Single-Phase Motors. Chapter (9) Introduction. 9.1 Types of Single-Phase Motors. 9.2 Single-Phase Induction Motors

Transcription

1 Chapter (9) Single-Phae Motor Introduction A the name ugget, thee motor are ued on ingle-phae upply. Singlephae motor are the mot familiar of all electric motor becaue they are extenively ued in home appliance, hop, office etc. It i true that inglephae motor are le efficient ubtitute for 3-phae motor but 3-phae power i normally not available except in large commercial and indutrial etablihment. Since electric power wa originally generated and ditributed for lighting only, million of home were given ingle-phae upply. Thi led to the development of ingle-phae motor. Even where 3-phae main are preent, the ingle-phae upply may be obtained by uing one of the three line and the neutral. In thi chapter, we hall focu our attention on the contruction, working and characteritic of commonly ued ingle-phae motor. 9.1 Type of Single-Phae Motor Single-phae motor are generally built in the fractional-horepower range and may be claified into the following four baic type: 1. Single-phae induction motor (i) plit-phae type (iii) haded-pole type (ii) capacitor type. A.C. erie motor or univeral motor 3. Repulion motor (i) Repulion-tart induction-run motor (ii) Repulion-induction motor 4. Synchronou motor (i) Reluctance motor (ii) Hyterei motor 9. Single-Phae Induction Motor A ingle phae induction motor i very imilar to a 3-phae quirrel cage induction motor. It ha (i) a quirrel-cage rotor identical to a 3-phae motor and (ii) a ingle-phae winding on the tator. 1

2 Unlike a 3-phae induction motor, a ingle-phae induction motor i not elftarting but require ome tarting mean. The ingle-phae tator winding produce a magnetic field that pulate in trength in a inuoidal manner. The field polarity revere after each half cycle but the field doe not rotate. Conequently, the alternating flux cannot produce rotation in a tationary quirrel-cage rotor. However, if the rotor of a ingle-phae motor i rotated in one direction by ome mechanical mean, it will continue to run in the direction of rotation. A a matter of fact, the rotor quickly accelerate until it reache a peed lightly below the ynchronou peed. Once the motor i running at thi peed, it will continue to rotate even though ingle-phae current i flowing through the tator winding. Thi method of tarting i generally not convenient for large motor. Nor can it be employed fur a motor located at ome inacceible pot. Fig. (9.1) how ingle-phae induction motor having a quirrel cage rotor and a inglephae ditributed tator winding. Such a motor inherently doc not develop any tarting torque and, therefore, will not tart to rotate if the tator winding i connected to ingle-phae a.c. upply. However, if the Fig.(9.1) rotor i tarted by auxiliary mean, the motor will quickly attain me final peed. Thi trange behaviour of ingle-phae induction motor can be explained on the bai of double-field revolving theory. 9.3 Double-Field Revolving Theory The double-field revolving theory i propoed to explain thi dilemma of no torque at tart and yet torque once rotated. Thi theory i baed on the fact that an alternating inuoidal flux ( = m co ωt) can be repreented by two revolving fluxe, each equal to one-half of the maximum value of alternating flux (i.e., m /) and each rotating at ynchronou peed (N = 10 f/p, ω = πf) in oppoite direction. The above tatement will now be proved. The intantaneou value of flux due to the tator current of a ingle-phae induction motor i given by; = m coωt Conider two rotating magnetic fluxe 1 and each of magnitude m / and rotating in oppoite direction with angular velocity ω [See Fig. (9.)]. Let the two fluxe tart rotating from OX axi at Fig.(9.)

3 t = 0. After time t econd, the angle through which the flux vector have rotated i at. Reolving the flux vector along-x-axi and Y-axi, we have, Total X-component m m = co ωt + co ωt = m m m Total Y-component = in ωt in ωt = 0 Reultant flux, ( co t) 0 co t = m ω + = m ω coωt Thu the reultant flux vector i = m co ωt along X-axi. Therefore, an alternating field can be replaced by two relating field of half it amplitude rotating in oppoite direction at ynchronou peed. Note that the reultant vector of two revolving flux vector i a tationary vector that ocillate in length with time along X-axi. When the Fig.(9.3) rotating flux vector are in phae [See Fig. (9.3 (i))], the reultant vector i = m ; when out of phae by 180 [See Fig. (9.3 (ii))], the reultant vector = 0. Let u explain the operation of ingle-phae induction motor by double-field revolving theory. (i) Rotor at tandtill Conider the cae that the rotor i tationary and the tator winding i connected to a ingle-phae upply. The alternating flux produced by the tator winding can be preented a the um of two rotating fluxe 1 and, each equal to one half of the maximum value of alternating flux and each rotating at ynchronou peed (N = 10 f/p) in oppoite direction a hown in Fig. (9.4 (i)). Let the flux 1 rotate in anti clockwie direction and flux in clockwie direction. The flux 1 will reult in the production of torque T 1 in the anti clockwie direction and flux will reult in the production of torque T In the clockwie direction. At tandtill, thee two torque are equal and oppoite and the net torque developed i zero. Therefore, ingle-phae induction motor i not elf-tarting. Thi fact i illutrated in Fig. (9.4 (ii)). Note that each rotating field tend to drive the rotor in the direction in which the field rotate. Thu the point of zero lip for one field correpond to 00% lip for the other a explained later. The value of 100% lip (tandtill condition) i the ame for both the field. 3

