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
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.
R2 = KR1, R3 = K R2 = KKR1 = K2 R1
R4 = K3 R1, .... .... r2 = Kn-1 R1