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

Transcription

1 Module 8 Three-phae Induction Motor Verion EE IIT, Kharagpur

2 Leon 33 Different Type of Starter for Induction Motor (IM Verion EE IIT, Kharagpur

3 Inructional Objective Need of uing arter for Induction motor Two (Star-Delta and Auto-tranformer type of arter ued for Squirrel cage Induction motor Starter uing additional reiance in rotor circuit, for Wound rotor (Slip-ring Induction motor Introduction In the previou, i.e. fourth, leon of thi module, the expreion of gro torque developed, a a function of lip (peed, in IM ha been derived fir. The ketche of the different torque-lip (peed characteriic, with the variation in input (ator voltage and rotor reiance, are preented, along with the explanation of their feature. Laly, the expreion of maximum torque developed and alo the lip, where it occur, have been derived. In thi leon, arting with the need for uing arter in IM to reduce the arting current, fir two (Star-Delta and Auto-tranformer type of arter ued for Squirrel cage IM and then, the arter uing additional reiance in rotor circuit, for Wound rotor (Slip-ring IM, are preented along with the arting current drawn from the input (upply voltage, and alo the arting torque developed uing the above arter. Keyword: Direct-on-Line (DOL arter, Star-delta arter, auto-tranformer arter, rotor reiance arter, arting current, arting torque, arter for quirrel cage and wound rotor induction motor, need for arter. Direct-on-Line (DOL Starter Induction motor can be arted Direct-on-Line (DOL, which mean that the rated voltage i upplied to the ator, with the rotor terminal hort-circuited in a wound rotor (lip-ring motor. For the cage rotor, the rotor bar are hort circuited via two end ring. Neglecting ator impedance, the arting current in the ator winding i (ee leon 3 i ( I1 = Er ( r + ( x where, ( I1 = ( I = ( I / a = Starting current in the motor (ator a = T /Tr = Effective turn ratio between ator and rotor winding E = Er = a Er = Input voltage per phae to the motor (ator E r = Induced emf per phae in the rotor winding r = a r = Rotor reiance in term of ator winding x = a x = Rotor reactance at andill in term of ator winding Verion EE IIT, Kharagpur

4 The input voltage per phae to the ator i equal to the induced emf per phae in the ator winding, a the ator impedance i neglected (alo hown in the la leon (#3. In the formula for arting current, no load current i neglected. It may be noted that the arting current i quite high, about 4-6 time the current at full load, may be higher, depending on the rating of IM, a compared to no load current. The arting torque i ( ( T [ ] 0 ( I1, which how that, a the arting current increae, the arting torque alo increae. Thi reult in higher accelerating torque (minu the load torque and the torque component of the loe, with the motor reaching rated or near rated peed quickly. Need for Starter in IM The main problem in arting induction motor having large or medium ize lie mainly in the requirement of high arting current, when arted direct-on-line (DOL. Aume that the diribution line i arting from a ubation (Fig. 33.1, where the upply voltage i conant. The line feed a no. of conumer, of which one conumer ha an induction motor with a DOL arter, drawing a high current from the line, which i higher than the current for which thi line i deigned. Thi will caue a drop (dip in the voltage, all along the line, both for the conumer between the ubation and thi conumer, and thoe, who are in the line after thi conumer. Thi drop in the voltage i more than the drop permitted, i.e. higher than the limit a per ISS, becaue the current drawn i more than the current for which the line i deigned. Only for the current lower the current for which the line i deigned, the drop in voltage i lower the limit. So, the upply authoritie et a limit on the rating or ize of IM, which can be arted DOL. Any motor exceeding the pecified rating, i not permitted to be arted DOL, for which a arter i to be ued to reduce the current drawn at arting. Starter for Cage IM ( 1 The arting current in IM i proportional to the input voltage per phae ( to the motor (ator, i.e. I E, where, V E, a the voltage drop in the ator impedance i mall compared to the input voltage, or V = E, if the ator impedance i neglected. Thi ha been hown earlier. So, in a (quirrel cage induction motor, the V Verion EE IIT, Kharagpur

5 arter i ued only to decreae the input voltage to the motor o a to decreae the arting current. A decribed later, thi alo reult in decreae of arting torque. Star-Delta Starter Verion EE IIT, Kharagpur

6 Thi type i ued for the induction motor, the ator winding of which i nominally delta-connected (Fig. 33.a. If the above winding i reconnected a ar (Fig. 33.b, the voltage per phae upplied to each winding i reduced by 1 / 3 ( Thi i a imple arter, which can be eaily reconfigured a hown in Fig. 33.c. A the voltage per phae in delta connection i V, the phae current in each ator winding i ( V / Z, where Z i the impedance of the motor per phae at andill or art (ator impedance and rotor impedance referred to the ator, at andill. The line current or the input current to the motor i [ ( I 1 = ( 3 V / Z ], which i the current, if the motor i arted direct-on-line (DOL. Now, if the ator winding i connected a ar, the phae or line current drawn from upply at art (andill i [ ( V / Z / 3 ], which i ( 1 / 3 = (1/ 3 of the arting current, if DOL arter i ued. The voltage per phae in each ator winding i now (. V / 3. So, the arting current uing ar-delta arter i reduced by 33.3%. A for arting torque, being proportional to the quare of the current in each of the ator winding in two different connection a hown earlier, i alo reduced by ( 1 / 3 = (1/ 3, a the ratio of the two current i ( 1 / 3, ame a that (ratio of the voltage applied to each winding a hown earlier. So, the arting torque i reduced by 33.3%, which i a diadvantage of the ue of thi arter. The load torque and the lo torque, mu be lower than the arting torque, if the motor i to be arted uing thi arter. The advantage i that, no extra component, except that hown in Fig. 33.c, need be ued, thu making it imple. A hown later, thi i an auto-tranformer arter with the voltage ratio a 57.7%. Alternatively, the arting current in the econd cae with the ator winding reconnected a ar, can be found by uing ar-delta converion a given in leon #18, with the impedance per phae after converting to delta, found a ( 3 Z, Verion EE IIT, Kharagpur