4 Fig.(9.4) (ii) Rotor running Now aume that the rotor i tarted by pinning the rotor or by uing auxiliary circuit, in ay clockwie direction. The flux rotating in the clockwie direction i the forward rotating flux ( f ) and that in the other direction i the backward rotating flux ( b ). The lip w.r.t. the forward flux will be where N N = N f = N = ynchronou peed N = peed of rotor in the direction of forward flux The rotor rotate oppoite to the rotation of the backward flux. Therefore, the lip w.r.t. the backward flux will be b b = N N = N = ( N) N + N N = = N N (N N) = N N N + N Thu fur forward rotating flux, lip i (le than unity) and for backward rotating flux, the lip i (greater than unity). Since for uual rotor reitance/reactance ratio, the torque at lip of le than unity arc greater than thoe at lip of more than unity, the reultant torque will be in the direction of the rotation of the forward flux. Thu if the motor i once tarted, it will develop net torque in the direction in which it ha been tarted and will function a a motor. 4

5 Fig. (9.5) how the rotor circuit for the forward and backward rotating fluxe. Note that r = R /, where R i the tandtill rotor reitance i.e., r i equal to half the tandtill rotor reitance. Similarly, x = X / where X i the tandtill rotor reactance. At tandtill, = 1 o that impedance of the two circuit are equal. Therefore, rotor current are equal i.e., I f = I b. However, when the rotor rotate, the impedance of the two rotor circuit are unequal and the rotor current I b i higher (and alo at a lower power factor) than the rotor current I f. Their m.m.f., which oppoe the tator m.m.f., will reult in a reduction of the backward rotating flux. Conequently, a peed increae, the forward flux increae, increaing the driving torque while the backward flux decreae, reducing the oppoing torque. The motor-quickly accelerate to the final peed. Fig.(9.5) 9.4 Making Single-Phae Induction Motor Self-Starting The ingle-phae induction motor i not elftarting and it i undeirable to reort to mechanical pinning of the haft or pulling a belt to tart it. To make a ingle-phae induction motor elf-tarting, we hould omehow produce a revolving tator magnetic field. Thi may be achieved by converting a ingle-phae upply into two-phae upply through the ue of an additional winding. When the motor attain ufficient peed, the tarting mean (i.e., additional winding) may be removed depending upon the type of the motor. A a matter of fact, ingle-phae Fig.(9.6) induction motor are claified and named according to the method employed to make them elf-tarting. (i) Split-phae motor-tarted by two phae motor action through the ue of an auxiliary or tarting winding. 5

6 (ii) Capacitor motor-tarted by two-phae motor action through the ue of an auxiliary winding and a capacitor. (iii) Shaded-pole motor-tarted by the motion of the magnetic field produced by mean of a hading coil around a portion of the pole tructure. 9.5 Rotating Magnetic Field From -Phae Supply A with a 3-phae upply, a -phae balanced upply alo produce a rotating magnetic field of contant magnitude. With the exception of the haded-pole motor, all ingle-phae induction motor are tarted a -phae machine. Once o tarted, the motor will continue to run on ingle-phae upply. Let u ee how -phae upply produce a rotating magnetic field of contant magnitude. Fig. (9.10 (i)) how -pole, -phae winding. The phae X and Y are energized from a two-phae ource and current in thee phae arc indicated a I x and I y [See Fig. (9.10 (ii))]. Referring to Fig. (9.10 (ii)), the fluxe produced by thee current arc given by; Y = m in ωt and X = m in( ωt + 90 ) = m coωt Here m i the maximum flux due to either phae. We hall now prove that thi -phae upply produce a rotating magnetic field of contant magnitude equal to m. (i) At intant 1 [See (Fig (ii)) and Fig. (9.10 (iii))], the current i zero in phae Y and maximum in phae X. With the current in the direction hown, a reultant flux i etablihed toward the right. The magnitude of the reultant flux i contant and i equal to m a proved under: At intant 1, ωt = 0 = 0 and = Fig.(9.7) Reultant flux, Y r = X + Y = m ) + (0) X m ( = (ii) At intant [See Fig. (9.10 (ii)) and Fig. (9.10 (iii))], the current i till in the ame direction in phae X and an equal current flowing in phae Y. Thi etablihe a reultant flux of the ame value (i.e., r = m ) a proved under: At intant, ωt = 45 Reultant flux, Y m = and r = ( X ) + ( Y ) X m = m Fig.(9.8) = m m + = m 6

