Collected by: Md. Mahfuzur Rahman
Facebook: Mahfuz Rasel ( http://www.facebook.com/mahfuz.rasel.5 http://www.facebook.com/mahfuz.rasel.5)) !"#rd $atch. %hsanullah &n'ers'ty of c'ence and *echnolo+y. *echnolo+y.
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Please Name some Single-phase induction motors? (i) split-phase type (ii) capacitor type (iii) shaded-pole type Please name two types of Repulsion motors? (i) Repulsion-start induction-run motor (ii) Repulsion-induction motor What are dierent types of single phase Synchronous motors? (i) Reluctance motor (ii) Hysteresis motor How a single phase motor is similar to 3 phase motor? A single phase induction motor is very similar to a 3-phase squirrel cage induction motor. It has (i) a squirrel-cage rotor identical to a 3-phase motor (ii) a single-phase winding on the stator What is major dierence etween a 3-phase induction motor and a single phase induction induction motor? Unlie a 3-phase induction motor! a single-phase induction motor is not sel"- starting #ut requires some starting means. Why single phase induction motor does not self starts? $he single-phase single-phase stator winding winding produces produces a magnetic magnetic %eld that pulsates pulsates in strength strength in a sinusoidal manner. $he %eld polarity reverses a"ter each hal" cycle #ut the %eld does not rotate. &onsequently! the alternating 'u cannot produce rotation in a stationary squirrel-cage rotor. What is asic method to start single phase induction motor and why it is not preferred? I" the rotor o" a single-phase motor is rotated in one direction #y some mechanical means! it will continue to run in the direction o" rotation. As a matter o" "act! the
rotor quicly accelerates until it reaches a speed slightly #elow the synchronous speed. nce the motor is running at this speed! it will continue to rotate even though single-phase current is 'owing through the stator winding. $his method o" starting is generally not convenient "or large motors. *or can it #e employed "ur a motor located at some inaccessi#le spot. !"plain why single phase induction motor does not start in the light of #oule-$ield Re%ol%ing &heory? $he dou#le-%eld revolving theory is proposed proposed to eplain eplain this dilemma dilemma o" no torque torque at start and yet torque once rotated. $his theory is #ased on the "act that an alternating sinusoidal 'u can #e represented #y two revolving 'ues! each equal to one-hal" o" the maimum value o" alternating alternating 'u (i.e.! m+,) and each rotating at synchronous speed in opposite directions. $here"ore! $here"ore! an alternating alternating %eld can #e replaced replaced #y two relating relating %elds o" o" hal" its amplitude rotating in opposite directions at synchronous speed. *ote that the resultant resultant vector o" two revolving revolving 'u vectors is a stationary vector that oscillates in length with time along -ais. What is the position of rotor at standstill? &onsider the case that the rotor is stationary and the stator winding is connected to a single-phase supply. $he alternating 'u produced #y the stator winding can #e presented as the sum o" two rotating 'ues and ,! each equal to one hal" o" the maimum value o" alternating 'u and each rotating at synchronous speed (*s / ,0 "+1) in opposite directions. At standstill! these two torques are equal and opposite and the net torque developed is 2ero. $here"ore! single-phase induction motor is not sel"-starting. What is the %alue of slip at standstill? *ote that each rotating %eld tends to drive the rotor in the direction in which the %eld rotates. $hus the point o" 2ero slip "or one %eld corresponds to ,00 slip "or the other . $he value o" 00 slip (standstill condition) is the same "or #oth the %elds. !"plain motor action from standstill to running condition? At standstill! s / so that impedances o" the two circuits are equal. $here"ore! rotor currents are equal i.e.! I," / I,#. However! when the rotor rotates! the impedances o" the two rotor circuits are unequal and the rotor current I,# is higher (and also at a lower power "actor) than the rotor current I,". $heir m.m.".s! which oppose the stator m.m.".s! will result in a reduction o" the #acward rotating 'u. &onsequently! as speed increases! the "orward 'u increases! increasing the driving torque while the #acward 'u decreases! reducing the opposing torque. $he motor-quicly accelerates to the %nal speed.
