Basic parts of an AC motor : rotor, stator, The stator and the rotor are electrical

Transcription

1 INDUCTION MOTO 1

2 CONSTUCTION Baic part of an AC motor : rotor, tator, encloure The tator and the rotor are electrical circuit that perform a electromagnet.

3 CONSTUCTION (tator) The tator - tationary part of the motor. Stator lamination are tacked together forming a hollow cylinder. Coil of inulated wire are inerted into lot of the tator core. Each grouping of coil, together with the teel core it urround, form an electromagnet. t 3

4 CONSTUCTION (rotor) The rotor i the rotating part of the motor It can be found in two type: Squirrel cage (mot common) Wound rotor Short circuit all rotor bar. /rotor winding 4

5 CONSTUCTION (rotor) Squirrel cage type: otor t winding i compoed of copper bar embedded in the rotor lot and horted at both end by end ring Simple, low cot, robut, low maintenance 5

6 CONSTUCTION (rotor) Wound rotor type: otor t winding i wound by wire. The winding terminal can be connected to external circuit through lip ring and bruhe. (imilar with DC motor, with the coil connected together that make contact with bruhe) Eay to control peed, more expenive. 6

7 CONSTUCTION (encloure) The encloure conit of a frame (or yoke) and two end bracket (or bearing houing). The tator i mounted inide the frame. The rotor fit inide the tator with a light air gap eparating it from the tator (NO direct phyical connection) Stator otor Air gap 7

8 CONSTUCTION (encloure) The encloure protect the electrical and operating part of the motor from harmful effect of the environment in which the motor operate. Bearing, mounted on the haft, upport the rotor and allow it to turn. A fan, alo mounted on the haft, i ued on the motor hown below for cooling. 8

9 otating Magnetic Field When a 3 phae tator winding i connected to a 3 phae voltage upply, 3 phae current will flow in the winding, which alo will induce 3 phae flux in the tator. Thee flux will rotate at a peed called a Synchronou Speed, n. The flux i called a otating ti magnetic Field Synchronou peed: peed of rotating flux n = 10 p f Where; p = i the number of pole, and f = the frequency of upply 9

10 Slip and otor Speed 1. Slip The rotor peed of an Induction machine i different from the peed of otating magnetic field. The % difference of the peed i called lip. = n n n n r O Where; n = ynchronou peed (rpm) n r = mechanical peed of rotor (rpm) under normal operating condition, = 0.01 ~ 0.05, which i very mall and the actual peed i very cloe to ynchronou peed. Note that : i not negligible n r = n ( 1 ) 10

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12 Induction Motor: otating Field Conider a imple tator with 6 alient pole - winding AN, BN, CN. The winding are mechanically paced at 10 from each other. The winding are connected to a 3-phae ource. AC current Ia, Ib and Ic will flow in the winding, but will be diplaced in time by 10. Each winding produce it own MMF,which create a flux acro the hollow interior of the tator. The 3 fluxe combine to produce a magnetic field that rotate at the ame frequency a the upply

13 Slip and otor Speed otor Speed When the rotor move at rotor peed, n r (rp), the tator flux will circulate the rotor conductor at a peed of (n -n r ) per econd. Hence, the frequency of the rotor i written a: f r = ( n n r ) = f p Where; Note : At tator : At otor : ( ii) ( i) : n n = n 10 f p = lip f = upply frequency n p f =...( i) 10 r r = 10 f p ( n nr ) p fr =...( ii) 10 f =. f 13

14 Principle of Operation Torque producing mechanim When a 3 phae tator winding i connected to a 3 phae voltage upply, 3 phae current will flow in the winding, hence the tator i energized. A rotating flux Φ i produced in the air gap.the flux Φ induce a voltage E a in the rotor winding (like a tranformer). The induced voltage produce rotor current, if rotor circuit i cloed. The rotor current interact with the flux Φ, producing torque. The rotor rotate in the direction of the rotating flux. 14

15 Direction of otor otate Q: How to change the direction of rotation? A: Change the phae equence of the power upply. 15

16 Equivalent Circuit of Induction Machine Conventional equivalent circuit Note: Never ue three-phae equivalent circuit. Alway ue per- phae equivalent circuit. The equivalent circuit alway bae on the Y connection regardle of the actual connection of the motor. Induction machine equivalent circuit i very imilar to the ingle-phae equivalent circuit of tranformer. It i compoed of tator circuit and rotor circuit 16

17 Equivalent Circuit of Induction Machine Step1 otor winding i open (The rotor will not rotate) t f f Note: the frequency of E i the ame a that of E 1 ince the rotor i at tandtill. At tandtill =1. 17

