Electrical Equipment of Machine Tools

Transcription

1 Electical Equipment of achine Tools Electification in machine tools today is vey extensive because it helps to simplify thei constuction, educe thei weight, and develop automatic contol. any advanced tends in moden machiney ae associated with the development of electical dive systems and electical contol gea. Among the main tends in the development of electic dive (consisting of an electic moto, its contol gea and a mechanical tansmission fom the moto to the opeative membes of the machine tool) ae impovement in electic moto chaacteistics, simplicity and high quality of contol gea and mechanical tansmissions, and poximity of the moto to the opeative machine-tool membes. The aangement of the moto neaby the opeative membe and also the use of vaiablespeed electic motos make it possible to simplify mechanical tansmissions as well as the constuction of the machine. The use of electical instead of mechanical contols helps to impove machine-tool design and consideably educe the physical foce equied to handle the machine. One o moe electic motos diving the opeative membes of the machine tool make up the machine s essential components. The geat majoity of machine tools ae diven by altenating-cuent thee-phase induction motos. These motos ae simple, eliable and inexpensive. Diect-cuent motos ae not so common and ae used mainly to dive heavy, machine tools. Induction otos Induction moto aangements ae classified with efeence to the method of thei mounting and envionmental influence potection. A conventional foot-mounted moto is shown in (Fig.a), Flange-type motos (Fig.b) fo hoizontal and vetical positioning ae extensively used. An example of a built-in moto is given in Fig. c. Hee one of the shafts of the machine (commonly the spindle) seves as the moto shaft. Fig. a Fig. b (c)

2 Electic motos used in machine tools ae equipped with vaious devices fo potection against envionment effects. To povide fo safety of opeation and pevent alien objects fom getting into motos, the latte have potective guads. Some motos have vent holes, eithe downwaddiected o located in vetical planes, as potection against liquid dops. Some motos ae fully enclosed and have no vent holes, but because of inadequate cooling, thei powe is much smalle than that of ventilated motos of the same size. Thee ae enclosed self-ventilated motos with a fan mounted on the ea end of the moto shaft and coveed with a lid. The fan cools down the moto duing opeation. otos of this type ae most widely used in machine tools. Electic motos ae designed to opeate at the standad voltages of 127, 380 and 500 V. A moto can be connected to the mains with two voltages diffeing fom each othe by 3 times, fo instance, 127 and 220 V o 220 and 380 V. The moto stato is delta-connected fo the lowe voltage of each of the two values, and sta-connected fo the uppe one; this esults in equal cuent intensity in stato windings fo the both connections. With motos used fo 500 V, the stato winding is invaiably sta-connected. The A02 seies of electic motos includes nine sizes of enclosed self-ventilated motos with cage otos fom 0.6 up to 100 kw, having otational (synchonous) speeds of 600, 750, 1000, 1500, and 3000 pm. The speed of an induction moto can be inceased by inceasing the altenating cuent fequency. When ginding small-diamete boes, vey high spindle speeds ae necessay to povide the equied cutting speeds, e.g. fo ginding with a 3 mm-diamete abasive wheel at the peipheal speed of 30 m/s, the otational spindle speed should be 200,000 pm. Fo this pupose units known as high-fequency diect-dive wheel-heads ae often used. Such a unit compises a ginding wheel spindle with a built-in high-fequency cage-oto induction moto. Wheel heads with mist lubicated beaings ae often used. The speed-toque chaacteistic of the moto is the elationship between the moto speed n and n f the toque it develops, and is expessed as Thee ae electic motos with speed-toque chaacteistics known as dooping, igid and absolutely igid. With dooping speed motos, change of toque (i.e., load) esults in substantial moto-speed change. It change of toque does not makedly affect the moto speed, such a moto is said to have a igid chaacteistic. A moto possesses an absolutely igid chaacteistic if its speed does not depend on load changes. The speed-toque chaacteistic featues a special facto called slippage, denoted by s, which epesents the atio between the fail in moto speed unde citical (maximum0 load (- c ) as compaed with the speed of idling (when = 0), the speed of idling being S no n n o Whee n o = speed of otating magnetic field (synchonous speed of electic moto), pm and 1/s n= oto speed (asynchonous), pm Slippage s is expessed as a pecentage o decimal faction, The toque developed by the induction moto can be oughly estimated by means of the following fomula:

