10. DC Drives. Prof. A. Binder : Electrical Machines and Drives 10/1. Institut für Elektrische Energiewandlung FB 18. Source: Siemens AG

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1 10. DC Drives Source: Siemens AG 10/1

2 10.1 Principles of Opertion of DC Mchines Source: H. Kleinrth, Studientext 10/2

3 Bsic function of DC mchine Rotor coil rottes in sttor DC mgnetic field; voltge is induced nd rectified by commuttor nd brushes In ech moving coil side (turns per coil N c, stck length l, speed n) n AC voltge u i,c is induced vi induction due to movement : Amplitude 10/3 Sttor ir gp mgnetic field distribution, electriclly excited Rectifiction (vi commuttor & brushes): ; verge DC voltge: v u u v B N l i, c, m 2, m c i, c ui u i, v e u i, m d n 2p n, v eb m Rotor dimeter d r :, verge ir gp flux density: Flux/pole: l B p 0 ( x) dx e plb, m r p Number of rotor conductors z = 2N c : B, u v i, 2z n

4 DC mchine excittion of sttor field Electricl excittion Source: ABB Sweden Exmple: Four-pole mchine: 1: Field coil, 2: Compenstion winding 3: Inter-pole winding, 4: Armture winding Source: Siemens AG, Germny Permnent mgnet excittion Exmple: Four-pole mchine: 1, 2: Field mgnets, 3: Pole shoe iron, 4: Housing s iron bck 10/4

5 DC mchine - components (1) Electricl field excittion / Exmple: Four-pole mchine Source: H. Kleinrth, Studientext 10/5

6 DC mchine - components (2) Source: H. Kleinrth, Studientext Electricl field excittion / Exmple: Four-pole mchine 10/6

7 Smoothed induced rotor DC voltge ) AC voltge of one coil u ic is rectified s DC voltge u i with deep sgs (here: 2p = 2) b) Incresed number of series connected rotor coils - displced by slot pitch ech -, rrnged in Q r rotor slots, led to sum of rectified coil voltges s smoothed totl DC voltge 10/7

8 Induced voltge (Bck EMF) Mx. voltge per coil: u v B i, c, m 2, m N l c u Averge voltge per coil: v eb, i, c, v 2 m N c l Flux per pole: Rotor circumference velocity: Totl number of rotor conductors: z v e plb, m 2p 2 K p N c n 2p poles, 2 prllel brnches: ui, c, v 22 u u i, v i, v u i,v (2 K p N N c K 22 p N 2 p U i z c z n n Averge voltge t K/(2p) coils: U i p c ) n n n 10/8

9 Rotor coils, commuttor, brushes, slot design Source: H. Kleinrth, Studientext - Two lyers per slot (Upper & lower lyer) = increses number of coils by 2 - Severl (= u) coil sides side by side in slot lyer = reduction of lot number possible - Two prllel rotor rmture brnches per pole pir. In both voltge u i,v is induced. - By dding p-1 pole pirs we get further prllel rmture brnches with induced voltge u i,v. This results in 2 = 2p prllel rmture brnches in 2p pole mchine. Fcit: With p pole pirs the induced voltge (bck EMF) U i = u i,v occurs between ech plus- nd minus brush (A = Plus, B = Minus). The totl number of rotor conductors is: z 2u N c Q r Source: F. Brenner, Bürstdt 10/9

10 Elements of lp-wound rmture winding Upper lyer Lower lyer Upper lyer Lower lyer Inserting n rmture coil into the rotor slots An rmture coil s bsic element of the winding y 1 : Width of coil = bout one pole pitch! y = y 1 y 2 = 1: Coil step t commuttor Connection to djcent rmture coil t the commuttor 10/10

11 Mnufcturing of rmture coils nd rotor with commuttor Armture coil: Rotor iron stck: Inserting rmture coils: Below: first step - unformed Insultion in slots Two-lyer winding Above: formed left: commuttor Upper nd lower lyer Soldering of the rmture coil ends to the commuttor segments: Upper nd lower lyer coil ends re soldered into the slits of the commuttor segments Source: F. Brenner/Bürstdt, Germny 10/11

12 Induced rotor rmture voltge = bck EMF A 6-pole (in generl: 2p pole mchine) mchine is derived from 2-pole rrngement by continution of the rmture coil sequence nd corresponding commuttor segments with brushes ( LAP WINDING"). Induced voltge: 2 2p p U i z n k 1 n Ui k2 m k1 k2 2 K u Q Ech coil strts / ends t djcent commuttor segments with totl number: r 10/12

13 Complete four-pole DC lp winding Source: Dr. Holzer, TU Wien Simplex lp winding: Dt: Q r = 26, 2p = 4, u = 1, N c = 1, = p = 2, K = 26, y 1 = 6, y 2 = 5, y = 1 10/13