7 Note that reultant flux ha the ame value (i.e. m ) but turned 45 clockwie from poition 1. (iii) At intant 3 [See Fig. (9.10 (ii)) and Fig. (9.10 (iii))], the current in phae. X ha decreaed to zero and current in phae Y ha increaed to maximum. Thi etablihe a reultant flux downward a proved under: Fig.(9.9) Fig.(9.10) 7

8 At intant 3, ωt = 90 = and 0 Y m X = r = X + Y ) = (0) + ( m ) ( = Note that reultant flux ha now turned 90 clockwie from poition 1. The reader may note that in the three intant conidered above, the reultant flux i contant and i equal to m. However, thi contant reultant flux i hining it poition (clockwie in thi cae). In other word, the rotating flux i produced. We hall continue to conider other intant to prove thi fact. (iv) At intant 4 [See Fig. (9.10 (ii)) and Fig. (9.10 (iii))], the current in phae X ha revered and ha the ame value a that of phae Y. Thi etablihe a reultant flux equal to m turned 45 clockwie from poition 3. At intant 4, r = ωt = 135 X + Y = Y m = Fig.(9.11) r = X + Y = ( m ) + (0) = m Fig.(9.1) (vi) Diagram 6, 7, and 8 [See Fig. (9.10 (iii))] indicate the direction of the reultant flux during the remaining ucceive intant. m and m + m = X m m = (v) At intant 5 [See Fig. (9.10 (ii)) and Fig. (9.10 (iii))], the current in phae X i maximum and in phae Y i zero. Thi etablihe a reultant flux equal to m toward left (or 90 clockwie from poition 3). At intant 5, ωt = 180 Y = 0 and X = It follow from the above dicuion that a -phae upply produce a rotating magnetic field of contant value (= m the maximum value of one of the field). Note: If the two winding arc diplaced 90 electrical but produce field that are not equal and that are not 90 apart in time, the reultant field i till rotating but i not contant in magnitude. One effect of thi nonuniform rotating field i the production of a torque that i non-uniform and that, therefore, caue noiy operation of the motor. Since -phae operation ceae once the motor i tarted, the operation of the motor then become mooth. m 8

9 9.6 Split-Phae Induction Motor The tator of a plit-phae induction motor i provided with an auxiliary or tarting winding S in addition to the main or running winding M. The tarting winding i located 90 electrical from the main winding [See Fig. (9.13 (i))] and operate only during the brief period when the motor tart up. The two winding are o reigned that the tarting winding S ha a high reitance and relatively mall reactance while the main winding M ha relatively low reitance and large reactance a hown in the chematic connection in Fig. (9.13 (ii)). Conequently, the current flowing in the two winding have reaonable phae difference c (5 to 30 ) a hown in the phaor diagram in Fig. (9.13 (iii)). Fig.(9.13) Operation (i) When the two tator winding are energized from a ingle-phae upply, the main winding carrie current I m while the tarting winding carrie current I. (ii) Since main winding i made highly inductive while the tarting winding highly reitive, the current I m and I have a reaonable phae angle a (5 to 30 ) between them a hown in Fig. (9.13 (iii)). Conequently, a weak revolving field approximating to that of a -phae machine i produced which tart the motor. The tarting torque i given by; T = ki I in α m where k i a contant whoe magnitude depend upon the deign of the motor. (iii) When the motor reache about 75% of ynchronou peed, the centrifugal witch open the circuit of the tarting winding. The motor then operate a a ingle-phae induction motor and continue to accelerate till it reache the 9

