This type of starter uses a current relay in series with the main winding and an auto-transformer in the auxiliary winding to increase the effective value of the capacitors in the auxiliary winding circuit. The circuit has been shown in Figure.
In this circuit, when contactor M is energized, a high inrush current flows through the main winding which energizes relay R. Its contact R1 closes and contact R2 opens. Due to closing of contact R1, the auto transformer winding comes in circuit and the effective value of capacitor is increased. The current relay is designed to pick up at 3 times the full load current and to drop at twice the full-load value. The motor therefore develops high starting torque during starting due to high effective capacitance. When the motor picks up speed, the current drops and the relay R drops out. Due to opening of contact R1 and closing of contact R2, the auto-transformer gets open circuited. The auxiliary winding then gets connected through the capacitor only. The relay R can pick up again, if load on the motor causes current to rise to its pick up value and thus re-inserts the auto-transformer. Thus, this increased capacitance permits the motor to develop more torque and prevents stalling at heavier loads.
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| Figure: Starter for a two value capacitor motor using an overcurrent relay and an auto-transformer |