14 Simplex lp winding: 2 = 2p Exmple: Q r = 26, 2p = 4, u = 1, N c = 1, = p = 2, K = 26, y 1 = 6, y 2 = 5, y = 1 U = 440V Pole count = Number of prllel winding brnches: 2 = 2p = 4-220V +220V Source: Dr. Holzer, TU Wien 10/14

15 Induced voltge = bck EMF = No-lod voltge (Genertor) Iron sturtion Remnence voltge U p 0 U i z n ( I f ) k n 1 No-lod chrcteristic: - Armture voltge mesured t open circuit nd constnt speed n: U 0 = U i = bck EMF (genertor nolod) - Bck EMF increses LINEAR with flux. Flux vrition by field current. Due to iron sturtion flux increses non-liner with field current If nd so does bck EMF. 10/15

16 Exmple: Rotor rmture winding of 200 kw-dc mchine Dt: Rotor rted DC voltge 430 V, rted speed n = 1470/min, rotor dimeter d r = 400 mm, Pole count 2p = 4, rotor iron stck length l = 190 mm, slot number Q r = 58, coil sides per slot nd lyer u = 4, number of turns per coil N c = 1, equivlent pole coverge rtio e = 0.7, mximum ir gp flux density: B,m = 0.86 T We clculte from tht: - Number of commuttor segments: K = Q r. u = = Totl number of rotor conductors z = 2. K. N c = = Pole pitch p = d r /4 = 400 /4 = mm - Flux per pole = e. p. l. B = = 35.9 mwb - Induced rotor voltge (U i = z. (p/). n. = 464. (2/2). (1470/60) = V Averge vlue of DC voltge between 2 commuttor segment must not exceed 408.5/(232/4) = 7 V < V (otherwise flsh-over between 2 segments!) Between djcent segments t 0.3 mm mic is plced s insultion, but hs in prllel ir (with crbon dust!) 10/16

17 The brush-copper contct Source: SKT, Gießen ) The brush-copper contct resistnce is the min prt of voltge drop t the brushes: U 1 is bout 80% of totl voltge drop U. Resulting brush voltge drop U b = U A + U B = c. 2 V. b) Brush voltge drop U b rises non-liner with brush current density J b ; nd decreses with incresing temperture. Brush current density J b : 1/100 of coil current density (< 10 A/cm 2 ). c) Brushes re short-circuiting rotor coils t tht moment, when coil sides re locted in neutrl zone (= inter-pole gp), where ir gp flux density is zero (B = 0), so induced voltge is zero. 10/17

18 Commuttor und grphite brushes Commuttorsegments Insultion Brush holder (bronze) copper litz wire 5 grphite brushes in prllel per holder spring force Source: Brenner, Bürstdt 10/18

19 Vrition of bck EMF p U i z n k 1 n - In cse of MOTOR opertion, pplied rmture voltge between the brushes U must be bigger thn bck EMF U i to drive DC current (Armture current) I in rotor winding: U U - Armture resistnce R is smll; brush voltge drop U b = c. 2V. i I R - At turned off field current remnence flux density B R of sttor iron poles remins, which induces smll bck EMF U R. - Bck EMF rises LINEAR with speed n. U b 10/19

20 Electromgnetic torque Me k2 I ) Per pole only one polrity of rotor current exists. Armture current is flowing in 2 prllel winding brnches: Ic I /( 2). In rotor winding it is n AC current i c in ech coil, t the brushes it s DC current I. b) Electromgnetic forces on ech rotor conductor due to ir gp field B : Fc IcB l. Averge force per conductor: Fc, v IceB, ml. Torque (lever dr / 2 p p / ) for ll z conductors: p p I z ( p / ) M e z eb, ml I e plb, m /20

21 Lp winding needs potentil equlizers (of 1 st kind) Exmple: Dt: 80% 120% Result: Brush A 1 is overloded, wers out very quickly. 2p = 4, 2 = 4, u = 2, Q r = 12, K = = 24, y V = K/p = 24/2 = 12: e. g. commuttor segments 1 nd 13 hve to be connected by equlizer. - Reson: In relity no mchine is idelly symmetric. So electric potentil (induced bck EMF) between prllel connected positive or negtive brushes is not exctly identicl. - Alredy smll voltge differences will led due to smll R to rther big symmetric current shring in prllel brushes I /. - Exmple: 2p = 4: 2 = 4: Brush A 1 shres 120%, brush A 2 only 80 % of rted brush current. - Counter-mesure: Potentil equlizers of 1 st kind Art = Copper wires, connecting commuttor segments, which (theoreticlly) hve identicl electric potentil. -Exmple: Current flow in equlizer is 20 % of rted brush current, wheres both brushes A 1 nd A 2 crry only 100% current. So brushes re not overloded. 10/21

