what is DOL Starter ( Direct ON Line ) construction working principle And advantages and disadvantages
DOL starter ( direct-on-line )
A direct-on-line (DOL) starter is a Three phase induction motor starter used to start and control the operation of a Three phase induction motors. It provides a simple and effective way to start the induction motor by connecting directly to the mains supply without any harm. While running, DOL starts applying full voltage to the motor, making it start abruptly, suitable for small and medium sized motors that do not need high starting power.
- The objective of this starting method, which is used with three-phase induction motors, is to reduce the starting current.
- In the starting position, the current supply to the stator windings is connected in star (Y) for starting.
- In the running position, the current supply is reconnected to the windings in delta (∆) once the motor has gained speed.
Construction of DOL starter
1. Circuit breaker
Circuit breakers or fuses are connected directly to the power supply and are used to prevent short circuits. In the event of a short circuit, it cuts off the electricity and protects the system from all kinds of dangers.
2. Contactor
- Contactor acts as a switch under normal working conditions by providing the means to switch ON and switch OFF the motor.
- It holds the switch ON condition when the motor is running by the use of a latch mechanism and interrupts the flow of current to the motor once the stop button is pressed.
- It also incorporates the overload tripping mechanism that can be operated by either thermal or magnetic overload trips. These tripping mechanisms are activated when there is a sustained increase of current flow through the starter.
3. Start and Stop Push Buttons:
A direct online starter consists of two buttons, a GREEN button for starting and a RED for stopping purpose of the motor.
These two push buttons Green and Red for starting and stop control the circuit.
4. Thermal Overload Relay:
- Provides additional motor protection by monitoring temperature.when temperature exceeds the threshold value then thermal overload relay come into action.
- Thermal overload relays provide protection for 1-phase or 3-phase motors.
- It is attached to the starter to protect the motor from overcurrent.
- The relay monitors the operating current of the motor and closes the contactor in case of overload.
Working principle of DOL starter
- The direct online starter has a common operating contactor C controlled by a start (normally open) button and a stop (normally closed) button. When the start button (S1) is pressed, the contactor coil C receives energy from the twoconductors R and Y. Main contact (M) and contact (A) are closed, x and y terminals are short. Therefore, the asynchronous motor is connected to the mains voltage.
- When the button is released, it moves backwards with the spring control. Despite this, contactor coil C is still energized from the x and y terminals. Therefore, the main contact (M) remains closed and the motor continues to receive electricity. For this reason, contact (A) is also called contact.
- When the stop button (S2) is pressed, the contactor coil C is de-energized, thus the contactor coil C is de-energized. Therefore, the main contact (M) and contact (A) are opened. The motor is cut off, so the motor stops.
Advantages and Disadvantages of DOL starter
Advantages
- It is a simple and most economical starter available in cheap price.
- DOL starter connects the supply to the delta winding of the motor device
- Provides nearly full starting torque at starting.
- DOI starter is cost effective.
- The designing, operating, and controlling of this starter is very easy.
Disadvantages
- DOL starter can't be used more than 5 HP motors.
- High starting torque and high starting current 4 to 6 times of full load current .
- DOL starter cause significant dip in voltage thus only used for small and medium size motors.
- High starting current .
- DOL starter reduce the lifespan of the machines
Applications
Direct on line starters are commonly used in industries , to start small water pumps, conveyor belts, fans, and compressors.