How to ma'e Single-Phase (nduction )otor Self-Starting? $he single-phase single-phase induction induction motor is not sel"sel"- starting and it is undesira#le undesira#le to resort resort to mechanical spinning o" the sha"t or pulling a #elt to start it. $o mae a singlephase induction motor sel"-starting! we should somehow produce a revolving stator magnetic %eld. $his may #e achieved #y converting a single-phase supply into twophase supply through the use o" an additional winding. 4hen the motor attains su5cient speed! the starting means (i.e.! additional winding) winding) may #e removed depending upon the type o" the motor. Name the types of single phase motors with respect to the method employed to ma'e them self-starting? (i) Split-phase motors-started motors-started #y two phase motor action through the use o" an auiliary or starting winding. (ii) Capacitor motors-started motors-started #y two-phase motor action through the use o" an auiliary winding and a capacitor. (iii) Shaded-pole motors-started motors-started #y the motion o" the magnetic %eld produced #y means o" a shading coil around a portion o" the pole structure. How to produce Rotating )agnetic $ield $rom *-Phase Supply? As with a 3-phase supply! a ,-phase #alanced supply also produces a rotating magnetic %eld o" constant magnitude. 4ith the eception o" the shaded-pole motor! all single-phase induction motors are started as ,-phase machine. nce so started! the motor will continue to run on single-phase supply. What is the major cause of noisy operation of the single phase induction motor? I" the two windings arc displaced 607 electrical #ut produce %elds that are not equal and that are not 607 apart in time! the resultant %eld is still rotating #ut is not constant in magnitude. ne e8ect o" this non uni"orm rotating %eld is the production o" a torque that is non-uni"orm and that! there"ore! causes noisy operation o" the motor. 9ince ,-phase operation ceases once the motor is started! the operation o" the motor then #ecomes smooth. #escrie the principal of operation of Split-Phase (nduction )otor? $he stator o" a split-phase split-phase induction motor motor is provided provided with an an auiliary or starting starting winding 9 in addition to the main or running winding :. $he starting winding is located 607 electrical "rom the main winding and operates only during the #rie" period when the motor starts up. $he two windings are so resigned that the starting
winding 9 has a high resistance and relatively small reactance while the main winding : has relatively low resistance and large reactance as shown in the schematic connections in ;ig. &onsequently! the currents 'owing in the two windings have reasona#le phase di8erence c (,<7 to 307) as shown in the phasor diagram in ;ig.
4hen the two stator windings are energi2ed "rom a single-phase supply! the main winding carries current Im while the starting winding carries current Is. 9ince main winding is made highly inductive while the starting winding highly resistive! the currents Im and Is have a reasona#le phase angle a (,<7 to 307) #etween them as shown in ;ig. &onsequently! a wea revolving %eld approimating to that o" a ,-phase machine is produced which starts the motor. where is a constant whose magnitude depends upon the design o" the motor. 4hen the motor reaches a#out =< o" synchronous speed! the centri"ugal switch opens the circuit o" the starting winding. $he motor then operates as a single-phase induction motor and continues to accelerate till it reaches the normal speed. $he normal speed o" the motor is #elow the synchronous speed and depends upon the load on the motor. What is starting tor+ue and starting current of split phase induction motor? $he spinning spinning torque torque is < to , times the "ull-loud torque mid (lie starting current current is > to ? times the "ull-load current. Why split-phase induction motors are most popular single- phase motors in the mar'et? @ue to their low cost! split-phase induction motors are most popular single- phase motors in the maret. Why uilt-in-thermal relay is necessary in split phase induction motor and what is its purpose? 9ince the starting winding is made o" %ne wire! the current density is high and the
winding heats up quicly. I" the starting period eceeds < seconds! the winding may #urn out unless the motor is protected #y #uilt-in-thermal relay. $his motor is! there"ore! suita#le where starting periods are not "requent. What is speed %ariation of split phase induction motor from no load to full load? An important characteristic o" these motors is that they are essentially constantspeed motors. $he speed variation is ,-< "rom no-load to "ull- load. ,sually where split phase induction motors are used? $hese motors are suita#le suita#le where where a moderate starting starting torque torque is required required and where starting periods are in"requent e.g.! to drive (a) "ans (#) washing machines (c) oil #urners (d) small machine tools etc. $he power rating rating o" such such motors generally generally lies #etween #etween >0 4 and ,<0 ,<0 4. 4.