22 Mounting of potentil equlizers (1 st kind) 80% 120% Potentil equlizers of 1 st kind for four-pole winding: Equlizer step = 2 pole pitches = hlf circumference Source: F. Brenner, Bürstdt Potentil equlizers of 1 st kind Potentil equlizers (1 st kind) beneth the bndge 10/22

23 The wve winding n lterntive to lp winding Upper lyer Lower lyer One rmture coil s bsic element Exmple: 2 = 2, u = 1, Q r = 25, 2p = 4, K = 25, step for one coil t commuttor y = (K-1)/p = 12: 1 (Upper lyer) 7 (Lower lyer) 13 (UL) 19 (LL) 25 (UL) 6 (LL)..., nd so on 10/23

24 Armture wve winding - Series connected wve-shped rmture coils: Beginning nd end of complete wve line re distnced by one segment pitch (segment 1 to segment K = 25). - Next series-connected wve-line strt t K, ends t K-1; it is shifted by one segment pitch. -One winding brnch covers ll upper lyer positions from 1 7, (Npoles), nd lower lyer positions in between (S-poles). - Second winding brnch does the sme to the left (strt t 7, ends t 13): Simplex wve winding hs lwys two prllel brnches: = 1, 2 = 2. Exmple: 2 = 2, u = 1, Q r = 25, 2p = 4, K = 25, Spn t commuttor y = (K-1)/p = 12: 1 (upper lyer) 7 (lower lyer) 13 (UL) 19 (LL) 25 (UL) 6 (LL)..., etc. 10/24

25 Complete simplex wve winding Simplex wve winding: Exmple: Source: Dr. Holzer, TU Wien Q r = 25, 2p = 4, u = 1, N c = 1, = 1, K = 25, y 1 = 6, y 2 = 6, y = 12 10/25

26 Simplex wve winding: 2 = 2 Exmple: Q r = 25, 2p = 4, u = 1, N c = 1, = 1, p = 2, K = 25, y 1 = 6, y 2 = 6, y = 12 U = 440V Number of prllel winding brnches ALWAYS 2: 2 = 2 Source: Dr. Holzer, TU Wien -220V +220V 10/26

27 Wve winding 2 brushes re sufficient, BUT - Only one Plus- nd Minus-brush sufficient, s only 2 prllel brnches. BUT: Big brush cross section necessry (should be voided!) - Brush-contcted coils re positioned in neutrl zone (= zero field), so no voltge induced there - Hence: Additionl A-Brushes my be plced, distnced by double pole pitch, nd connected in prllel: e.g.: t commuttor segments 7 & 19 or 8 & 20 ): THUS: - p Plus- nd p Minus-brushes with reduced cross section 1/p re used. N S N S +220V +220V +220V +220V - 220V - 220V - 220V - 220V 10/27

28 Wve winding NO potentil equlizers necessry - The coils in neutrl zone (no voltge) ct s potentil equlizers 1 st kind ( = they connect equl potentils A1 nd A2 t 2p = 4, B1 nd B2). Fcit: The wve winding is self-equlizing. N S N S At the sme potentil - 220V +220V +220V - 220V 10/28

29 Comprison: Lp nd wve rmture winding U i Lp winding Number of prllel winding brnches = pole count 2 = 2p Equlizers of 1 st kind necessry High currents possible due to mny prllel brnches Voltge increses in proportion to z. High rted power possible (typiclly up to 12 MW) z p n Result: - DC mchines for big power re designed with lp winding. Wve winding Number of prllel winding brnches lwys 2 2 = 2 No equlizers necessry Current limited to c. 500 A, s mximum c. 250 A / prllel brnch High voltge, becuse it increses in proportion to z. p. Limited power (c. 300 kw) Exmple: 6 MW-cold strip mill drive. Mchine with 18 poles, so with 18 prllel winding brnches. - DC mchines with smller power re designed with wve winding (cheper); sufficient high voltge lso t smll flux per pole. 10/29

30 Big DC mchine 1 st stge of mill strip motor 12 MW Lp winding Second DC mchine Commuttor S N S Source: Siemens AG 10/30

31 Lines of force re the flux lines Only rmture field Field wekening Field increse geometric neutrl zone Mgneticlly neutrl zone Superposition of rmture field with min field of sttor poles gives resulting mgnetic field. Flux lines ct like rubber strings (MAXWELL s mgnetic pull) nd move the rotor nti-clockwise (MOTOR opertion). 10/31

