Direct on-line Starter

Direct on-line Starter Points : Direct on-line Starter, Definition Most three-phase, squirrel-cage induction motors with ratings up to 10 H.P are connected directly across the line. Across the line starting usually is accomplished using a magnetic starting switch controlled from a push-button. In fig an elementary control circuit with start and stop button and a sealing circuit are shown. First you understand this simple circuit.

When we press the start button, current flows from L1 through normally closed stop button, through start button and through coil (M) to line L2. The current in coil M causes the contact M to close. As a result, the sealing circuit around start button closes. Now, if we remove the finger from the start button, we see that, coil (M) remains energized and hold contacts M closed to maintain a sealing circuit are and the start button. Coil (M) being enegized, also closes M contacts in the power circuit to the motor. If we press the stop button, the circuit is interrupted and coil (M) is deenergized. Contacts M then open and coil (M) cannot be energized until the start button again closes. Now, we study D.O.L starter. The 3-phase induction motor is connected across full line voltage for operation in one direction only. The magnetic switch used for starting has three heavy contacts, one auxiliary contact, three motor overload relays, and an operating coil, the operation of circuit, which shown in fig is very simple.

When we press the start push button, the contacts coil (M) is energized and its normally open contacts m₁, m₂, m₃ and m₄ are closed. When these contacts (mi, tm, mi) are closed, power is applied to the induction motor and the motor starts. Contact m closed to maintain a sealing circuit around the start button. If we remove the finger from the start button, but the coil M remains energized because the contact on made a sealing circuit across the start button. When we press the stop button, the M coil is de energized and the m₁, m₂, m₃ and m₄contacts open, stopping the motor.
A magnetic motor starter circuit of this sort has several built-in protective features.
(1) Short-circuit protection
(2) Overload protection
(3) Under-voltage protection

Short circuit protection for the motor is provided by fuses f₁, f₂ and f₃. If a short circuit is occurred in motor, cause’s high current flows, as a result, fuses will blow, disconnecting the motor from the power supply. These fuses must not burn up during normal motor starting.

Overload protection for the motor is provided by the devices labeled O/L in the figure. Under normal condition, the contacts of the overload remain in closed position. When the overload is occur in the motor circuit, the, temperature of heater element rises for enough, the O/L contacts open, de-energized the M coil of the contactor and as a result motor is disconnected from the supply.

Should the voltage fail or drop to a very low value while the motor is running, the coil (M) will not have enough power to keep the contacts closed. The contactor will open and stopping the motor. This is a safety feature, for a motor running at greatly reduced voltage will probably bum out. In fig a power and control circuit of a D.O.L starter is shown. The circuit commonly used is industries. Short circuit protection for the motor is provided by fuses f₁, f₂ and f₃. If a short circuit is occurred in motor, cause’s high current flows, as a result, fuses will blow, disconnecting the motor from the power supply. These fuses must not burn up during normal motor starting.

Overload protection for the motor is provided by the devices labeled O/L in the figure. Under normal condition, the contacts of the overload remain in closed position. When the overload is occur in the motor circuit, the, temperature of heater element rises for enough, the O/L contacts open, de-energized the M coil of the contactor and as a result motor is disconnected from the supply.

Should the voltage fail or drop to a very low value while the motor is running, the coil (M) will not have enough power to keep the contacts closed. The contactor will open and stopping the motor. This is a safety feature, for a motor running at greatly reduced voltage will probably bum out. In fig a power and control circuit of a D.O.L starter is shown. The circuit commonly used is industries.