10 normal peed. The normal peed of the motor i below the ynchronou peed and depend upon the load on the motor. Characteritic (i) The inning torque i 15 to time the full-loud torque mid (lie tarting current i 6 to 8 time the full-load current. (ii) Due to their low cot, plit-phae induction motor are mot popular inglephae motor in the market. (iii) Since the tarting winding i made of fine wire, the current denity i high and the winding heat up quickly. If the tarting period exceed 5 econd, the winding may burn out unle the motor i protected by built-in-thermal relay. Thi motor i, therefore, uitable where tarting period are not frequent. (iv) An important characteritic of thee motor i that they are eentially contant-peed motor. The peed variation i -5% from no-load to fullload. (v) Thee motor are uitable where a moderate tarting torque i required and where tarting period are infrequent e.g., to drive: (a) fan (b) wahing machine (c) oil burner (d) mall machine tool etc. The power rating of uch motor generally lie between 60 W and 50 W. 9.7 Capacitor-Start Motor The capacitor-tart motor i identical to a plit-phae motor except that the tarting winding ha a many turn a the main winding. Moreover, a capacitor C i connected in erie with the tarting winding a hown in Fig. (9.14 (i)). The value of capacitor i o choen that I lead I m by about 80 (i.e., α ~ 80 ) which i coniderably greater than 5 found in plit-phae motor [See Fig. (9.14 (ii))]. Conequently, tarting torque (T = k I m I in α) i much more than that of a plit-phae motor Again, the tarting winding i opened by the centrifugal witch when the motor attain about 75% of ynchronou peed. The motor then operate a a ingle-phae induction motor and continue to accelerate till it reache the normal peed. Characteritic (i) Although tarting characteritic of a capacitor-tart motor are better than thoe of a plit-phae motor, both machine poe the ame running characteritic becaue the main winding are identical. (ii) The phae angle between the two current i about 80 compared to about 5 in a plit-phae motor. Conequently, for the ame tarting torque, the current in the tarting winding i only about half that in a plit-phae motor. Therefore, the tarting winding of a capacitor tart motor heat up le 10

11 quickly and i well uited to application involving either frequent or prolonged tarting period. Fig.(9.14) (iii) Capacitor-tart motor are ued where high tarting torque i required and where the tarting period may be long e.g., to drive: (a) compreor (b) large fan (c) pump (d) high inertia load The power rating of uch motor lie between 10 W and 7-5 kw. 9.8 Capacitor-Start Capacitor-Run Motor Thi motor i identical to a capacitor-tart motor except that tarting winding i not opened after tarting o that both the winding remain connected to the upply when running a well a at tarting. Two deign are generally ued. (i) In one deign, a ingle capacitor C i ued for both tarting and running a hown in Fig.(9.15 (i)). Thi deign eliminate the need of a centrifugal witch and at the ame time improve the power factor and efficiency of the motor. Fig.(9.15) (ii) In the other deign, two capacitor C 1 and C are ued in the tarting winding a hown in Fig. (9.15 (ii)). The maller capacitor C 1 required for optimum running condition i permanently connected in erie with the 11

12 tarting winding. The much larger capacitor C i connected in parallel with C 1 for optimum tarting and remain in the circuit during tarting. The tarting capacitor C 1 i diconnected when the motor approache about 75% of ynchronou peed. The motor then run a a ingle-phae induction motor. Characteritic (i) The tarting winding and the capacitor can be deigned for perfect -phae operation at any load. The motor then produce a contant torque and not a pulating torque a in other ingle-phae motor. (ii) Becaue of contant torque, the motor i vibration free and can be ued in: (a) hopital (6) tudio and (c) other place where ilence i important. 9.9 Shaded-Pole Motor The haded-pole motor i very popular for rating below 0.05 H.P. (~ 40 W) becaue of it extremely imple contruction. It ha alient pole on the tator excited by ingle-phae upply and a quirrelcage rotor a hown in Fig. (9.16). A portion of each pole i urrounded by a hort-circuited turn of copper trip called hading coil. Fig.(9.16) Operation The operation of the motor can be undertood by referring to Fig. (9.17) which how one pole of the motor with a hading coil. (i) During the portion OA of the alternating-current cycle [See Fig. (9.17)], the flux begin to increae and an e.m.f. i induced in the hading coil. The reulting current in the hading coil will be in uch a direction (Lenz law) o a to oppoe the change in flux. Thu the flux in the haded portion of the pole i weakened while that in the unhaded portion i trengthened a hown in Fig. (9.17 (ii)). (ii) During the portion AB of the alternating-current cycle, the flux ha reached almot maximum value and i not changing. Conequently, the flux ditribution acro the pole i uniform [See Fig. (9.17 (iii))] ince no current i flowing in the hading coil. A the flux decreae (portion BC of the alternating current cycle), current i induced in the hading coil o a to oppoe the decreae in current. Thu the flux in the haded portion of the 1