32 Field distortion due to rmture field I = 0 : Air gp field B 0 t no-lod (rmture current = 0) beneth poles nerly constnt, becuse of constnt ir gp. I > 0 : At lod (rmture current flows) the rmture field B is excited. It is super-imposed on the min field nd results in field distortion (Armture rection). B ( x) 0 V f V ( x) ( x) - Left hlf of pole: B = B 0 B - Right hlf of pole: B = B 0 + B 10/32

33 Flux reduction due to rmture field rection - Increse of flux density t right pole side leds to iron sturtion. So resulting field is NOT B = B 0 + B, but B < B 0 + B. -Thus field increse on right pole side is smller then field decrese on left pole side. Hence per pole decrese of flux occurs. Result: With incresing rmture current I the mgnetic flux per pole is decresing t constnt field current I f. Additionl sturtion Counter-mesure: Compenstion winding in the sttor pole shoes. This winding hs to be excited by the rmture current. 10/33

34 Compenstion winding Comp C H ds 0 Comp Armture field B is cncelled! Compenstion winding & inter-pole winding in series with rmture winding - Armture current I feeds compenstion winding in the sttor pole shoes; direction of current flow opposite to current flow direction in rotor winding: Ampere-turns of rotor rmture nd of compenstion winding cncel: B = 0. 10/34

35 Compensted four-pole DC mchine Field coil Compenstion winding Inter-pole winding Armture winding Source: ABB Density of flux lines t left nd right pole edge IDENTICAL = NO field distortion! Compenstion winding necessry bove kw! 10/35

36 DC-mchine sttor poles without winding Inter pole Min pole with slots for compenstion winding Source: Brenner, Bürstdt 10/36

37 Four-pole sttor with sttor windings Field winding Compenstion winding Interpole with winding Source: Brenner, Bürstdt 10/37

38 Commuttion (current reversl) of rmture current The rmture coil current i c is n c current. It chnges from its positive vlue I /(2) to its negtive vlue -I /(2) nd vice vers, when the brush short-circuits the two coil ends ( = neighbouring commuttor segments). At this time, both coil sides re locted in the neutrl zone (B = 0). No voltge is induced by motion induction. Moving direction of rotor 10/38

39 Commuttion (current reversl) of rmture current An rmture coil hs the inductnce L c (slot nd winding overhng lekge field). A current chnge cuses self-induced voltge ( rectnce voltge of commuttion ) u R. With pproximtion of "liner commuttion": u R L c di dt c L c I T com k R ni Tcom bb / vc ~1/ n => u R k R n I 10/39

40 Inter-poles reduce rectnce voltge of commuttion u R u R increses with ) incresing lod (M resp. I ), b) with speed n. u R ignites" sprks between brush nd commuttor rpid brush erosion. Remedy: Inter-poles, excited by rmture current (commuttion winding, number of turns N W,Pole ). Commutting field B W induces vi motion induction compole voltge u W opposite to the rectnce voltge nd cncels the effect of u R. l u 2N ( v B ) ds 2N v l B v ~ n, BW ~ I W c 0 W c W Demnd: u W k W u R u W n I 0 Circuit of the inter-pole winding B1-B2 in series with the rmture A1-A2 with opposite winding direction. 10/40

41 Dimensioning of the commuttion winding u R nd u W depend on n nd I : t EACH opertion point (n, I ) vlid: k W = k R. C u R u W ( k k ) n I 0 R W Inter-pole Ampere-turns W : Demnd: Inter-pole mgnetic circuit is unsturted H ds H 2 2 2( ) 2, W W W f f W W NW, PolI N, PolI B W 0 0 ~ W W I Inter-pole Ampere turns N W, PolI must be chosen bigger thn rmture Ampereturns N, PolI z /(8p) I to get in the inter-pole ir gp W positive commutting field B W : (c. 10% 12% bigger). 10/41

42 Connection of commutting winding - Commutting field must be opposite to rmture field - Hence: Opposite sense of winding direction Resulting commutting field Commutting field Armture field For k W = k R inter-pole ir gp W nd N W,Pol /N,Pol must be chosen properly. Opticl check, if brushes re sprking. If so, then commutting field is either too strong or too wek ( Over-/Under-Commuttion")! Removing/Plcing of dditionl smll iron sheets t the inter-poles increses/ /decreses inter-pole ir gp W nd decreses/increses commutting field. 10/42

43 Inter-poles of four pole DC mchine Plcing of smll iron sheets Field coil Compenstion winding Inter-pole winding Armture winding Source: ABB Inter-poles necessry bove c. 1 kw! 10/43

44 Inter-poles of four pole DC mchine Source: ABB Compenstion winding & inter-pole winding in series with rmture winding 10/44

45 Seprtely excited DC mchine Four-pole DC mchine with inter-poles, but NO compenstion winding 400 V, 250 A, 100 kw, 2000/min Externl fn, driven by 2-pole, grid-fed induction motor (2950/min) Shft end Terminl box 160 mm Out-let of cooling ir flow 10/45 Source: Siemens AG, Bd Neustdt/Sle, Germny

