What is Direct online starter?Different types of techniques are used to start induction motor with safe starting current because these motors utilize more power for starting. During starting when the flow of current through windings is high then there will be a chance to damage the motor winding To overcome this problem, different methods of starter techniques are used. The simple type of starter method is DOL (Direct on Line Starter).
DOL starter include circuit breaker or MCCB( Molded case circuit breaker), overload relay and contactor for the protection of motor. The operation of an electromagnetic contactor can be done through the thermal overload relay normally closed contact .
The contactor can be controlled by using separate buttons like start and stop. An auxiliary contact is used on the contactor across the start button because the contactor is electrically latched while the working of an induction motor.
Direct online starter (DOL)
The most easiest method for running a three-phase induction motor is named as the DOL starter. This starter connect the motor directly across the supply of three phases.. The Direct Online Starter consists of a protective device as well as main contacts.
Construction of DOL Starter
The DOL starter consists of two switches namely Green and Red & start and stop )where the green switch is used to start the motor and red switch is used to stop the motor.
The DOL starter include circuit breaker (or) MCCB( Molded case circuit Breaker), thermal overload relay and electromagnetic contactor for protecting the motor. With the help of the two switches we can control the motor. The motor can be started by pressing the green and the full-line voltage are connected to the induction motor.
The contactors are normally 3-poles contactors or 4-pole contactors.Usually a 4-pole type electromagnetic contactor consists of three normally open contacts and one is auxiliary or holds on contact are used to make DOL starter. The 3 pole of the contactor ( NO) are used to connect the induction motor to supply lines whereas the auxiliary contact is used to boost the contactor coil when the start button is opened.
If any fault occur then holds on contactor gets deactivated and due to which the DOL starter separates the induction motor from the mains supply by protecting the fault current ..
DOL circuit Diagram
DOL circuit diagram and wiring is consist main contact, normally open contacts, normally closed contacts, and connections of relay coil as well as thermal overload relay.
Main Contactor Connection.
The main contactor is installed between thermal overload relay and voltage supply or MCCB.The L1 pole of contactor is connected to R-phase o by using MCCB.The L2 pole of the contactor is connected to Y-phase by using MCCB.The L3 pole of the Contactor is connected to B- phase by using MCCB.
Connection of Actuating Coil of Contactor
The Actuating Coil (A1) is connecting to any supply segment & A2 actuating coil is connected to the NC Contact of the thermal overload relay.
Connection of Thermal Overload Relay
The L1 .L2 and L3 pole of contactor is connected to the thermal overload Relay. This relay is connected with the motor and main contactor. The normally closed ( NC ) of this relay is connected to the stop switch as well as mutual connection of start or stop switch.
DOL Starter Working
When we pressed the start button then DOL starter connect the L1 ,L2 and L3 of the main contactor with the induction motor The working of DOL starter is done in two different stage namely DOL starter control circuit and DOL starter power circuit .
The control circuit is connected to any two of the phases or with one phase and neutral wire depending upon the electromagnetic contactor actuating coil.
Whenever we pressed the start switch, then the current will flows through the control circuit as well as contactor actuating coil also. The contactor is operated due to which the contactor normally opened contact of coil become closed and motor connected to the supply line .
When we pressed the stop button, then the flow of current through the contactor actuating coil is disconnected as a result the become off stopping the power supply to the induction motor as well as same operation occur while the thermal overload relay work during the fault of overheated
The contactor actuating coil gets power supply even though we open start switch because it will obtain supply from the main contacts.
Advantages & Disadvantages of DOL Starter
The advantages of DOL starter include the following.
- It is an inexpensive starter and easy to understand for electrician
- It gives almost complete starting torque at the beginning.
- It has easy designing, operation and controlling.
- It is too much easy to understanding and troubleshooting
The disadvantages of DOL starter include the following.
- It has very high starting current due to which may damage the motor winding
- It is suitable for small motor only
- Due to direct starting the life of machine is reduced.
Applications of DOL Starter
This type of starter is used where maximum current do not cause any damage such as small compressor ,conveyor belts, water pumps ,fans etc.DOL starter is useful in motor when the 3- phase induction motor is at low rating and the input is very small .
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