13 pole i trengthened while that in the unhaded portion i weakened a hown in Fig. (9.17 (iv)). Fig.(9.17) (iii) The effect of the hading coil i to caue the field flux to hift acro the pole face from the unhaded to the haded portion. Thi hifting flux i like a rotating weak field moving in the direction from unhaded portion to the haded portion of the pole. (iv) The rotor i of the quirrel-cage type and i under the influence of thi moving field. Conequently, a mall tarting torque i developed. A oon a thi torque tart to revolve the rotor, additional torque i produced by ingle-phae induction-motor action. The motor accelerate to a peed lightly below the ynchronou peed and run a a ingle-phae induction motor. Characteritic (i) The alient feature of thi motor are extremely imple contruction and abence of centrifugal witch. (ii) Since tarting torque, efficiency and power factor are very low, thee motor are only uitable for low power application e.g., to drive: (a) mall fan (6) toy (c) hair drier (d) dek fan etc. The power rating of uch motor i upto about 30 W. 13

14 9.10 Equivalent Circuit of Single-Phae Induction Motor It wa tated earlier that when the tator of a ingle-phae induction motor i con heeled to ingle-phae upply, the tator current produce a pulating flux that i equivalent to two-contant-amplitude fluxe revolving in oppoite direction at the ynchronou peed (double-field revolving theory). Each of thee fluxe induce current in the rotor circuit and produce induction motor action imilar to that in a 3-phae induction motor Therefore, a ingle-phae induction motor can to imagined to be coniting of two motor, having a common tator winding but with their repective rotor revolving in oppoite direction. Each rotor ha reitance and reactance half the actual rotor value. Let R 1 = reitance of tator winding X 1 = leakage reactance of tator winding X m = total magnetizing reactance R' = reitance of the rotor referred to the tator X' = leakage reactance of the rotor referred to the tator (i) revolving theory. At tandtill. At tandtill, the motor i imply a tranformer with it econdary hort-circuited. Therefore, the equivalent circuit of ingle-phae motor at tandtill will be a hown in Fig. (9.18). The double-field revolving theory ugget that characteritic aociated with each revolving field will be jut one-half of the characteritic aociated with the actual total flux. Therefore, each rotor ha reitance and reactance equal to R' / and X' / repectively. Each rotor i aociated with half the Fig.(9.18) 14

15 total magnetizing reactance. Note that in the equivalent circuit, the core lo ha been neglected. However, core lo can be repreented by an equivalent reitance in parallel with the magnetizing reactance. At tandtill, f = b. Therefore, E f = E b. Now E f = 4.44 f N f ; E b = 4.44 f N b where V 1 ~ E f + E b = I 1 Z f + I 1 Z b Z f = impedance of forward parallel branch Z b = impedance of backward parallel branch (ii) Rotor running. Now conider that the motor i pinning at ome peed in the direction of the forward revolving field, the lip being. The rotor current produced by the forward field will have a frequency f where f i the tator frequency. Alo, the rotor current produced by the backward field will have a frequency of ( )f. Fig. (9.19) how the equivalent circuit of a ingle-phae induction motor when the rotor i rotating at lip. It i clear, from the equivalent circuit that under running condition, E f become much greater than E b becaue the term R' / increae very much a tend toward zero. Converely, E^ fall becaue the term R' /( ) decreae ince ( ) tend toward. Conequently, the forward field increae, increaing the driving torque while the backward field decreae reducing the oppoing torque. Fig.(9.19) Total impedance of the circuit.i given by; 15

16 Z r = Z 1 + Z f + Z b where Z 1 = R 1 + j X 1 Z Z f b I = V Xm R' X' j + j = R' Xm X' + j + Xm R' X' j + j ( ) = R' Xm X' + j + ( ) 1 1 / Zr 9.11 A.C. Serie Motor or Univeral Motor A d.c. erie motor will rotate in the ame direction regardle of the polarity of the upply. One can expect that a d.c. erie motor would alo operate on a ingle-phae upply. It i then called an a.c. erie motor. However, ome change mut be made in a d.c. motor that i to operate atifactorily on a.c. upply. The change effected are: (i) The entire magnetic circuit i laminated in order to reduce the eddy current lo. Hence an a.c. erie motor require a more expenive contruction than a d.c. erie motor. (ii) The erie field winding ue a few turn a poible to reduce the reactance of the field winding to a minimum. Thi reduce the voltage drop acro the field winding. (iii) A high field flux i obtained by uing a low-reluctance magnetic circuit. (iv) There i coniderable parking between the bruhe and the commutator when the motor i ued on a.c. upply. It i becaue the alternating flux etablihe high current in the coil hort-circuited by the bruhe. When the hort-circuited coil break contact from the commutator, exceive parking i produced. Thi can be eliminated by uing high-reitance lead to connect the coil to the commutator egment. Contruction 16