3 c 2 s sc s s o Whee c = citical toque (maximum load) of moto S c = citical slippage coesponding to c Figue 53 depicts the induction moto speed-toque chaacteistics constucted by means of the toque fomula. With n being equal to n o, =0 which coesponds to the idling of the moto. When the moto is stated and the oto is still immovable (s=1), the moto develops stiating toque s which is highe than the ated (o nominal) toque. The values of c and S c detemine the citical point (o the maximum) of the chaacteistic. The inteval between the idling point and the citical point of the chaacteistic is called the woking inteval. It is this inteval that is suitable fo stable opeation with the moto speed not depending on the toque change (as is the case with induction motos used in machine tools). The ated value of slippage depends on the type of moto and its ated powe, and is within the limits of ; the highe the moto powe, the lowe the spillage. In addition to plain induction motos, thee ae motos with highe stating toque and slippage ( ). While motos with nomal slippage have a igid speed-toque chaacteistic and ae used in most machine tools, motos with highe spillage have a dooping chaacteistic and ae used in machine-tool dives with fequent moto stats and consideable stating loads. Cuve 1 shown in fig 53 is fo a igid chaacteistics moto, and cuve 2-fo a dooping chaacteistic moto. It is evident fom the Figue that, all othe things being equal, the dooping chaacteistic moto has lowe ated speed and highe stating toque s. Point A epesents the ated value of load. The following induction moto specifications ae usually given in catalogues: ated powe c N, kw; ated speed n, pm; synchonous speed n o, pm; atios and s (whee and s ae ated and stating toques, espectively). The value of is detemined fom the fomula. N 9550 N. m kgf. m n Ratio c defines the value of pemissible mechanical oveload of the moto. c =

4 The value of c depends on mains voltage. As the voltage value may change, the maximum pemissible value of oveload is assumed to be 0.85 c. With geneal-pupose induction motos having cage otos, atio s = Pincipal movement dive motos ae stated up unde no load, so the stating toque s < s is sufficient. otos stated up unde load should develop highe stating toques. Stating up induction motos. When a moto with cage oto is stated up, the intensity of the stating cuent exceeds its ated value 4 to 8 times. This ise in the cuent causes the voltage in the mains to dop. When a highe-powe moto is being stated up and the voltage in the mains dops, its stating toque also dops, while othe motos opeated unde oveload fom the same mains may stop (change to a shot-cicuit egime). So it is only whee the ated moto powe comes to not moe than 25 pecent of the powe of shop mains tansfomes that such a moto can be stated up without potective devices limiting the stating cuent intensity. Would-oto induction motos ae stated up by means of a heostat connected to the oto cicuit. Changing moto speed. The oto speed of a cage-oto induction moto is found fom the fomula. n 60 f 1 s pm Whee f = altenating cuent fequency, Hz p = numbe of pole pais s = slippage of oto It is evident fom the fomula that moto speed can be changed by changing cuent fequency, slippage o the numbe of pole pais. With the fequency of altenating cuent in the mains being constant, the fist method can be applied only if thee is a sepaate a.c. geneato to feed the electic moto. The second method of speed changing by change of slippage is accomplished by intoducing effective esistance into the oto cicuit, which can be done only with wound-oto induction motos. The thid method of speed changing by changing the numbe of pole pais-is the most widely used in machine tools. It involves the use of multi-speed pole-change motos. oto baking can be accomplished mechanically o electically. Electical methods of baking include egeneative baking, injection baking, evese-cuent baking etc.