46 10.2 Drive technology with DC mchines 10/46

47 Equivlent circuit of the seprtely excited dc mchine Armture conductors, commuttion- nd compensting winding = totl rmture resistnce R. Seprte excittion: field current I f djustble independent of I. U R I U b U i, Ui k2m ( I f ) U b : brush voltge drop c. 2 V Brking rotor losses: Iron losses, friction losses, dditionl losses (AC skin effect in conductors) will be neglected here! Internl power P : Air gp power P is converted vi the LORENTZ-forces into mechnicl power P m (vi the electromgnetic torque M e ). P UiI mme Pm Me UiI / m ( k2 m / m) I k2 I 10/47

48 Exmple: Power flow in DC motor Exmple : 200 kw motor, U = 430 V, n = 1470/min, = 92%, U i = V - electricl input power P P / 200/ kW e, in m, out - rmture current I Pe / U / A - internl power P U I i kW Pm! - brking rotor losses cuse: P δ > P m - Electromgnetic torque: M e = 206.7/( /60) = knm U I - Torque t the shft: M = P m,out / m = 200/( /60) = knm 10/48

49 Blnce of losses of the dc mchine Exmple: 200 kw dc motor, seprtely excited, I = 506 A, - Totl losses: = 17.4 kw converted into het 17.4 kw 1. Hereof in the rmture circuit = 10.7 kw 10.7 kw 2. In the brushes: 2V. 506A = 1.0 kw 1.0 kw 3. In the rmture resistnce: PCu, Pd, Pb kw 9.7 kw 4. Mechnicl brking torque M d of the rotor s difference between electromgnetic nd shft torque: M d = M e M = = knm, 5. This corresponds to the rotor losses P Fe PR Pz 2nMd P Pm, out = = 6.7 kw. 6.7 kw 6. Iron losses P Fe : Eddy-current nd hysteresis losses in the rotor iron sheets 7. Additionl losses P z : Eddy-currents in the slot conductors due to current displcement, s the conductors crry n AC current 8. Friction nd windge losses P R in the berings nd brushes nd cused by the cooling ir flow. - Additionlly: Excittion losses P f = 1.5 kw 1.5 kw 10/49

50 10/50 Sttionry bsic equtions of seprtely excited DC mchine s e f e f f f f m i b i M M I I k M I R U I k U U U R I U ) ( ) ( ) ( 2 2 Sttionry bsic equtions (consumer reference system): Neglecting friction losses, iron losses, dditionl rotor losses: Shft torque M s internl (electromgnetic) torque M e

51 coil Dynmic equtions of seprtely excited DC mchine Dynmic bsic equtions: Speed, rmture current, rmture voltge, field excittion current & voltge nd min flux re subject to chnge. Armture field: Armture self-inductnce L Min field: Field self-inductnce L f. Mutul inductnce M f only between ) commutting rmture coils (= shortcircuited by brushes) nd b) field coil, otherwise zero. min pole re coil: N c brush u( t) u u i M i ( t) f ( t) e k ( t) R i 2 ( t) R k ( t) i J d ( t) / dt m ( t) f 2 m f L ( t) M ( t) ( t), L e di f ( t) / dt ( t) ( i di f ( t) M ( t) / dt s ( t) u f i ( t) ( t)) 10/51

52 Seprtely excited dc genertor ) Mchine operted with n = const., field winding F1-F2 supplied by seprte dc voltge source U f. n = const. b1) Open-circuit chrcteristic: no-lod voltge U 0 (= induced voltge) mesured t vried field current I f (I f chnged by field regulting resistor): U k n( I ) 0 1 f b2) Internl chrcteristic: in cse of uncompensted mchine the flux is reduced due to sturtion by the vlue cused by incresing rmture current : U i U( I k f 1 n I f, I) ) U ( I ) I i ( U b3) Lod chrcteristic: rmture voltge U( I ) U I R U i depending on I f t I = const. f c) Externl chrcteristic: rmture voltge R b U depending on I t I f = const. 0 b 10/52

53 Shunt wound-/seprtely excited dc motor U = const. ) mchine supplied with U = const., field current I f is djustble. terminls: E1-E2: shunt wound opertion terminls: F1-F2: seprte U f -source b) motor chrcteristic n(m): From U U we get: I R ( U ), U k n Ui U IR n k1 k1 M er n n0 2 k k i b i 1 no lod: motor is only loded by its smll loss torque M d (friction, iron losses): M e = M d 0. No lod speed n 0 : n U /( k 1 ( I )) 0 f Result: Seprtely excited nd shunt-wound motors hve decresing speed-torquechrcteristic with smll slope, s the rmture voltge drop is smll compred to the rmture terminl voltge. (Fig b) curve 1: compensted mchine) /53