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What is the Three Phase induction motor Working Principle?
Like any electric motor, a 3-phase induction motor has a stator and rotor. The The stator carries a 3 phase winding (called stator winding) while the rotor carries a short short circuited winding (called rotor winding). Only the stator winding is fed from 3-phase supply. The rotor winding derives its voltage and power from the eternally energi!ed stator winding through electromagnetic induction and hence the name ac"uired. The induction motor may #e considered to #e a transformer with a rotating secondary and it can, therefore, #e descri#ed as a $transformer-type$ a.c. machine in which electrical elec trical energy is converted into mechanical energy. What are the advantages and disadvantages of three phase induction motor? %danta+es are:
(i) %t has simple and rugged construction. (ii) %t is relatively cheap. (iii) %t re"uires little maintenance. (iv) %t has high efficiency and reasona#ly good power factor.
(v) %t has self-starting tor"ue. 2'sadanta+es are:
(i) %t is essentially a constant speed motor and its speed cannot #e changed easily. (ii) %ts starting tor"ue is inferior to d.c. shunt motor.
Describe the construction construction of a three phase phase motor?
& 3-phase induction motor has two main parts (i) stator and (ii) rotor. The The rotor is separated from the stator #y a small air-gap which ranges from '. mm to mm, depending on the power of the motor.
With what material the Stator of three phase induction motor is made?
%t consists of a steel frame which encloses a hollow, cylindrical core made up of thin laminations of silicon steel to reduce hysteresis and eddy current loss. & num#er of evenly spaced slots are provided on the inner periphery of the laminations. The insulated conductors are placed in the stator slots and are suita#ly connected to form a #alanced 3-phase star or delta de lta connected circuit. The 3-phase %*+%* stator winding is wound for a definite num#er of poles as per re"uirement of speed. What is the effect of no. of poles on speed of motor?
reater the num#er of poles, lesser is the speed o f the motor and vice-versa. hen 3-phase supply is given to the stator winding, a rotating magnetic field of constant magnitude is produced. This rotating field induces currents in the rotor #y electromagnetic electromagnetic induction.
With what material the rotor of 3 phase motor is made?
otor. The The rotor, mounted on a shaft, is a hollow laminated core having slots on its outer periphery. In which motor motor both both stator and and rotor winding winding are connected connected to to voltage source?
%n d.c. motor where #oth stator winding (i.e., field winding) and the rotor winding (i.e., armature winding) are connected to a voltage source. What are the maor t!pes of "otor Winding placed in 3 phase induction motor?
The winding placed in these slots (called rotor winding) may #e one of the following two type (/) 0"uirrel cage type (ii) ound type
Differentiate Differ entiate between between S#uirrel S#uirrel cage t!pe rotor rotor and Wou Wound nd t!pe rotor? rotor?
0"uirrel cage rotor consists of a laminated cylindrical core having parallel slots on its outer periphery. One copper or aluminum #ar is placed in each slot. &ll &ll these #ars are 1oined at each end #y metal rings called end rings. This forms a permanently short-circuited winding which is indestructi#le. The entire construction (#ars and end rings) resem#les a. s"uirrel cage and hence the name. The rotor is not connected electrically to the supply #ut has current induced in it #y transformer action from the stator. hile wound rotor consists of a laminated cylindrical core and carries a 3-phase winding, similar to the one on the stator. $ow rotor rotor windings windings are connected connected to suppl! suppl! and wh! wh! wound rotor t!pe is preferred preferred to s#uirrel s#uirrel cage t!pe induction motor?
The rotor winding is uniformly distri#uted in the slots and is usually star-connected. The open ends of the rotor winding are #rought out and 1oined to three insulated slip rings mounted on the rotor shaft with one #rush resting on each slip ring. The three #rushes are connected to a 3-phase star-connected rheostat. &t starting, the eternal resistances are included in the rotor circuit to give a large starting tor"ue. These resistances are gradually reduced to !ero as the motor runs up
to speed. The eternal resistances are used during starting period onl y. hen the motor attains normal speed, the three #rushes are short-circuited so that wound rotor runs like a s"uirrel cage rotor. What do !ou mean b! s#uirrel cage induction motors?
