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Authors: Singh, Yashwant
Issue Date: 1964
Abstract: Tha induction meter compares favourably with d*c, motors as r• *rd* costs alts, saintefeacrt,, and wear and tears but suffers from the Limitations that its speed of operation reagins more or less constant# sad can not be varied as desired. A number of s~t1 Is for its speed control, are known but the variation of speed is neither smooth nor suitable for certain drives * especially heist crane etc* A standard torque-spaod, characteristic of an induction m rotor is obtained when a positivou sequence system of voltages is impressed on its teralAals. Under negative.sequeaca operation the characteristic is the inverse unige of that at the previous on.. When a sero. sequence system of voltago3 is applied $ the motor behaviour could be determined by recourse to single-phase motor theory #in** the resultant field caused by cophaaal voltages Is pulsating,. If the induction motor is subjected to the positive and negative-sequence systems of voltages sis~zltaAeousiy, the resultant terqva.speed characteristics will be the sl,debric sun of the two separate characteristics, and this resultant characteristic could be modified, as desired, by varying the relative magnitude of the two systems. This suggests one of the ways it smooth speed control of the induction motor, A st .ltaneous production of positive and negative-sequence system of voltages could be achieved by a number of says. One of the easier method Is an unbalanced operation of the induction motor. Under unbalanced operation, system, if not eliminated, is also present alongwitit the positive and negative- sequence systems. The system Could be easily oiiainatad by providing a winding arrangement such that the atgobric sum of the phase vectors Is sero. The unbalancing could be produced, simply by introducing an impedance in one of the lines, or using an auto«tranatorasr. The doffs of unbalance produced depends to some extent on the load conditions on the motor, but to general a variable it sdancs is required to control the degree of unbalance intro. duced. A saturable core reactor (8 C ! ) Can be used as a circuit element having a variable taped ace. The S C R consists of two Windings • a load winding connected to the alternating current supply p ad a control winding connected to a direct current sourest and providing a variable bias. The 5 C R is driven electrically by the ,lns voltage and the bias,, and the flux density in the core reaches a saturatisaa value Bs sometimes during one of the halt cycle of the lino voltage.. if - the core material Is rectangular the transition into saturation is sharp, the impedance of the coli drops abruptly and most of the lie* voltage appears across the loads - and hence provides use ful power. The reversal of line •oltage drives the flux out of. saturations lead conduction *eases# and the direction of flux change, is opposite to that of the previous half cyclo. Lead powers and conduction angle are controll. ad by waving the initial flux density 'B' towards positive Bs (direct curroat), in general, the above technique controls laducteaa.o by applying as ad3uatable 4.0. magnetisation. The speed of an induction voter can be controlled by a.Marcciat; tip S.C. (load) winding of the 8CR is one or the supply lines, and obtaining d.c, for the control winding either with the help of rectifiers or directly from a does. Soum availablo. By varying the control current of the C R the relative magnitude of aegativa•ssquence voltage can be varied as desired# As such$ this method Is not effective# because the a ter operation is stable only in a very limited range of spse4. However, with a motor having a high roter resistances the speed control Is possible in all the four quadrants, The high rotor resistance also limits the high input current caused by the unbalanced opera-tine, In case of a slip ring induction motors the secondary resistance can easily be Increased by adding an additional resistor In the rotor circuit, and a wide range of speed control can be obtained by rasing a capacitor In addition to the resistor, or by using a combination of external resistance f capao2tanc. and induct . elements in the secondary circuit. The speed control +sae also be ade automatic and responsive to speed as suggested by W1ak.rhat min the case of slip.ring induction motors, In case, of squirrel cage induction motor the current in the control winding of the 6 C R is automatically varied by the d.c. tachometer genera. tar (fixed on the shaft of the induction motor) and comparing It with a reference direct voltage* Also by using a capacitor in parallel with the 6 C 1 f the external reactance may be varied npto a greater extent alongeith the facility of reversing its sign too# the direction of rotation of the motor can be reversed without interrupting the mains circuit, and a 4y*a.mic braking Is possible without an extra equip a*t. This fort of speed control of Induction Voter, however,, causes problems of magnetic vibrations, solsst overheating, and high .Input line, current. However, all 4be. these difficulties tsuld be alleviated to some extent. In the laboratory standard tests on a slip-ring induction motor# and a squirrel-►cage Induction motor were performed to 'in obtain ter parameters of the motor windings* The possibility of speed eo*trot of the induction motors,, by mixd.ssquancs eperati©a, was studied by measuring the torque-speed, and torque-input current charsotsristles for the *bove motors under the following different condition$$ (a) with balanced applied voltages (b) with various unbalanced v©ltagea obtained with thy+ .(6), with various unba3eiived voltages obtained with the single phase Varise. ad : (d) In the case of slip-ring induction motor (a), (b),, and (0) with the different externs, resistanoee in the rotor circuit.
Other Identifiers: M.Tech
Appears in Collections:MASTERS' DISSERTATIONS (Electrical Engg)

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