17 The contruction of en a.c. erie motor i very imilar to a d.c. erie motor except that above modification are incorporated [See Fig. (9.0)]. Such a motor can be operated either on a.c. or d.c. upply and the reulting torque-peed curve i about the ame in each cae. For thi reaon, it i ometime called a univeral motor. Operation When the motor i connected to an a.c. upply, the ame alternating current flow through the field and armature winding. The field winding produce an alternating Fig.(9.0) flux that react with the current flowing in the armature to produce a torque. Since both armature current and flux revere imultaneouly, the torque alway act in the ame direction. It may be noted that no rotating flux i produced in thi type of machine; the principle of operation i the ame a that of a d.c. erie motor. Characteritic The operating characteritic of an a.c. erie motor are imilar to thoe of a d.c. erie motor. (i) The peed increae to a high value with a decreae in load. In very mall erie motor, the loe are uually large enough at no load that limit the peed to a definite value ( ,000 r.p.m.). (ii) The motor torque i high for large armature current, thu giving a high tarting torque. (iii) At full-load, the power factor i about 90%. However, at tarting or when carrying an overload, the power factor i lower. Application The fractional horepower a.c. erie motor have high-peed (and correponding mall ize) and large tarting torque. They can, therefore, be ued to drive: (a) high-peed vacuum cleaner (b) ewing machine (c) electric haver (d) drill (e) machine tool etc. 9.1 Single-Phae Repulion Motor A repulion motor i imilar to an a.c. erie motor except that: 17

18 (i) bruhe are not connected to upply but are hort-circuited [See Fig. (9.1)]. Conequently, current are induced in the armature conductor by tranformer action. (ii) the field tructure ha non-alient pole contruction. By adjuting the poition of hort-circuited bruhe on the commutator, the tarting torque can be developed in the motor. Contruction The field of tator winding i wound like the main winding of a plit-phae motor and i connected directly to a ingle-phae ource. The armature or rotor i imilar to a d.c. motor armature with drum type winding connected to a commutator (not hown in the figure). However, the bruhe are not connected to upply but are connected to each other or hort-circuited. Short-circuiting the bruhe effectively make the rotor into a type of quirrel cage. The major difficulty with an ordinary ingle-phae induction motor i the low tarting torque. By uing a commutator motor with bruhe hort-circuited, it i poible to vary the tarting torque by changing the bruh axi. It ha alo better power factor than the conventional ingle-phae motor. Principle of operation Fig.(9.1) The principle of operation i illutrated in Fig. (9.1) which how a two-pole repulion motor with it two hort-circuited bruhe. The two drawing of Fig. (9.1) repreent a time at which the field current i increaing in the direction hown o that the left-hand pole i N-pole and the right-hand pole i S-pole at the intant hown. (i) In Fig. (9.1 (i)), the bruh axi i parallel to the tator field. When the tator winding i energized from ingle-phae upply, e.m.f. i induced in the armature conductor (rotor) by induction. By Lenz law, the direction of the e.m.f. i uch that the magnetic effect of the reulting armature current will oppoe the increae in flux. The direction of current in armature conductor will be a hown in Fig. (9.1 (i)). With the bruh axi in the poition hown in Fig. (9.1 (i)), current will flow from bruh B to 18

19 bruh A where it enter the armature and flow back to bruh B through the two path ACB and ADB. With bruhe et in thi poition, half of the armature conductor under the N-pole carry current inward and half carry current outward. The ame i true under S-pole. Therefore, a much torque i developed in one direction a in the other and the armature remain tationary. The armature will alo remain tationary if the bruh axi i perpendicular to the tator field axi. It i becaue even then net torque i zero. (ii) If the bruh axi i at ome angle other than 0 or 90 to the axi of the tator field, a net torque i developed on the rotor and the rotor accelerate to it final peed. Fig. (9.1 (ii)) repreent the motor at the ame intant a that in Fig. (9.1 (i)) but the bruhe have been hifted clockwie through ome angle from the tator field axi. Now e.m.f. i till induced in the direction indicated in Fig. (9.1 (i)) and current flow through the two path of the armature winding from bruh A to bruh B. However, becaue of the new bruh poition, the greater part of the conductor under the N- pole carry current in one direction while the greater part of conductor under S-pole carry current in the oppoite direction. With bruhe in the poition hown in Fig. (9.1 (ii), torque i developed in the clockwie direction and the rotor quickly attain the final peed. (iii) The direction of rotation of the rotor depend upon the direction in which the bruhe are hifted. If the bruhe are hifted in clockwie direction from the tator field axi, the net torque act in the clockwie direction and the rotor accelerate in the clockwie direction. If the bruhe Fig.(9.) are hifted in anti-clockwie direction a in Fig. (9.). the armature current under the pole face i revered and the net torque i developed in the anti-clockwie direction. Thu a repulion motor may be made to rotate in either direction depending upon the direction in which the bruhe are hifted. (iv) The total armature torque in a repulion motor can be hown to be T a in α where α = angle between bruh axi and tator field axi For maximum torque, α = 90 or α = 45 19

