Module 8. Three-phase Induction Motor. Version 2 EE IIT, Kharagpur

Transcription

1 Module 8 Thee-phase Induction Moto

2 Lesson 30 Constuction and Pinciple of Opeation of IM

3 In the pevious, i.e. fist, lesson of this module, the fomation of otating magnetic field in the ai gap of an induction moto (IM), has been descibed, when the thee-phase balanced winding of the stato is supplied with thee-phase balanced voltage. The constuction of the stato and two types of oto squiel cage and wound (slip-ing) one, used fo thee-phase Induction moto will be pesented. Also descibed is the pinciple of opeation, i.e. how the toque is poduced. Keywods: Thee-phase induction moto, cage and wound (slip-ing) oto, synchonous and oto speed, slip, induced voltages in stato winding and oto ba/winding. Afte going though this lesson, the students will be able to answe the following questions: 1. How would you identify the two types (cage and wound, o slip-ing) of otos in thee-phase induction moto? 2. What ae the meits and demeits of the two types (cage and wound, o slip-ing) of otos in IM? 3. How is the toque poduced in the oto of the thee-phase induction moto? 4. How does the oto speed diffe fom synchonous speed? Also what is meant by the tem slip? Constuction of Thee-phase Induction Moto This is a otating machine, unlike the tansfome, descibed in the pevious module, which is a static machine. Both the machines opeate on ac supply. This machine mainly woks as a moto, but it can also be un as a geneato, which is not much used. Like all otating machines, it consists of two pats stato and oto. In the stato (Fig. 30.1), the winding used is a balanced thee-phase one, which means that the numbe of tuns in each phase, connected in sta/delta, is equal. The windings of the thee phases ae placed 120 (electical) apat, the mechanical angle between the adjacent phases being [ ( ) / p ], whee p is no. of poles. Fo a 4-pole (p = 4) stato, the mechanical angle

4 between the winding of the adjacent phases, is [( ) / 4] = 120 / 2 = 60, as shown in Fig The conductos, mostly multi-tun, ae placed in the slots, which may be closed, o semi-closed, to keep the leakage inductance low. The stat and etun pats of the winding ae placed nealy 180, o ( 180 β ) apat. The angle of shot choding ( β ) is nealy equal to 30, o close to that value. The shot choding esults in educing the amount of coppe used fo the winding, as the length of the conducto needed fo ovehang pat is educed. Thee ae also othe advantages. The section of the stampings used fo both stato and oto, is shown in Fig The coe is needed below the teeth to educe the eluctance of the magnetic path, which caies the flux in the moto (machine). The stato is kept nomally inside a suppot. Thee ae two types of oto used in IM, viz. squiel cage and wound (slip-ing) one. The cage oto (Fig. 30.3a) is mainly used, as it is cheap, ugged and needs little o no maintainance. It consists of coppe bas placed in the slots of the oto, shot cicuited at the two ends by end ings, bazed with the bas. This type of oto is equivalent to a wound (slip-ing) one, with the advantage that this may be used fo the stato with diffeent no. of poles. The cuents in the bas of a cage oto, inseted inside the stato, follow the patten of cuents in the stato winding, when the moto (IM) develops toque, such that no. of poles in the oto is same as that in the stato. If the stato winding of IM is changed, with no. of poles fo the new one being diffeent fom the ealie one, the cage oto used need not be changed, thus, can be same, as the cuent patten in the oto bas changes. But the no. of poles in the oto due to the above cuents in the bas is same as no. of poles in the new stato winding. The only poblem hee is that the equivalent esistance of the oto is constant. So, at the design stage, the value is so chosen, so as to obtain a cetain value of the stating toque, and also the slip at full load toque is kept within limits as needed. The othe type of oto, i.e., a wound oto (slip ing) used has a balanced thee-phase winding (Fig. 30.3b), being same as the stato winding, but no. of tuns used depends on the voltage in the oto. The thee ends of the winding ae bought at the thee slip-ings, at which points extenal esistance can be inseted to incease the stating toque equiement. Othe thee ends ae shoted inside. The moto with additional stating