54 Instbility of shunt-wound/seprtely excited dc motors In uncompensted mchines, operted with currents bove roughly rted current, the min flux drops to = - with incresing rmture current due to dditionl sturtion cused by rmture rection. With big currents I ( = big flux loss ) speed increses gin, becuse the first ddend in the speed eqution increses fster thn the second one decreses. m U R M k2 )) 2 ( ( I)) k2 ( ( I 2 Stbility criterion (derivtion see IM): dm d e m dm d s m 0 stble Exmple: UNSTABLE: Increse of speed with incresing lod: mchine overspeeds, without ny brking it ccelertes to very high speed up to self-destruction. Counter-mesure: compenstion winding or speed control! 10/54

55 Instbility of over-commutted motors ) Over-commuttion: Commutting coil hs lredy reversed mpere-turns. It excites coil flux which reduces the min flux. b) Over-commuttion: Current reversl too quick, cused by too strong commuttion field! Over-commutting cuses coil flux ~ I, which opposes nd thus reduces the min flux. ~ I occurs lredy t smll currents! Speed chrcteristic hs positive inclintion lredy t no-lod speed. This my led to instbility! m k U M ( ( I)) k2 ( ( I)) Counter-mesure: Reduction of the commuttion field by djusting the inter-poles! R 10/55

56 Strting resistor in rmture circuit to strt dc mchine Motor t stndstill: n = 0: induced voltge is zero: U U I R 0 I R I U / R i Armture resistnce is very smll (except for smll motors) : rmture current t stnd still VERY BIG: motor winding would burn! Counter-mesures: Current limiting strting resistor in the rmture circuit: offers the opportunity to strt the motor with rted current. ( R strter R ) I N U 10/56 R strter After the strt-up the induced voltge limits the current; the strting resistor is then shortcircuited to void unnecessry resistive losses. U I N R Exmple: DC motor: U N = 430 V, P N = 200 kw, = 92% (without excittion losses), R = 37.9 m rted current: IN PN / UN 506 A Strting without strting resistor: I U / R 430/ A I Required strting resistor: R strter U N N / I R 430/ N N

57 Shunt-wound genertor (self-excittion) ) Excittion in prllel ( = shuntcircuit) to the rmture. b) Driven genertor cn generte voltge without ny uxiliry voltge source. Self-excittion: Remnence flux of the sttor poles R induces smll remnence voltge into Remnence U the rotting rmture coil. rem n = const. Urem k 1 nr U rem cuses field current I f = U rem /(R + R f + R v ). The corresponding min flux (I f ) increses remnence flux. This increses the induced voltge, so field current, which gin increses the field nd so on = SELF EXCITATION! Process stops in operting point A (voltge equilibrium). First published 1866 by Werner von SIEMENS s dynmoelectric principle. Suicide Control: With exchnged terminls E1, E2 the field current cuses flux tht opposes the remnence flux insted of supporting it: NO self-excittion! 10/57

58 n = const. Series-wound genertor ) Series-connection of rmture nd field: I = I f b) no-lod voltge: U i ( I 0) mesurble in cse of seprte excittion. Internl chrcteristic: Lod chrcteristic: U Remnence voltge U rem is the initil voltge. The voltge U increses only with incresing lod ( = rmture current I ), s the lod current is the field current lso. U i I U i ( I 0) R U Incresing I : Liner rise of rmture voltge drop I R, due to iron sturtion the induced voltge U i rises less thn liner: Terminl voltge U drops gin fter mximum. Appliction: Regenertive brking of series-wound mchines (e.g. electric trins, electric crs). b 10/58

59 U = const. Series-wound motor n Field current = rmture current Torque: L M e M e k2( I) I Approximtion: sturtion = constnt: I k, L = const., Torque rises with the squre of the rmture current: L I 2 2 With smll rmture currents nd thus smll flux this is vlid exctly, becuse iron sturtion occurs t stronger flux only. n(m)-chrcteristic: U IR 1 U U 1 R R k k L 2 I k L M 2k 2 e The speed of series-wound motor decreses t constnt sturtion hyperboliclly with the lod M e to the vlue zero during strting. 2 L 10/59

60 Importnce of the series-wound motor Series-wound motors must not be operted t no-lod, s t M s = 0 the motor would ccelerte to theoreticlly infinite speed ( overspeeding ) nd would be destroyed. The strong decrese of speed with incresing lod is clled soft chrcteristic ( series chrcteristic ). Appliction: DC trction (rilwy: e.g. Itly 3 kv, DC-grid), electric cr (DC bttery grid) ) low speed ( strting ): high torque = good ccelertion b) wheel-ril contct (rolling resistnce) nd erodynmic resistnce lwys lod the mchine, preventing the mchine from over-speeding under norml operting conditions. In cse of slipping wheels (e.g. wet rils) n over-speed protection hs to protect the motor. 10/60