20 Thu adjuting α to 45 at tarting, maximum torque can be obtained during the tarting period. However, α ha to be adjuted to give a uitable running peed. Characteritic (i) The repulion motor ha characteritic very imilar to thoe of an a.c. erie motor i.e., it ha a high tarting torque and a high peed at no load. (ii) The peed which the repulion motor develop for any given load will depend upon the poition of the bruhe. (iii) In comparion with other ingle-phae motor, the repulion motor ha a high tarring torque and relatively low tarting current Repulion-Start Induction-Run Motor Sometime the action of a repulion motor i combined with that of a inglephae induction motor to produce repulion-tart induction-run motor (alo called repulion-tart motor). The machine i tarted a a repulion motor with a correponding high tarting torque. At ome predetermined peed, a centrifugal device hort-circuit the commutator o that the machine then operate a a ingle-phae induction motor. The repulion-tart induction-run motor ha the ame general contruction of a repulion motor. The only difference i that in addition to the baic repulionmotor contruction, it i equipped with a centrifugal device fitted on the armature haft. When the motor reache 75% of it full pinning peed, the centrifugal device force a hort-circuiting ring to come in contact with the inner urface of the commutator. Thi nort-circuit all the commutator bar. The rotor then reemble quirrel-cage type and the motor run a a ingle-phae induction motor. At the ame time, the centrifugal device raie the bruhe from the commutator which reduce the wear of the bruhe and commutator a well a make the operation quiet. Characteritic (i) The tarting torque i.5 to 4.5 time the full-load torque and the tarting current i 3.75 time the full-load value. (ii) Due to their high tarting torque, repulion-motor were ued to operate device uch a refrigerator, pump, compreor etc. However, they poed a eriou problem of maintenance of bruhe, commutator arid the centrifugal device. Conequently, manufacturer have topped making them in view of the development of capacitor motor which are mall in ize, reliable and low-priced. 0

21 9.14 Repulion-Induction Motor The repulion-induction motor produce a high tarting torque entirely due to repulion motor action. When running, it function through a combination of induction-motor and repulion motor action. 1

22 Contruction Fig. (9.3) how the connection of a 4-pole repulion-induction motor for 30 V operation. It conit of a tator and a rotor (or armature). (i) The tator carrie a ingle ditributed winding fed from ingle-phae upply. (ii) The rotor i provided with two independent winding placed one inide the other. The inner winding i a quirrel-cage winding with rotor bar permanently hort-circuited. Placed over the quirrel cage winding i a repulion commutator armature winding. The repulion winding i connected to a commutator on which ride hort-circuited bruhe. There i no centrifugal device and the repulion winding function at all time. Operation Fig.(9.3) (i) When ingle-phae upply i given to the tator winding, the repulion winding (i.e., outer winding) i active. Conequently, the motor tart a a repulion motor with a correponding high tarting torque. (ii) A the motor peed increae, the current hift from the outer to inner winding due to the decreaing impedance of the inner winding with increaing peed. Conequently, at running peed, the quirrel cage winding carrie the greater part of rotor current. Thi hifting of repulionmotor action to induction-motor action i thu achieved without any witching arrangement. (iii) It may be een that the motor tart a a repulion motor. When running, it function through a combination of principle of induction and repulion; the former being predominant. Characteritic (i) The no-load peed of a repulion-induction motor i omewhat above the ynchronou peed becaue of the effect of repulion winding. However,

23 the peed at full-load i lightly le than the ynchronou peed a in an induction motor. (ii) The peed regulation of the motor i about 6%. (iii) The tarting torque i.5 to 3 time the full-load torque; the lower value being for large motor. The tarting current i 3 to 4 time the full-load current. Thi type of motor i ued for application requiring a high tarting torque with eentially a contant running peed. The common ize are 0.5 to 5 H.P Single-Phae Synchronou Motor Very mall ingle-phae motor have been developed which run at true ynchronou peed. They do not require d.c. excitation for the rotor. Becaue of thee characteritic, they are called unexcited ingle-phae ynchronou motor. The mot commonly ued type are: (i) Reluctance motor (ii) Hyterei motor The efficiency and torque-developing ability of thee motor i low; The output of mot of the commercial motor i only a few watt Reluctance Motor It i a ingle-phae ynchronou motor which doe not require d.c. excitation to the rotor. It operation i baed upon the following principle: Whenever a piece of ferromagnetic material i located in a magnetic field; a force i exerted on the material, tending to align the material o that reluctance of the magnetic path that pae through the material i minimum. Contruction Fig.(9.4) A reluctance motor (alo called ynchronou reluctance motor) conit of: 3

