STARTER FOR A TWO VALUE CAPACITOR MOTOR USING A CURRENT RELAY AND AN AUTO TRANSFORMER

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.

Figure: Starter for a two value capacitor motor using an overcurrent relay and an auto-transformer

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