Those induction motors which employ s"uirrel cage rotor are called s"uirrel cage induction motors. motors. 2ost of 3-phase induction motors use s"uirrel cage rotor as it has a remarka#ly simple and ro#ust construction ena#ling it to operate in the most adverse circumstances. Wh! the starting tor#ue of S#uirrel cage induction motor is %&W?
%t suffers from the disadvantage of a low starting tor"ue. %t is #ecause the rotor #ars are permanently short-circuited and it is not possi#le to to add any eternal resistance to the rotor circuit to have a large starting tor"ue.
$ow "otating "otating 'agnetic 'agnetic (ield (ield is produced produced in three phase induction induction motor? motor?
hen a 3-phase winding is energi!ed from a 3-phase supply, a rotating magnetic field is produced. This field is such that its poles do not remain remain in a fied position on the stator #ut go on shifting their positions around the stator. or this reason, it is called a rotating field.
What is S!nchronous speed?
speed at which the revolving flu rotates is called synchronous speed (N s ). ). its value depends upon the num#er of poles and the supply fre"uency. 0ince revolutions per second is e"ual to the revolutions per minute (*s) divided #y 4' and the num#er of cycles per second is the fre"uency f, *05(/6')78 'agneticc flu) is said 'agneti said to rotate at at s!nchronous s!nchronous speed speed wh!?
The speed of the rotating magnetic field is the same as the speed of the alternator that is supplying power to the motor if the two have the same num#er of poles. 9ence the magnetic flu is said to rotate at synchronous speed.
Please draw draw the *#uivalent *#uivalent +ircuit +ircuit of Induction Induction 'otor 'otor
in case of a transformer, the approimate e"uivalent circuit of an induction motor is o#tained #y shifting the shunt #ranch (c :m) to the input terminals . This step has #een taken on the assumption that voltage drop in / and :/ is small and the terminal voltage ;/ does not apprecia#ly differ from the induced voltage
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Why the e"citing current of induction motor is so high as compared to the power transformer? transformer? Unlie that o" a power trans"ormer! the magnetic circuit o" the induction motor has an air-gap. $here"ore! the eciting current o" induction motor (3 to E o" "ull-load current) is much higher than that o" the power trans"ormer. &onsequently! the eact equivalent circuit must #e used "or accurate results. How transformation ratio of induction motor is dierent as compared to power transformer? transformer? In a trans"ormer! the windings are concentrated whereas in an induction motor! the windings are distri#uted. $his a8ects the trans"ormation ratio. Why it is re+uired to start 3-Phase (nduction )otors? $he induction induction motor is "undamentally "undamentally a trans"ormer trans"ormer in which the stator stator is the primary and the rotor is short-circuited secondary. At starting! the voltage induced in the induction motor rotor is maimum ( s / ). 9ince the rotor impedance is low! the rotor current is ecessively large. $his large rotor current is re'ected in the stator #ecause o" trans"ormer action. $his results in high starting current (E to times the "ull-load current) current) in the stator at low power "actor and consequently the value o" starting torque is low. Fecause o" the short duration! this value o" large current does not harm the motor i" the motor accelerates normally. What is the eect of staring of induction motor on connected line? Garge starting current will produce large line-voltage drop. $his will adversely a8ect the operation o" other electrical equipment connected to the same lines. $here"ore!