24 (i) (ii) a tator carrying a ingle-phae winding along with an auxiliary winding to produce a ynchronou-revolving magnetic field. a quirrel-cage rotor having unymmetrical magnetic contruction. Thi i achieved by ymmetrically removing ome of the teeth from the quirrelcage rotor to produce alient pole on the rotor. A hown in Fig. (9.4 (i)), 4 ailent pole have been produced on me rotor. The alient pole created on the rotor mut be equal to the pole on the tator. Note that rotor alient pole offer low reductance to the tator flux and, therefore, become trongly magnetized. Operation (i) When ingle-phae tator having an auxiliary winding i energized, a ynchronouly-revolving field i produced. The motor tart a a tandard quirrel-cage induction motor and will accelerate to near it ynchronou peed. (ii) A the rotor approache ynchronou peed, the rotating tator flux will exert reluctance torque on the rotor pole tending to align the alient-pole axi with the axi of the rotating field. The rotor aume a poition where it alient pole lock with the pole of the revolving field [See Fig. (9.4 (ii))]. Conequently, the motor will continue to run at the peed of revolving flux i.e., at the ynchronou peed. (iii) When we apply a mechanical load, the rotor pole fall lightly behind the tator pole, while continuing to turn at ynchronou peed. A the load on the motor i increaed, the mechanical angle between the pole increae progreively. Neverthele, magnetic attraction keep the rotor locked to the rotating flux. If the load i increaed beyond the amount under which the reluctance torque can maintain ynchronou peed, the rotor drop out of tep with the revolving field. The peed, then, drop to ome value at which the lip i ufficient to develop the neceary torque to drive the load by induction-motor action. Characteritic (i) Thee motor have poor torque, power factor and efficiency. (ii) Thee motor cannot accelerate high-inertia load to ynchronou peed. (iii) The pull-in and pull-out torque of uch motor are weak. Depite the above drawback, the reluctance motor i cheaper than any other type of ynchronou motor. They are widely ued for contant-peed application uch a timing device, ignalling device etc. 4

25 9.17 Hyterei Motor It i a ingle-phae motor whoe operation depend upon the hyterei effect i.e., magnetization produced in a ferromagnetic material lag behind the magnetizing force. Contruction It conit of: (i) (ii) a tator deigned to produce a ynchronouly-revolving field from a ingle-phae upply. Thi i accomplihed by uing permanent-plit capacitor type contruction. Conequently, both the winding (i.e., tarting a well a main winding) remain connected in the circuit during running operation a well a at tarting. The value of capacitance i o adjuted a to reult in a flux revolving at ynchronou peed. a rotor coniting of a mooth cylinder of magnetically hard teel, without winding or teeth. Operation (i) When the tator i energized from a ingle-phae upply, a ynchronoulyrevolving field (aumed in anti-clockwie direction) i produced due to plit-phae operation. (ii) The revolving tator flux magnetize the rotor. Due to hyterei effect, the axi of magnetization of rotor will lag behind the axi of tator field by hyterei lag angle a a hown in Fig. (9.5). Thu the rotor and tator pole are locked. If the rotor i tationary, the tarting torque produced i given by: T r in α where = tator flux. r = rotor flux. From now onward, the rotor accelerate to ynchronou peed with a uniform torque. (iii) After reaching ynchronim, the motor continue to run at ynchronou peed and adjut it torque angle o a to develop the torque required by the load. Characteritic (i) A hyterei motor can ynchronize any load which it can accelerate, no matter how great the inertia. It i becaue the torque i uniform from tandtill to ynchronou peed. (ii) Since the rotor ha no teeth or alient pole or winding, a hyterei motor i inherently quiet and produce mooth rotation of the load. 5

26 Fig.(9.5) (iii) The rotor take on the ame number of pole a the tator field. Thu by changing the number of tator pole through pole-changing connection, we can get a et of ynchronou peed for the motor. Application Due to their quiet operation and ability to drive high-inertia toad, hyterei motor are particularly well uited for driving (i) electric clock (ii) timing device (iii) tape-deck (iv)from-table and other preciion audio-equipment. 6