61 Speed vrition of series-wound DC motor Resistor R sh in prllel to the field winding (shunt resistor) Reduction of field current = reduction of flux = increse of speed. I f Rsh I f R f IshRsh ( I I f ) Rsh 1 I R R sh f U = const. Opertion with single phse c current: - Trction (e.g. Deutsche Bhn, 16.7 Hz) - domestic pplinces: universl motor: vcuum clener, n mx = c /min, hir dryer, drilling mchine,... 10/61

62 Single phse AC commuttor mchine Excittion nd rmture winding re SERIES connected, being operted t single phse AC grid (Single phse series-wound motor). Field current = rmture current i = AC current (frequency f). Armture current excites min flux, which pulstes in phse with i : nd i reverse polrity t the sme time. M e ~ i ( ) ( i ) Torque hs lwys sme polrity, but pulstes with double frequency 2f. 10/62

63 Source: R. Fischer, hnser-verlg Universl motor Smll two-pole motors for high speed (up to c /min), low cost for mss production, no inter-poles (low number of operting hours, consumer drives). Opertion t e.g. 230 V/ 50 Hz: rmture current is AC current! AC flux nd rmture current ~ i give pulsting torque t 50 Hz-grid with double frequency 2f = 2x50 = 100 Hz! M M e( 2 e(, Field winding Averge torque vlue my only be used for driving. Therml AC power IS ONLY 70% of DC opertion! i ( t) Iˆ sin(2 f t) t) k ( ) ( ) ˆ sin(2 ) ˆ 2 t i t k ft I sin(2 ft) ˆ t) k2 I rms (1 cos(2 2 f t)) M k 2 ˆ e 2 I, rms / 2 10/63

64 n Vrible speed DC drive (1) U k I R U k 0 1 ( I f ) 1 k1 k1 I R n R I Seprtely excited dc motor: speed vrition by ) Vrition of rmture voltge U: no-lod speed n 0 chnges, n(i )-chrcteristic prllelly shifted, bse speed rnge": 0 < n < n N corresponds to 0 < U < U mx = U N t = mx = const. b) Flux wekening : no-lod speed n 0 increses, slope of n(i ) increses, field wekening rnge": n N < n < n mx corresponds to mx > > min t U = U mx = U N = const. c) Increse of resistnce R+R : no-lod speed n 0 constnt, slope of n(i ) increses, e.g. strting with strting resistor" (otherwise not used, due to dditionl losses in R) 10/64

65 Vrible speed DC drive (2) speed reversl by (A) polrity reversl of the rmture voltge from +U to U or (B) polrity reversl of the flux to -. (A) is quicker thn (B), s the rmture time constnt T = L /R is considerbly smller thn the field time constnt T f = L f /R f. Opertionl limits cused by mximum speed n mx nd mximum rmture current I,mx! Four-qudrnt opertion: 2. qudrnt: n > 0, M < 0: U > 0, I < 0 GEN. 3. qudrnt: n < 0, M < 0: U < 0, I < 0 MOT. 1. qudrnt: n > 0, M > 0: U > 0, I > 0 MOT. 4. qudrnt: n < 0, M > 0: U < 0, I > 0 GEN. 10/65

66 Vrible speed DC drive (3) 1. qudrnt Field wekening Four qudrnts Chnging of rmture voltge U : n(m)-chrcteristics re shifted in prllel = speed Vrition (constnt flux) If the rmture voltge cnnot be incresed ny further, speed cn be incresed using field wekening. At constnt rmture current the torque decreses (field wekening opertion). 10/66

67 Four qudrnt opertion n(m)-chrcteristic: n U M R 2k e ( k2 ) Vi converter U = U d is vrible: Speed n is chnged between +U d,mx nd -U d,mx. At -U d (chnge of polrity) speed is reversed (speed reversl). Mximum speed n 0 = U d,mx /(2k 2 ) cn be rised, if flux is reduced (flux wekening). Hence torque M e is decresing. 10/67

68 Limiting curves of seprtely excited DC mchine - Limiting curves = mximum vlues of the operting prmeters = envelope curves! - Up to rted speed n N the rmture voltge U cn be incresed. - For higher speed the flux needs to be wekened. - Above n R the mximum rmture current must be reduced to limit sprking. The rectnce voltge of commuttion is limited. 10/68