it is desira#le and necessary to reduce the magnitude o" stator current at starting and several methods are availa#le "or this purpose. Please descrie the )ethods of Starting 3-Phase (nduction )otors? $he common methods methods used to start start induction induction motors are are (i)
@irect-on-line starting
(ii) 9tator resistance starting
(iii) iii)
Autot utotrrans ans"or "ormer star tartin ting
(iv) iv) 9tar tar-del -delta ta start tartin ing g
(v) (v)
Rotor otor resis esista tanc nce e star starti ting ng
How slip ring motors are started? 9lip ring motors are invaria#ly started #y rotor resistance starting. What is Direct-on-line starting of induction motor? $his method o" starting in ust what the name implies the motor is started #y connecting conn ecting it directly to t o 3-phase supply. supply. $he impedance o" the motor at standstill is relatively low and when it is directly connected to the supply system! the starting current will #e high (E to times the "ull-load current) and at a low power "actor. "actor. &onsequently! this method o" starting is suita#le "or relatively small (up to =.< 4) machines. Does starting torque is higher than full load torque? *o! starting current is as large as %ve times the "ull-load current #ut starting torque is ust equal to the "ull-load torque. $here"ore! starting current is very high and the starting torque is comparatively low. low. I" this large starting current 'ows "or a long time! it may overheat the motor and damage the insulation. What is Stator resistance starting method? In this method! eternal resistances are connected in series with each phase o" stator winding during starting. $his causes voltage drop across the resistances so that voltage availa#le across motor terminals is reduced and hence the starting current. $he starting resistances are gradually cut out in steps (two or more steps) "rom the stator circuit as the motor pics up speed. 4hen the
motor attains rated speed! the resistances are completely cut out and "ull line voltage is applied to the rotor. rotor. Why Stator resistance starting is not recommended? $his method su8ers "rom two draw#acs. ;irst! ;irst! the reduced voltage applied to the motor during the starting period lowers the starting torque and hence increases the accelerating time. 9econdly! a lot o" power is wasted in the starting resistances. $here"ore! this method is used "or starting small motors only. only. What is Autotransformer starting method? $his method also aims at connecting the induction motor to a reduced supply at starting and then connecting it to the "ull voltage as the motor pics up su5cient speed. ;ig. shows the circuit arrangement "or autotrans"ormer starting.
$he tapping on the autotrans"ormer is so set that when it is in the circuit! >< to ? o" line voltage is applied to the th e motor. motor. At the instant o" starting! the change-over switch is thrown to startJ position. $his puts the autotrans"ormer in the circuit and thus reduced voltage is applied to the circuit. &onsequently! starting current is limited to sa"e value. 4hen the motor attains a#out ? o" normal speed! the changeover switch is thrown to runJ position. $his taes out the autotrans"ormer "rom the circuit and puts the motor to "ull line voltage.
What are the advantages of Autotransformer starting? Autotrans"ormer starting has several advantages lie low power loss! low starting current and less radiated heat. ;or large machines (over ,< H.1.)! H.1.)! this method met hod o" starting star ting is o"ten used. $his method can #e used "or #oth star and delta connected motors. What is Star-delta starting method of starting of 3 phase induction motor? $he stator winding o" the motor is designed "or delta operation and is connected in star during the starting period. 4hen the machine is up to speed! the connections are changed to delta. $he circuit arrangement "or star-delta star-delta starting is shown #elow #elow
$he si leads o" the stator windings are connected to the changeover switch as shown. At the instant o" starting! the changeover switch is thrown to 9tartJ position which connects the stator windings in star. $here"ore! each stator phase gets volts where C is the line voltage. $his reduces the starting current. 4hen the motor pics up speed! the changeover switch is thrown to RunJ RunJ position which connects the stator windings in delta. *ow each stator phase gets "ull "u ll line voltage C. Eplain in details the Starting of Slip-!ing "otors? 9lip-ring motors are invaria#ly started #y rotor resistance starting. In this method! a varia#le star-connected rheostat is connected in the rotor circuit through slip rings and "ull voltage is applied to the stator winding as shown in ;ig. ;ig.