5 esistance is costlie, as this type of oto is itself costlie than the cage oto of same powe ating, and additional cost of the stating esistance is incued to incease the stating toque as equied. But the slip at full load toque is lowe than that of a cage oto with identical ating, when no additional esistance is used, with diect shotcicuiting at the thee slip-ing teminals. In both types of oto, below the teeth, in which bas of a cage oto, o the conductos of the oto winding, ae placed, lies the ion coe, which caies the flux as is the case of the coe in the stato. The shaft of the oto passes below the oto coe. Fo lage diamete of the oto, a spide is used between the oto coe and the shaft. Fo a wound (slip-ing) oto, the oto winding must be designed fo same no. of poles as used fo the stato winding. If the no. of poles in the oto winding is diffeent fom no. of poles in the stato winding, no toque will be developed in the moto. It may be noted that this was not the case with cage oto, as explained ealie. The wound oto (slip ing) shown in Fig (b) is shown as sta-connected, wheeas the oto windings can also be connected in delta, which can be conveted into its equivalent sta configuation. This shows that the oto need not always be connected in sta as shown. The No. of oto tuns changes, as the delta-connected oto is conveted into sta-connected equivalent. This point may be kept in mind, while deiving the equivalent cicuit as shown in the next lesson (#31), if the additional esistance (being in sta) is connected though the slip ings, in seies with the oto winding

6 Pinciple of Opeation The balanced thee-phase winding of the stato is supplied with a balanced theephase voltage. As shown in the pevious lesson (#29), the cuent in the stato winding poduces a otating magnetic field, the magnitude of which emains constant. The axis of the magnetic field otates at a synchonous speed ( n s = ( 2 f ) / p ), a function of the supply fequency (f), and numbe of poles (p) in the stato winding. The magnetic flux lines in the ai gap cut both stato and oto (being stationay, as the moto speed is zeo) conductos at the same speed. The emfs in both stato and oto conductos ae induced at the same fequency, i.e. line o supply fequency, with No. of poles fo both stato and oto windings (assuming wound one) being same. The stato conductos ae always stationay, with the fequency in the stato winding being same as line fequency. As the oto winding is shot-cicuited at the slip-ings, cuent flows in the oto windings. The electomagnetic toque in the moto is in the same diection as that of the otating magnetic field, due to the inteaction between the otating flux poduced in the ai gap by the cuent in the stato winding, and the cuent in the oto winding. This is as pe Lenz s law, as the developed toque is in such diection that it will oppose the cause, which esults in the cuent flowing in the oto winding. This is iespective of the oto type used cage o wound one, with the cage oto, with the bas shot-cicuited by two end-ings, is consideed equivalent to a wound one The cuent in the oto bas inteacts with the ai-gap flux to develop the toque, iespective of the no. of poles fo which the winding in the stato is designed. Thus, the cage oto may be temed as univesal one. The induced emf and the cuent in the oto ae due to the elative velocity between the oto conductos and the otating flux in the ai-gap, which is maximum, when the oto is stationay ( n = 0.0 ). As the oto stats otating in the same diection, as that of the otating magnetic field due to poduction of the toque as stated ealie, the elative velocity deceases, along with lowe values of induced emf and cuent in the oto. If the oto speed is equal that of the otating magnetic field, which is temed as synchonous speed, and also in the same diection, the elative velocity is zeo, which causes both the induced emf and cuent in the oto to be educed to zeo. Unde this condition, toque will not be poduced. So, fo poduction of positive (motoing) toque, the oto speed must always be lowe than the synchonous speed. The oto speed is neve equal to the synchonous speed in an IM. The oto speed is detemined by the mechanical load on the shaft and the total oto losses, mainly compising of coppe loss. The diffeence between the synchonous speed and oto speed, expessed as a atio of the synchonous speed, is temed as slip in an IM. So, slip (s) in pu is ns n n s = = 1 o, n = ( 1 s) ns ns ns whee, ns and n ae synchonous and oto speeds in ev/s. In tems of N s = 60 n s and N = 60 n, both in ev/min (pm), slip is s = ( Ns N)/ Ns If the slip is expessed in %, then s = [( N s N ) / N s ] 100 Nomally, fo toques vaying fom no-load ( zeo) to full load value, the slip is popotional to toque. The slip at full load is 4-5% ( ).