5 Regeneative baking is possible only with multi-speed motos. Its pinciple consists in that the moto, while still connected to the mains, is switched ove to a lowe speed step and so stats woking as a geneato etuning electical enegy into the mains. This esults in the moto slowing down to the speed of the changed step. Futhe baking is accomplished mechanically o othewise. Injection baking is caied out by injecting diect cuent into the stato winding, thus foming a constant magnetic field theein. This field etads the otating field of the moto and stops the late. Once being fully stopped, the moto is automatically disconnected fom the mains by a special devices. Revese-cuent baking is caied out by intechanging two phases in the stato winding. Hee, the diection of the otating magnetic field is evesed, which etads the initially otating oto. When the baking ends, the moto is automatically disconnected fom the mains. The dawback of this method is that it is attended with load peaks and poduces impacts in machine tansmissions. Howeve, this method is widely used in machine tools due to its simplicity and eliability. oto evesal is accomplished by changing any two extenal teminals (phases) of the moto. Diect Cuent otos Diect cuent motos with shunted excitation (shunt-wound motos) ae extensively used in heavy machine-tool dives. They ae connected accoding to the cicuit diagam shown in Fig. d. The amatue winding A is connected to the mains though stating heostat 1, exciting (shunt) winding SW though heostat 2 used fo speed vaiation. The toque and speed values fo the d.c. moto ae detemined by means of the following fomulas: ki a, N. m v 1 a a kgf. cm; n pm c Whee = toque developed by moto, N.m n= moto speed, pm V == mains voltage, V F a = cuent intensity in amatue winding A a = amatue cicuit esistance, ohm c = constant of given moto k = popotionality facto =magnetic flux of moto, Wb.s

6 Fig. d The speed toque chaacteistics of the moto ae given in Fig. d. Numeal f denotes the line coesponding to the ated speed-toque chaacteistic. The elatively small value of amatue winding esistance detemines a sufficiently igid ated chaacteistic of the shunt-wound moto, which is expessed gaphically by the modest slope of line 1. With the moto in opeation, the esistance the esistance of heostat 1 can be inceased; this will esult in an incease of the total amatue cicuit esistance and the slope of the chaacteistic s line. In this way, a numbe of modified heostat-poduced chaacteistics 2,3,4 is obtained. Powe losses in the exciting cicuit depend on the moto powe and ae within the limits of 1-8 pe cent; the lowe the powe, the highe the losses. The ated value of amatue cuent is detemined as the diffeence between the ated value of moto cuent and the value of exciting cuent. Howeve, the exciting cuent value in the shunt-wound moto is small, so it is often not taken into account in design. Shunt-wound motos can endue shot-tem opeation unde oveload with pemissible oveload facto = This value is limited by the appeaance of consideable bush spaking. Shown in Fig. 55 by dotted line 9 is the speed-toque chaacteistic of moto with evesed amatue polaity, in which the diection of moto otation is evesed. Stating up the shunt-wound moto is caied out only with the aid of a stating heostat 1 is connected to the cicuit with all its steps and the moto begins speeding up in accodance with chaacteistic 4. The esistance of the heostat steps is detemined in such a way that on being stated up the moto should develop pedetemined toque 1 (usually m 1 2 ). as the moto acceleates, its toque dops and when it eaches the pedetemined value 2 ( ), one heostat step is disconnected. The moto then changes ove to un in accodance with chaacteistic 3. Gadually the steps of the heostat ae all disconnected until the moto uns accoding to its ated speed-toque chaacteistic.