At starting! the handle o" rheostat is set in the ;; position so that maimum resistance is placed in each phase o" the rotor circuit. $his reduces the starting current and at the same time starting torque is increased. As the motor pics up speed! the handle o" rheostat is gradually moved in clocwise direction and cuts out the eternal resistance in each phase o" the rotor circuit. 4hen the motor attains normal speed! the change-over switch is in the * position and the whole eternal resistance is cut out "rom " rom the rotor circuit. What are the advantages of slip-ring induction motors over the squirrel cage motors? (i) High starting torque with low starting current. (ii) 9mooth acceleration under heavy loads. (iii) *o a#normal heating during starting. (iv) Kood running characteristics a"ter eternal rotor resistances are cut out. (v) Adusta#le speed #s there any disadvantage of slip-ring motors? (i) $he initial and maintenance costs are greater than those o" squirrel cage motors. (ii) $he speed regulation is poor when run with resistance in the rotor circuit
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What are the di$erent !atings used for #nduction "otor? $he nameplate o" a 3-phase induction motor provides the "ollowing in"ormation (i) Horsepower (ii) Gine voltage (iii) Gine current (iv) 9peed (v) ;requency (vi) $emperature rise What do you mean %y horsepower rating and does it shows motor synchronous speed? $he horsepower rating is the mechanical output o" the motor when it is operated at rated line voltage! rated "requency and rated speed. Under these conditions! the line current is that speci%ed on the nameplate and the temperature rise does not eceed that speci%ed. $he speed given on the nameplate is the actual speed o" the motor at rated "ull- loadL it is not the synchronous speed. Why dou%le-cage construction is used in "otors? In order to provide high starting torque at low starting current! dou#le-cage construction is used. As the name suggests! the rotor o" this motor has two squirrelcage windings located one a#ove the other. What is the function of outer winding of dou%le cage construction? $he outer winding consists o" #ars o" smaller cross-section shortcircuited #y end rings. $here"ore! the resistance o" o " this winding is high. 9ince the outer winding has relatively open slots and a poorer 'u path around its #ars so it has a low inductance. $hus the resistance o" the outer squirrel-cage winding is high and its inductance is low. What is the function of inner winding of dou%le cage construction? $he inner winding consists o" #ars o" greater cross-section cross-section short-
circuited #y end rings. $here"ore! the resistance o" o " this winding is low. low. 9ince the #ars o" the inner winding are thoroughly #uried in iron! it has a high inductance M9ee ;ig. (?.3< (ii))N. $hus the resistance o" the inner squirrel- cage winding is low and its inductance is high.
What do you mean %y synchronous speed of a 3-phase induction motor ? $he speed at which the 'u produced #y 3-phase stator windings o" induction motor rotates is called the synchronous speed o" the motor. It is given #y *s / ,0 f&p where *s/ synchronous speed in r.p.m. r.p.m. f ' supply "requency in H2 ( ' num#er o" poles Why is the )eld producing winding of a 3-phase induction motor made stationary ? A 3-phase induction motor has two windings vi* stator stator winding supported #y the stationary part o" the machine and the rotor winding placed on the rotor. rotor. 9o "ar as the #asic operation o" the motor is concerned! it maes no di8erence which wh ich winding is located on the stator. $he machine will "unction equally well with %eld producing winding as either the stationary or rotating element. :aing the %eld winding stationary element eliminates the use o" slip rings and #rushes and! there"ore! will result in a very trou#le-"ree construction. why does the rotor of a 3-phase induction motor rotate in the same direction as the rotating )eld ?
4hen three-phase stator winding is "ed "rom 3-phase supply! a rotating magnetic %eld is set up which cuts the rotor conductors. 9ince the rotor circuit is closed! currents start 'owing in the rotor conductors. *ow! rotor conductors are a re carrying currents and are in the magnetic %eld. $here"ore! mechanical "orce acts on the rotor! tending to move it in the same direction as the stator %eld. $he "act that the rotor is urged to "ollow the stator %eld (i.e. rotor moves in the direction o" stator %eld) can #e eplained #y Gen2Os law. law. According to Gen2Os law! the direction o" rotor currents will #e such so as to oppose the cause producing them. *ow! the cause producing the rotor currents is the relative speed #etween the rotating %eld and stationary station ary rotor. rotor. Hence to reduce this relative speed! the rotor starts running in the same direction as the stator %eld and tries to catch it. Why cannot 3-phase induction motor run at synchronous +speed ? $he rotor "ollows the stator %eld. In actual practice! rotor! can never reach the speed o" stator %eld (i.e. synchronous speed). I" it did! there would #e no relative movement #etween the stator %eld and rotor conductors and! there"ore! t here"ore! no torque to drive the motor. motor. Hence 3-phase induction motor can never run at synchronous speed. Why is the air gap %etween the rotor and stator of a 3-phase induction motor ,ept as short as possi%le ? $he air gap #etween the rotor and stator o" a 3-phase induction motor is made as small as possi#le in order that i. mutual 'u may #e produced with a minimum eciting current. it. leaage reactance are as small as possi#le. How is magneti*ing current ,ept small in a 3-phase induction motor ? $he magneti2ing current drawn #y a 3-phase induction motor motor is very large (30-<0 o" "ull-load stator winding current) owing to the presence o" air-gap #etween the stator and rotor o" the
