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 I_2max and I_2min.

       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, ....R_n-1.

Fig. 2 Steps of rotor resistance starter

       It consists of rotor resistance r₂ and the external resistances R_x1, R_x2...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 I_2min.

       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 I_2min given by,

       Just reaching to stud 2, the current again increases to I_2min as the part of external resistance R_x1 gets cut-off.

       While leaving stud 2, the slip changes to s₃ and current again reduces to,

       While just reaching to stud 3, R_x2 gets cut off completely and current again increases to,

        Hence at the last n^th stud, the maximum current is,

       where s_n = Slip under normal running condition

       At n^th stud no external resistance is in series with rotor.

        From (1) and (2) we can write,

       where K = Constant

       From (1),   R₁ = s₁r₂/s_n    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 = K^n-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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