Rotor Resistance Starter

       To limit the rotor current which consequently reduces the current drawn by the motor from the supply, the resistance can be inserted in the rotor circuit at start. This addition of the resistance in rotor in the form of 3 phase star connected rheostat. The arrangement is shown in the Fig. 1.
Fig. 1 Rotor resistance starter

       The external resistance is inserted in each phase of the rotor winding through slip ring and brush assembly. Initially maximum resistance is in the circuit. As motor gather speed, the resistance is gradually cut-off. The operation may be manual or automatic.
       We have seen that the starting torque is proportional to the rotor resistance. Hence important advantage of this method is not only the starting current is limited but starting torque of the motor also gets improved.
       Note : The only limitation of the starter that it can be used only for slip ring induction motors as in squirrel cage motors, the rotor is permanently short circuited.
1.1 Calculation of Steps of Rotor Resistance Starter
       The calculation of steps of rotor resistance starter is based on the assumptions that,
1. The motor starts against a constant torque
2. The rotor current fluctuates between two fixed values, a maximum and a minimum, denoted as I2max and I2min.
       The Fig. 2, shows a single phase of a three phase of a three phase rheostat to be inserted in the rotor. The starter has n steps, equally divided into the section AB. The contact point after each step is called stud. The total resistances upto each stud from the star point of star connected rotor as denoted as R1, R2, ....Rn-1.
Fig. 2 Steps of rotor resistance starter
       It consists of rotor resistance r2 and the external resistances Rx1, Rx2...etc. At the time of reaching to the next step, current is maximum . Then motor gathers speed, slip reduces and hence while leaving a stud, the current is I2min.
       Let   E2 = Standstill rotor e.m.f. per phase
       When handle is moved to stud 1, the current is maximum given by,
       where   s1 = Slip at start = 1
       while moving to stud 2, the current reduces to I2min given by,
       Just reaching to stud 2, the current again increases to I2min as the part of external resistance Rx1 gets cut-off.
       While leaving stud 2, the slip changes to s3 and current again reduces to,
       While just reaching to stud 3, Rx2 gets cut off completely and current again increases to,
        Hence at the last nth stud, the maximum current is,
       where sn = Slip under normal running condition
       At nth stud no external resistance is in series with rotor.

        From (1) and (2) we can write,
       where K = Constant
       From (1),   R1 = s1r2/sn    but s1 = 1 at start
       Once R1 is known, other resistances can be calculated.
       R2 = KR1,           R3 = K R2 = KKR1 = K2 R1
       R4 = K3 R1, ....  .... r2 = Kn-1 R1
       From last expression of r2,
       where n = Number of starter studs
       Thus the resistances of various sections can be obtained as,
       In this way the various steps of rotor resistance starter can be calculated.

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    vaisakh s