7 In machine tools this opeation is accomplished automatically. Changing the speed of d.c. motos can be effected by changing the amatue cicuit esistance, by changing the amatue cicuit esistance, by changing the magnetic flux, and by changing the input voltage. The fist method is aely used because it involves enegy losses. The second method by changing the magnetic flux is the most commonly used. The magnetic flux value is changed by heostat 2 (Fig. e ). The heostat esistance being inceased, the exciting cuent and magnetic flux ae educed, which esults in an incease in idling moto speed and slope of moto speed-toque chaacteistics, epesented by a numbe of staight lines (5,6,7,8). These lines ae not paallel to the ated chaacteistic 1 of the moto and have the geate inclination, the lowe values of magnetic fluxes they coespond to. The numbe of these chaacteistics depends on the numbe of steps on heostat 2. Whee the numbe of heostat steps is lage, moto speed changing becomes pactically stepless. The thid method of speed changing- by changing the input voltage-involves the use of special cicuity and is employed in geneato-moto systems. The baking of d.c. motos is caied out by methods simila to those used fo baking induction motos. Regeneative baking is accomplished by means of a shunt cicuit heostat, whose opeation causes the amatue speed to dop to a minimum. Hee, the moto begins opeating as a geneato etuning electic enegy into the mains. The moto is bought to a full stop by being disconnected fom the mains. Dynamic baking, the most common method of baking, consists in the amatue being disconnected fom the mains, while the exciting cuent is on, and closed though the ballast esisto (o heostat). Revese-cuent baking is done by changing ove the diection of cuent in the amatue cicuit. The Geneato-oto System This system known also as the Wad-Leonad system, is used in heavy and high-powe machine tools with fequent moto evesal o whee infinitely vaiable speed o feed ae equied. This system also facilitates the stating of the machine tool. The system (Fig. f ) consists of induction a.c. moto 2; d.c. geneato 3 diven by moto 2; self-excited diect-cuent geneato 1 fo the excitation of geneato 3 and moto 4; d.c. moto 4 as the dive moto of the machine tool. oto 4 is stated up by means of shuntcicuit heostat 6 connected to the exciting winding of geneato 3, whose excitation magnetic flux it educes. This gives a vey small value of voltage on the geneato bushes, which is then gadually inceased. As moto 4 is sped up, it develops a back electomotive foce (emf) and the shunt-cicuit heostat is gadually switched off, inceasing the geneato voltage. The geneato-moto system allows the speed of moto 4 to be vaied in two ways: (a) by changing the input voltage fed into moto 4 with the aid of heostat 6 (by changing the

8 Fig. f magnetic flux of geneato 3); and (b) by changing the excitation magnetic flux of moto 4 by means of heostat 5. The evesal of moto 4 is accomplished by changing ove the diection of cuent in the exciting winding of geneato 3 with the aid of switch 7. The baking of moto 4 is accomplished by the egeneative method by means of heostat 5. Hee, the magnetic flux of moto 4 inceases, the moto emf exceeds the input voltage and the moto begins opeating as a geneato. Low efficiency (about 0.65), lage size, and high cost ae among the impotant disadvantages of the geneato-moto system. Selection of the Powe Rating of achine-tool oto It is highly impotant to make a coect estimation of the moto powe. With insufficient moto powe, the machine tool will be undeloaded, while the moto itself will be vulneable to oveloads, edundant moto powe will esult in its being systematically undeloaded, which means opeation at low efficiency and with low powe facto (a.c. induction motos). In opeation, the electic moto tends to heat up. The eat is geneated duing the tansfomation of electical into mechanical enegy. Some of input electical enegy is wasted on the heating of the electic-moto windings and magnetic cicuit, and to a lesse degee-on fiction in moto beaings. The loss of powe is the diffeence between the input (the powe of feeding) and output (the useful powe) of the electic moto. The powe losses on the heating of the moto windings ae popotional to the squae of the cuent intensity and ae called vaiable losses. Othe losses ae conventionally called constant losses. The acceptable value of moto beating is limited by the themal stability of the winding insulation mateial, USSR-made electic motos ae mostly equipped with class A insulation (cotton fabic impegnated with insulating compound). This insulation allow a