$HRBB 1HA9B I*@U&$I* :$R I*$BRCIB4 I*$BRCIB4 DUB9$I* 9B$ E What is the importance of slip in a 3-phase induction motor ? $he rate at which the 'u cuts the rotor conductors is directly proportional to the di8erence #etween the speed o" rotating %eld / . s. and that o" the rotor (*). I" the rotor speed were to #ecome equal to the speed o" the rotating %eld! there would #e no generated e.m.". (and hence current) in the rotor conductors. &onsequently! there would #e no motor action. $hus it is the slip o" the rotor /s 0 / r1p1m1. which causes e.m.".s to #e generated and currents to 'ow in the rotor conductors. $his is precisely the principle o" operation o" 3-phase induction motor. motor. What is the di$erence %etween squirrel cage rotor and wound rotor ? Bssentially! there is little di8erence #etween the squirrel cage and wound rotors. $he o#ect o" the latter is merely to #ring out the ends o" polyphase rotor winding to slip rings in order that additional eternal resistance may #e connected in series to improve the starting torque. What are the advantages of wound rotor motors over squirrel cage motors ? Ans. $he wound-rotor motors have the "ollowing advantages over the squirrel cage motors (i) High starting torque and low starting current. (ii) 9mooth acceleration under heavy load. (iii) *o a#normal heating during starting. (iv) Kood running characteristics a"ter rotor resistance is cut out. (v) Adusting speed. What are the disadvantages of wound rotor motors compared to squirrel cage motors? $he wound-rotor motors have the "ollowing disadvantages as compared to squirrel cage motors (i) $he initial and maintenance costs are greater than that o"
squirrel cage motors. (ii) 1oor speed regulation when run with resistance in the rotor circuit. What is the origin of the name squirrel cage rotor ? 4hen this type o" rotor %rst appeared! the common squirrel squirrel was a "requent house pet. $he usual cage that housed him contained a rotating wheel that the animal could enter en ter.. $his wheel a8orded a 8orded eercise eercise and amusement to the pet. 9ince the rotor resem#led squirrelOs eercise cage. it was named squirrel cage rotor. Why is the power factor of a 3-phase induction motor low at notoad? Fecause o" air-gap! the reluctance o" the magnetic circuit o" 3phase induction motor is very high. &onsequently! the current drawn #y the motor on no load is largely magneti2ing currentL the no load current lagging the applied voltage #y a large angle. ;or this reason! the p.". o" a lightly loaded 3-phase induction motor is very low. Why is the power factor of a fully loaded 3-phase induction motor not very high? A 3-phase induction motor draws a large magneti2ing current #ecause o" high reluctance o" the magnetic circuitL air-gap air-gap #eing the main reason. As load is added! the active component o" current increases! resulting in higher power "actor. "actor. However!#ecause o" the large value o" magneti2ing current! which is present regardless o" load! the p.". o" 3-phase induction motor even at "ull-load "u ll-load seldom eceeds 0.?<. What are the advantages of s,ewed slots in the rotor of a squirrel cage motor ? It is a usual practice to use the t he rotor o" a squirrel cage motor with sewed slots i1e1 slots that are not parallel to the sha"t ais. $his arrangement o8ers the "ollowing advantages i. It reduces motor noise and vi#ration. (ii) It increases the starting torque and reduces the starting current.
(iii) It increases the rotor resistance due I increased length o" rotor #ars. How will you design the rotor of a squirrel cage motor to have high starting torque ? 4hen a high starting torque is required! a squirrel cage machine with specially designed rotor can #e used without much sacri%ce o" e5ciency #ut with some reduction o" rated output. $his can #e achieved #y either o" the "ollowing two ways (i) $he rotor #ars can #e made very deep so that owing to sin e8ect! their resistance will #e high at starting when the rotor "requency is equal e qual to the supply "requency. "requency. (ii) Fy using dou#le squirrel cage rotor Why is the maimum torque of a squirrel cage induction motor called pull-out torque ? $he maimum torque o" a squirrel squirrel cage induction motor is also called pull-out torque #ecause o" the way the motor reacts to an overload. A"ter the maimum torque point (which is three to "our times the "ull-load torque)! to rque)! the decrease in rotorP p.". is greater than the increase in rotor current resulting in a decreasing torque and the motor quicly comes to stop. It is generally eperienced that a lathe will stall on a heavy cut. $he machine will slow down as its cutting load is increased until suddenly it will stall and hum hu m or growl loudly. loudly. $his condition will persist until the load is removed or a "use #lows. When will rotor resistance dominate rotor reactance and viceversa? i. 4hen a 3-phase induction motor is in normal operation! the rotor "requency " O/ s f where f is the supply "requency) is low and so is the rotor reactance. $he current that 'ows is then largely limited #y rotor resistance rather than reactance. ii. 4hen the rotor is stationary (i.e. at standstill)! s ' . $his means that rotor sees the "ullline "requency (i.e. f+ ' f. and its reactance is dominant compared to its resistance.