7 and the arting current now being reduced to ( 1/ 3 of the arting current obtained uing DOL arter, with the ator winding connected in delta. Auto-tranformer Starter Verion EE IIT, Kharagpur

8 An auto-tranformer, whoe output i fed to the ator and input i from the upply (Fig. 33.3, i ued to art the induction motor. The input voltage of IM i x V, which i the output voltage of the auto-tranformer, the input voltage being V. The output voltage/input voltage ratio i x, the value of which lie between 0.0 and 1.0 ( 0.0 < x < 1.0. Let ( I 1 be the arting current, when the motor i arted uing DOL arter, i.e applying rated input voltage. The input current of IM, which i the output current of auto-tranformer, i ( I, when thi arter i ued with input voltage a x 1 x V. The input current of auto-tranformer, which i the arting current drawn from the upply, i x ( I1, obtained by equating input and output volt-ampere, neglecting loe and auming nearly ame power factor on both ide. A dicued earlier, the arting torque, being proportional to the quare of the input current to IM in two cae, with and without auto-tranformer (i.e. direct, i alo reduced by x, a the ratio of the two current i x, ame a that (ratio of the voltage applied to the motor a hown earlier. So, the arting torque i reduced by the ame ratio a that of the arting current. If the ratio i x = 0.8 (80%, both arting current and torque are x = (0.8 = 0.64 (64% time the value of arting current and torque with DOL arting, which i nearly time the value obtained uing ar-delta arter. So, the diadvantage i that arting current i increaed, with the reult that lower rated motor can now be arted, a the current drawn from the upply i to be kept within limit, while the advantage i that the arting torque i now doubled, uch that the motor can art again higher load torque. The ar-delta arter can be conidered equivalent to an autotranformer arter with the ratio, x = (57.7%. If x = 0.7 (70%, both arting current and torque are x = (0.7 = (50% time the value of arting current and torque with DOL arting, which i nearly 1.5 time the value obtained uing ardelta arter. By varying the value of the voltage ratio x of the auto-tranformer, the Verion EE IIT, Kharagpur

9 value of the arting current and torque can be changed. But additional co of autotranformer with intermittent rating i to be incurred for thi purpoe. Rotor Reiance Starter for Slip-ring (wound rotor IM In a lip-ring (wound rotor induction motor, reiance can be inerted in the rotor circuit via lip ring (Fig. 33.4, o a to increae the arting torque. The arting current in the rotor winding i Er ( I = ( r + R + ( x where ext = Additional reiance per phae in the rotor circuit. R ext The input (ator current i proportional to the rotor current a hown earlier. The arting current (input reduce, a reiance i inerted in the rotor circuit. But the arting torque, [ ( T 0 = 3 [ ( I ] ( r + Rext ] increae, a the total reiance in the rotor circuit i increaed. Though the arting current decreae, the total reiance increae, thu reulting in increae of arting torque a hown in Fig. 3.b, and alo obtained by uing the expreion given earlier, for increaing value of the reiance in the rotor circuit. If the additional reiance i ued only for arting, being rated for intermittent duty, the reiance i to be decreaed in ep, a the motor peed increae. Finally, the external reiance i to be completely cut out, i.e. to be made equal to zero (0.0, thu leaving the lip-ring hort-circuited. Here, alo the additional co of the external reiance with intermittent rating i to be incurred, which reult in decreae of arting current, along with increae of arting torque, both being advantageou. Alo it may be noted that the co of a lip-ring induction i higher than that of IM with cage rotor, having ame power rating. So, in both cae, additional co i to be incurred to obtain the above advantage. Thi i only ued in cae higher arting torque i needed to art IM with high load torque. It may be oberved from Fig. 3.b that the arting torque increae till it reache maximum value, i.e. (( T 0 < ( T0 m, a the external reiance in the rotor circuit i increaed, the range of total reiance being [ r < ( r + Rext < x ]. Verion EE IIT, Kharagpur

10 The range of external reiance i between zero (0.0 and ( x r. The arting torque i equal to the maximum value, i.e. (( T 0 = ( T0 m, if the external reiance inerted i equal to ( x r. But, if the external reiance in the rotor circuit i increaed further, i.e. [ R ( ext > x r ], the arting torque decreae ( ( T 0 < ( T0 m. Thi i, becaue the arting current decreae at a faer rate, even if the total reiance in the rotor circuit i increaed. In thi leon the fifth one of thi module, the direct-on-line (DOL arter ued for IM, along with the need for other type of arter, ha been decribed fir. Then, two type of arter ar-delta and auto-tranformer, for cage type IM, are preented. Laly, the rotor reiance arter for lip-ring (wound rotor IM i briefly decribed. In the next (ixth and la leon of thi module, the variou type of ingle phae induction motor, along with the arting method, will be preented. Verion EE IIT, Kharagpur