9 maximum heat tempeatue of up to 105 o C, but due to the difficulty in measuing maximum tempeatue, maginal tempeatue fo moto windings is limited to 95 o C, when measued by means of a plain themomete, and to 100 o C when measued by means of a esistance themomete. Estimation of powe fo motos opeating unde constant long tem load. In longtem opeation unde constant load, thee is enough time fo the tempeatue of the moto to be stabilized. This happens with motos in heavy lathes, vetical lathes, boing machines, gea cuttes, etc., with long duation of sepaate machining opeations. The ated powe o output of an electic moto opeating unde such conditions should be equal to the machine-tool powe consumption. Since it is pactically impossible to find in any catalogue a moto with ated powe matching its powe consumption, the moto with the neaest highe value of ated powe is chosen. The cutting powe is found fom the following fomula: N c Pz. 102 v 60 kw Whee P z = cutting foce, tangential when tuning and milling, longitudinal when shaping, boaching etc. v = cutting speed Hence, the powe consumption of the machine tool, with mechanical tansmission losses taken into account, will be N c Nc Pz v. kw Whee == mechanical efficiency of machine-tool tansmissions. Estimation of powe fo motos opeating unde shot-tem load. In shot-tem opeation, thee is not enough time fo the moto tempeatue to be stabilized. The point is that shot peiods of opeation unde load ae followed by long peiods of down time, when the moto tempeatue, which has isen duing opeation, dops to the tempeatue of the envionment. This happens in motos used fo the auxiliay dives of some machine-tool membes, such as apid tavese dives of caiages, coss ails, spindle heads, dives of clamping devices, etc. The opeational duation of such dives does not nomally exceed 5 to 15 s, while in heavy machines it may each 1 to 1.5 min. With oveloads being within the acceptable limits, this time is too shot fo the moto to become even nomally oveheated. The powe consumption in this case is detemined by oveload conditions. The fomula fo calculating powe consumption is N p. c. G kw 6120

10 Whee G == weight of tavelling machine-tool membe, N == coefficient of fiction of motion v== speed of tavel, m/min == efficiency of tansmission fom moto to tavelling membe == oveload facto The toque of esistance on stating will be es Gov s 0 Whee o = coefficient of static fiction n o = speed of moto idling otation, pm s = slippage of moto once N p.c. and es ae calculated, the electic moto is chosen fom the catalogue accoding to the equied ated powe. Stating toque s is then found fo this moto and compaed with es. If s > es, the choice is coect. Estimation of powe fo motos opeating unde vaiable long-tem load. These woking conditions ae common in machine tools incopoated stating clutches in the pincipal movement dives and used fo poducing pats of the same type, and also in machine tools on automatic tansfe line. Hee the electic moto otates continuously. Cutting peiods ae followed by idling peiods when the cutting tool is etacted and the next wok piece is loaded. Accodingly, a cetain load of the moto coesponds to each stage of the machining cycle. Since motos used in machine-tool dives ae ated on the basis of long-tem load, these long-tem constant-load conditions, with the heating of the moto equivalent to the heating of the moto unde vaiable long-tem load, need to be found so as to estimate the machine powe consumption. Without going into the details, let us note among the methods of equivalent cuent, toque o powe. Estimation of powe fo motos opeating unde intemittent load. These woking conditions involve shot peiods of opeation unde load, which ae not long enough fo the moto tempeatue to be stabilized, followed by shot peiods of moto shut-downs, which ae not long enough fo the moto to cool down to the envionment tempeatue. This change of moto tempeatue can be visualized as a boken cuve consisting of altenating ise and fall sections which epesent the heating-up and cooling-down of the moto. These conditions occu in most machine-tool dives. One such cycle should not last fo oe than10 min. The moto powe is best estimated by means of the aveage losses method.