18 Equivalent Circuit of Induction Machine 18

19 Equivalent Circuit of Induction Machine Step otor winding i horted (Under normal operating condition, the rotor winding i horted. The lip i ) f f r Note: the frequency of E i f r =f becaue rotor i rotating. 19

20 Equivalent Circuit of Induction Machine Step3 Eliminate f Keep the rotor current ame: 0

21 Equivalent Circuit of Induction Machine Step 4 eferred to the tator ide Note: X and will be given or meaured. In practice, we do not have to calculate them from above equation. Alway refer the rotor ide parameter to tator ide. c repreent core lo, which i the core lo of tator ide. 1

22 Equivalent Circuit of Induction Machine IEEE recommended equivalent circuit

23 Equivalent Circuit of Induction Machine IEEE recommended equivalent circuit X I ' X ' V 1 Xm ' 1 Note: can be eparated into PATS (1 ) = + Purpoe : to obtain the developed mechanical 3

24 Power Flow Diagram 3 V I coθ P in (Motor) P in (Stator) P in in (otor) P air Gap (P (P ag ) P developed P mechanical P converted (P m ) 1 3 I ' 3 I ' ' ' 1 hp = 746 P out, P o W P tator copper lo, (P cu ) 3I 3 P core lo (P c ) V M c P rotor copper lo (P rcu ) 3 I ' ' P windage, friction, etc (P μ - Given) 4

25 Torque-Equation Torque, can be derived from power equation in term of mechanical power or electrical power. π n Power, P = ω T, whereω = ( rad/ ) 60 60P Hence, T = π n Thu, Mechanical Output Torque, T Torque, T o = m = 60 Po πnπ n r 60 P πn m r 5

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27 Type of Electric Motor AC Motor Induction motor elationhip load, peed and torque At 80% of full peed: highet pull- out torque and current drop At tart: high current and low pull-up up torque At full peed: torque and tator current are zero

28 Speed Control There are 3 type of peed control of 3 phae induction machine i. Varying rotor reitance ii. Varying upply voltage iii. Varying upply voltage and upply frequency 8

29 Varying rotor reitance For wound rotor only Speed i decreaing Contant maximum torque The peed at which max torque occur change Diadvantage: large peed regulation Power lo in ext reduce the efficiency T T 3 n r3 1 1 < < 3 n r1 < n r < n r3 n r n r1 n n ~n NL 9

30 Varying upply voltage Maximum torque change The peed which at max torque occur i contant (at max torque, X = / elatively imple method ue power electronic circuit for voltage controller Suitable for fan type load Diadvantage : Large peed regulation ince T T V 1 V V 3 V decreaing V 1 > V > V 3 n r1 > n r > n r3 ~n n r3 n r n r1 n n ~n NL 30

31 Varying upply voltage and upply frequency The bet method ince upply voltage and upply frequency i varied to keep V / f contant Maintain i peed regulation ue power electronic circuit for frequency and voltage controller Contant maximum torque T T f decreaing n r3 n r n r1 n NL3 n NL n NL1 n 31

32 Torque-Equation Note that, Mechanical torque can written in term of circuit parameter. Thi i determined by uing approximation method ' Hence, Plot T Pm = 3I ' (1 ) and Pm = ωrt m v m T ' T max m 3 ' I (1 ) P m T = = m ωr ωr... T t = V ' ( ') + ( X 3( Mφ ) T m π n ') =1 max max max i the lip for T max to occur n 3

33 Power Flow Diagram atio: P ag P rcu P m 3I ' 1 ' 3I ' 1 ' 3I ' ' 1 1 atio make the analyi impler to find the value of the particular power if we have another particular power. For example: P rcu = P m 1 33

34 Torque-Equation Torque Equation = ' ) 3( 1, V Starting Torque = ') ( ') ( 60 ) 3( t X X n V T π φ + = ± max ') ( ) ( ' X 1 ) 3( ) ( ) ( V = max ') ( ) ( 1 60 ) 3( X X n V T π φ 34 60

35 COGGING AND CAWLING When rotor bar are made to run parallel with tator, the torque rie & fall correpondingly cauing more pulation. Thi i termed a cogging in other word magnetic locking. Thi i reduced by making the rotor bar run at an angle to the tator i.e crawling in order to make the torque uniform. Crawling on the other hand ignifie running of motor at almot one eventh of the rated pped due to interference of eventh harmonic.

36 OTATING FIELD

37 OVEVIEW OF SINGLE PHASE IM Home air conditioner Kitchen fan Wahing W machine Indutrial machine Compreor efrigerator

38 OVEVIEW OF SINGLE PHASE IM Type of 1φ induction Motor Split Phae Motor Capacitor Start Motor Capacitor Start, Capacitor un Shaded Pole Induction Motor Univeral Motor (ac erie motor)