Primary resistors starting of Induction motor
Primary resistors starting:
Their purpose is to drop some voltage and hence reduce the voltage applied across the motor terminals. In this way, the initial current drawn by the motor is reduced. However, it should be noted that whereas current varies directly as the voltage, the torque varies as square of applied voltage. When applied voltage is reduced, the rotating flux Φ is reduced which, in turn, decreases rotor e.m.f. and hence rotor current I2. Starting torque, which depends both on Φ and I2 suffers on two counts when impressed voltage is reduced. If the voltage applied across the motor terminals is reduced by 50%, starting current is reduced by 50%, but torque is reduced to 25% of the full-voltage value. By using primary resistors (Fig), the applied voltage/phase can be reduced by a fraction ‘x’ (and it additionally improves the power factor of the line slightly).
From Direct on line starting of induction motor we have,
It is obvious that the ratio of the starting torque to full-load torque is x2 of that obtained with direct switching or across-the-line starting. This method is useful for the smooth starting of small machines only.