Why is the sum of rotor core loss and friction and windage losses of a 3-phase induction motor nearly constant at all loads? $his is eplained as under (i) At no load! the rotor speed is maimum. $he "requency and hence the rotor core loss is practically 2ero. However! the "riction and windage losses are maimum. (ii) 4hen the load is increased! the rotor speed decreases and hence rotor "requency increases. &onsequently! the rotor core loss increases while "riction and 4indage losses decrease. It is "ound that at all loads! the rotor core loss plus "riction and windage losses remain nearly constant. Eplain the statement that induction motor is fundamentally a transformer ? Ans. $he induction motor is "undamentally a trans"ormer in which the stator is the primary and the rotor is a short-circuited secondary. secondary. $his is evident! particularly when wh en the rotor is stationary. stationary. $he rotor current esta#lishes a 'u which opposes and! there"ore! tends to weaen the stator 'u. $his causes more current to 'ow in the stator winding ust as as increase in secondary current in a trans"ormer tr ans"ormer causes a corresponding increase in primary current. Cery Cery o"ten the analysis o" an induction motor is made on the same lines as the trans"ormer with the modi%cation that short-circuited secondary is considered rotating. What are the essential di$erences %etween 3-phase induction motor and a transformer ? $he essential di8erences #etween a 3-phase induction motor and a power trans"ormer are as under 2. Unlie that o" a trans"ormer! the magnetic circuit o" a 3-phase induction motor has an air-gap. air-gap. $his increases the reluctance o" the magnetic circuit o" the motor to a great etent. &onsequently! the magneti2ing current drawn #y an induction motor is much more than that o" a power trans"ormer. trans"ormer. (ii) $he windings o" a power trans"ormer are cylindrical whereas
those o" induction motor are distri#uted. $his a8ects the turn ratio. (iii) In a 3-phase induction motor! electrical energy is converted into mechanical energy e nergy.. However! in a trans"ormer! tr ans"ormer! electrical energy is trans"erred "rom one circuit to another! usually with a change in voltage level. (iv) A trans"ormer is a static device and! there"ore! "riction and windage losses are a#sent. Howev- r! 3-phase induction motor is a rotating machine and is attended #y "riction and windage losses. ;or this reason! the e5ciency o" a trans"ormer is more than that o" an induction motor. What is the advantage of a dou%le squirrel cage induction motor ? $he advantage o" a dou#le-squirrel cage motor motor is that it provides high starting torque and low starting current. How does a dou%le-squirrel cage induction motor operate i. at starting ii. under running conditions ? A dou#le-squirrel cage motor has two rotor windings! one inside the other. impedance o" the outer winding is less than that o" the inner winding! resulting in a large proportion o" current 'owing in the outer high-resistance winding. $his provides good starting torque. As the motor accelerates! the rotor "requency decreases! there#y lowering the reactance o" inner winding! allowing it to carry a largy proportion o" the total current. At the normal operating speed! the rotor "requency is so low that nearly all the rotor current 'ows in the low-resistance inner cage! resulting in high e5ciency and good speed regulation. How is the speed of a squirrel cage induction motor changed? Ans. $he "ormula "or the synchronous speed o" a 3-phase induction motor is given #y /s '24f &( It is clear that supply "requency and the num#er o" poles are the only varia#le "actors determining the synchronous speed. A
variation o" "requency is impossi#le #ecause motor is connected to commercial supply which has a %ed "requency. "requency. $here"ore! speed o" a squirrel cage induction motor can #e changed #y varying the num#er o" poles. :ultispeed squirrel cage motors are provided with stator windings that can #e reconnected to "orm di8erent num#er o" poles.