69 Thyristor-converter supplied dc mchine Genertion of vrible rmture voltge: A dc voltge U d for the dc drive is obtined by rectifiction of the three-phse grid L1, L2, L3. Bridge for one current direction! Controlled three-phse bridge rectifier B6C: Armture voltge depends on. If current reversl is desired, second nti-prllel converter is necessry. Thyristors conduct current, if there is positive voltge between node A nd cthode K AND firing impulse is supplied t to Gte G. If this firing impulse is delyed with respect to the first moment of positive voltge between A nd K by the time t ~, the rectified voltge decreses = vrition of rmture voltge. firing ngle" t, 2f U U cos, U 2U (3/ ) 0: 90 : 180 : grid d d, mx d, mx grid mx. voltge, voltge is zero, mx. negtive voltge t 10/69

70 (B6C)A(B6C)- Two nti-prllel thyristor bridges For current reversl second, nti-prllel converter is necessry! nti-prllel converter 10/70

71 Trnsistor-chopper supplied dc drive Disdvntge of the thyristor-converter B6C: dc voltge nd current show ripple with 6 times grid frequency: e.g.: t 50 Hz: Hz Alterntive to B6C-bridge: DC chopper converter: From constnt dc voltge (bttery, diode rectifier u U btt ) vrible verge dc voltge is generted using pulse width modultion (PWM). DC chopper: Chopped rmture voltge, verge i,pp voltge nd the rmture current with ripple one-qudrnt converter Free-wheeling diode required, becuse current pth needs to be provided for the inductive current tht cnnot be switched off immeditely due to the time constnt T. Current ripple i,pp : Due to the high switching frequency of the trnsistors (e.g. f P = 2 khz) the current ripple is smll: i, pp Ubtt (1 k) k /( fpl), k Ud / Ubtt T on / T 10/71

72 WARD-LEONARD-converter Voltge vrition for n idel dc voltge (e.g. test by) is done with rotting mchines: WARD-LEONARD-converter! A three-phse induction motor, supplied by the grid, drives seprtely excited dc genertor ( control genertor ) t lmost constnt speed n IM. The field current is supplied by n dditionl rotting converter or by bttery. This genertor supplies vrible dc voltge U to the dc motor, which cn be chnged vi I fg. Disdvntges of the WARD-LEONARD-converter: ) three times rted mchine power needs to be instlled (expensive!) b) three times the losses (e.g. efficiency per mchine 90 %: totl = 0.73 = 73 %) c) poor dynmic: U-chnge is slow, s the time constnt T f = L f / R f of control genertor is big. 10/72

73 Smll motors Europen mrket: Applictions: Automobil Automotive Hushlt Home pplince Industrie Industry Pumpen/Kompressoren Pumps Heting/ventiltion Heiz/Klimtechnik Bureu Bürogeräte Portble tools Trgbre Werkzeuge Medicl cre Medizintechnik 4,4 Mrd US $ 5,4 Mrd US $ +3% p.. 35,1% 12,3% 11,5% 9,8% 9,7% 5,9% 5,1% 5,1% Else Sonstige 5,5% Source: Frost & Sullivn Source: Fulhber 10/73

74 Opertionl limits of the dc mchine Frme size resp. vible power per mchine set ( unit power ) is limited by the commuttor. Source: F. Brenner, Bürstdt - centrifugl force limit: prevent commuttor deformtion, brushes bounce! - commuttion: rectnce voltge u R < 10 V in stedy-stte, < 20 V trnsient, otherwise strong sprking! - brush current density: stedy-stte J b < 12 A/cm 2, < 20 A/cm 2 trnsient, otherwise brush dmge - segment voltge limit: verge segment voltge U s,v < 20 V, locl segment voltge < 35 V, otherwise flshover. Uncontrolled opertion: Stbility limit needs to be considered, s the seprtely excited motor usully my only be operted in the rnge of negtive slope of the n(m)-chrcteristic. 10/74

75 Lrge DC mchines Biggest DC mchines for strip mills s 1 st stge drive units with typiclly 6 MW MW t speed rnge c. 100/min. To increse power, two mchines re coupled in tndem ( Tndem -opertion). Mounting of DC rotor of big DC mchine 12 MW Commuttor Second DC mchine for tndem opertion Source: Siemens AG 10/75

76 DC mchines - perspectives Lrge DC mchines re replced due to power limits - by ) converter-fed synchronous mchines (up to c. 100 MW!) nd b) inverter-fed induction mchines (up to c. 40 MW). Also in lower power rnge the converter-fed DC mchine is replced by the inverter-fed, robust cge induction mchine with field-oriented control (due to brush mintennce!). Smll DC motors: In utomotive ppliction nd household pplince still stedily incresing numbers (crs: 12 V / 24 V.) Source: Siemens AG 10/76

77 Tht s ll, folks! Source: F. Brenner, Bürstdt 10/77