7 An altenative explanation fo the poduction of toque in a thee-phase induction moto is given hee, using two ules (ight hand and left hand) of Fleming. The stato and oto, along with ai-gap, is shown in Fig. 30.4a. Both stato and oto is shown thee as sufaces, but without the slots as given in Fig, Also shown is the path of the flux in the ai gap. This is fo a section, which is unde Noth pole, as the flux lines move fom stato to oto. The oto conducto shown in the figue is at est, i.e., zeo speed (standstill). The otating magnetic field moves past the conducto at synchonous speed in the clockwise diection. Thus, thee is elative movement between the flux and the oto conducto. Now, if the magnetic field, which is otating, is assumed to be at standstill as shown in Fig. 30.4b, the conducto will move in the diection shown. So, an emf is induced in the oto conducto as pe Faaday s law, due to change in flux linkage. The diection of the induced emf as shown in the figue can be detemined using Fleming s ight hand ule.

8 As descibed ealie, the oto bas in the cage oto ae shot cicuited via end ings. Similaly, in the wound oto, the oto windings ae nomally shot-cicuited extenally via the slip ings. In both cases, as emf is induced in the oto conducto (ba), cuent flows thee, as it is shot cicuited. The flux in the ai gap, due to the cuent in the oto conducto is shown in Fig. 30.4c. The flux patten in the ai gap, due to the magnetic fields poduced by the stato windings and the cuent caying oto conducto, is shown in Fig. 304d. The flux lines bend as shown thee. The popety of the flux lines is to tavel via shotest path as shown in Fig. 30.4a. If the flux lines ty to move to fom staight line, then the oto conducto has to move in the diection of the otating magnetic field, but not at the same speed, as explained ealie. The cuent caying oto conducto and the diection of flux ae shown in Fig. 30.4e. It is known that foce is poduced on the conducto caying cuent, when it is placed in a magnetic field. The diection of the foce on the oto conducto is obtained by using Fleming s left hand ule, being same as that of the otating magnetic field. Thus, the oto expeiences a motoing toque in the same diection as that of the otating magnetic field. This biefly descibes how toque is poduced in a thee-phase induction moto. The fequency of the induced emf and cuent in the oto As given ealie, both the induced emf and the cuent in the oto ae due to the elative velocity between the oto conductos and the otating flux in the ai-gap, the speed of which is the synchonous speed ( N s = ( 120 f ) / p ). The oto speed is N = ( 1 s) N s The fequency of the induced emf and cuent in the oto is f = p ( ns n ) = s ( p ns ) = s f Fo nomal values of slip, the above fequency is small. Taking an example, with full load slip as 4% (0.04), and supply (line) fequency as 50 Hz, the fequency (Hz) of the oto induced emf and cuent, is f = = 2. 0, which is vey small, wheeas the fequency (f) of the stato induced emf and cuent is 50 Hz, i.e. line fequency. At standstill, i.e. oto stationay ( n = 0. 0 ), the oto fequency is same as line fequency, as shown ealie, with slip [s = 1.0 (100%)]. The eade is equested to ead the next lesson (#31), whee some additional points ae included in this matte. Also to note that the poblems ae given thee (#31). In this lesson the second one of this module, the constuction of a thee-phase Induction Moto has been pesented in bief. Two types of oto squiel cage and wound (slip-ing) ones, along with the stato pat, ae descibed. Then, the poduction of toque in IM, when the balanced stato winding is fed fom balanced thee-phase voltage, with the balanced oto winding in a wound one being shot-cicuited, is taken up. In the next lesson, the equivalent cicuit pe phase of IM will be deived fist. Then, the complete powe flow diagam is pesented.