) Eiciency cont.