11 Hand-Opeated Contol Gea The hand-opeated contol gea includes knife switches, otay switches, contolles, tumble switches, and hand-opeated states. Thee ae single, double, and thee-pole knife witches accoding to the numbe of knives they contain. These switches featue a casing and a side leve handle. To effect an instant beak in the electic cicuit, the knives of the switch ae equipped with a sping-actuated device. A thee-pole knife switch with side hand leve, ated fo 100 o 200 A, is most commonly used Fig. (ga). Rotay switches ae moe compact and easy to opeate than knife switches. The otay switch shown in Fig. (gb) contains a set of stacked sections each section being a singlepole otay cicuit beake (o change-ove switch). All the sections ae contolled by one otay shaft and ae aanged in such a way that when the shaft is otated some cicuits ae connected and othes-disconnected. Rotay switches ae used to connect machine tools to the mains, to stat up aely enegized motos, etc. Fig. (ga) Fig. (gb) Fig. (gc) Contolles ae used to commutate a lage numbe of cicuits. Thee ae cam and dumtype contolles. Shown in fig. (gc) is a dum contolle with two woking position I and II and one neutal position 0. The contolle contains shaft I and, fixed on it, dum 2, which is made of dielectic. The dum accommodates coppe lamina 3, while ba 5 also made of dielectic and placed beside the dum, holds contact finges 4. Each position of the dum coesponds to the commutation of a cetain cicuit. Tumble switches ae used in the electic lighting systems of machine tools. In pinciple and design they ae simila to the switches used in lighting mains. Hand-opeated states ae sometimes used to stat up one o thee-phase electic motos. The opeating pinciple of the state shown in Fig. h is as follows: when button 5 is pessed contacts 2 and 1 ae closed and contact 2 is locked by means of sping-actuated stop 3, theeby connecting the moto to the mains. When button 6 is pessed stop 3 eleases the contacts and they beak, disconnecting the moto. Casing 4, the buttons and othe components of the state ae made as dialectic.

12 Stating Gea Contactos ae widely used in machine tools to contol electic motos. In the cicuit shown in Fig. ia, the moto is stated by a special device called the contacto. When button 2 is pessed, coil 3 is enegized and coe 5 attacts amatue 4. This esults in shaft 6 tuning and closing powe contacts i. Figue ib shows the scheme of a contacto with staight movement of amatue. Fig. ja Fig. jb The devices simila in constuction to contactos, but with weak-cuent contacts to opeate in contol cicuits, ae called auxiliay elays. Buttons to contol contactos can have nomally open (o font) contacts (Fig ja), and nomally closed (o back) contacts (Fig. jb). some buttons have one font and one back contact (Fig. jc). buttons ae assembled in blocks, two of which, consisting of two and thee buttons, ae pesented in Fig jd. The blocks ae designed fo two contol commands: Stat and Stop, o fo thee: Fowad, Back and Stop, whee thee ae seveal Stat buttons to contol one machine-tool membe, these ae connected in paallel. They have one common etaining contact which is also connected in paallel and is closed when the cicuit is enegized. The Stop buttons ae connected in seies. Combination states seve to contol a.c. cage-oto induction motos. These states compise sets of contol gea, such as contactos, button blocks and themal-oveload elays fo moto potection. The cicuit diagam of such a state is shown in Fig. je.

13 The state contains a double-button block fo stating and stopping the moto. To stat up the moto, the Stat button in pushed, closing the cicuit of the contacto coil K 1, contacts K ae closed and connect the moto to the mains. To pevent coil K 1 fom deenegizing when the Stat button is eleased, the cicuit is povided with etaining contact K, connected in paallel with the Stat button and closed simultaneously with contacts K. The moto is stopped by pessing the Stop button which causes coil K 1 to be de-enegized, theeby beaking contacts K and disconnecting the moto fom the mains. Combination states pevent the moto fom stating spontaneously when the cicuit voltage suddenly fails and then eappeas. Subsequent stating can be effected only by pushing the Stat button. Depending on pupose, states come in two vesions: plain and evesible. Automatic Contol Gea Limit switches ae used to contol the tavel of machine-tool mechanisms automatically. They contol both ecipocating and otating movements of machine opeative membes by closing and beaking the machine-tool contol cicuit at the equied intemediate o teminal points of tavel. A limit switch consisting of two back contacts `1 and two font contacts 2, is opeated by od 3 being engaged by the tip dog fixed on a machine s tavelling membe. When eleased, the od is etuned to its initial position by sping 4. The micoswitch is a small-size vesion of the limit switch, featuing one independent contact. icoswtiches povide high accuacy of opeation with modest opeational foce (3-7 N, o kgf). Electomagnetic Devices Plunge electomagnets (etact solenoids) ae used in vaious contol systems of machine tools, e.g. contactos, emote-contol begins opeating as geneato. Low efficiency (about 0.65), lage size, and high cost ae among the impotant disadvantages of the geneato-moto system.