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DOL Starter for Motors – Direct Online Starter Diagram, Working, Types & Applications

The induction motor draws a huge amount of current at startup. This starting current can damage the motor windings. In order to avoid any damage, we use different techniques to reduce the starting current using Motor Starter. These techniques depend on the motor ratings and the load connected to the motor. Apart from this, the motor starter also protects the motor from overloading and overcurrent.

The Direct Online or DOL starter employs full voltage or across the line starting technique where the motor is directly connected to full voltage through MCCB or circuit breaker and relays for overload protection. This is why such a starter is used with induction motors rated below 5 hp.

What is Direct Online (DOL) Starter?

DOL Starter (Direct Online Starter) is also known as “across the line starter”. DOL starter is a device consisting of main contactor, protective devices and overload relay which is used for motor starting operations. It is used for low rating usually below 5HP motors.

In direct online starter method of motor starting, the motor stator windings is directly connected to the main supply where the DOL protect the motor circuit from high inrush current which may damage the overall circuit as the initial current is much higher than the full rated current .

Protection Offered by DOL Starter:

The Motor starters not only provide the safe starting current but also provide protection to keep the motor safe during operation. It is clear that the DOL starter provides the full line voltage but it does provide the following protection:

Overcurrent Protection:

The condition that causes the flow of a faulty current in a large amount mostly due to a short circuit or ground fault is called overcurrent.

The overcurrent condition can cause damage to the motor, power lines and can be a hazard for operators. Such an amount of current is too dangerous for a brief moment.

In the DOL starter, we use a circuit breaker or fuses for protection against overcurrent. They open the circuit and breaks the current flow in an instant until the problem in the system is resolved. The fuse or circuit breaker is carefully selected with its rating kept in mind. Because we do not want the fuse to break but to tolerate the starting current as well as the heavy load current. The overcurrent breaker’s rating is kept a bit higher than the rated starting current of the motor.

Related Post: What is Soft Starter? Its Working, Diagram and Applications

Overload Protection:

The condition where the load connected to the motor increases beyond its limit and the motor draws an excessive amount of current is called overload condition. During overload, the current flow is beyond the safe limits which damage the wires as well as the motor windings. It melts the windings and may cause fire hazards.

In order to protect the motor from overloading, we use an overload relay that trips the power supply and protects the system from overheating. The overload relay monitors the current and breaks the current flow when it exceeds a certain limit for a period of time. The tripping mechanism may vary and depends on the application of the motor.

Following are a few types of overload relays used for motor protection:

Thermal Overload Relay: This type of overload relay works on the principle of expansion due to the heat generated by the current flow. A bimetallic strip is used with different thermal expansion to break or make the circuit based on the temperature.

Magnetic overload relay: such relays work on the principle of the magnetic field generated by the current flow through a coil. An excessive current drawn by the motor (that is a predetermined amount) generates enough magnetic field to trips the contact terminals and breaks the current supply.

Electronic Overload Relay: Electronic relay is a solid-state device without any movable parts or contacts. It uses current sensors to monitor the motor current and has an adjustable setting that allows the tripping at a wide range of current ratings.

Related Post: Why We Need to Install a Starter with a Motor?

Construction of DOL Starter:

A DOL or Direct Online starter has simply two buttons; Green and Red, where the green button is used for starting and the red button is used for stopping the motor. The green button connects the terminals and closes the circuit while the red button disconnects the terminals and breaks the circuit.

The DOL starter is made of a circuit breaker or MCCB or fuse, an overload relay and contactor or coil. The circuit breaker is used for protection against short circuits while the overload relay protects the motor from overloading. The contactor is used for starting and stopping the motor where the green and red buttons are connected. The wiring for the start and stop button is briefly explained in this article below.

Parts of DOL Starter:

A DOL starter is made of following parts:

The circuit breaker or fuse is directly connected to the power mains and it is used for protection against short circuits. It trips the power supply in case of short circuit to protect the system from any potential hazards.

Magnetic Contactors:

A magnetic contactor is an electromagnetic switch that operates electromagnetically to switch the power supplied to the motor. It connects and disconnects multiple contacts conveniently by providing remote control over the operation.

The magnetic field generated by the coil is used for switching the terminals. The current passing through the coil magnetizes the iron core that is surrounded by the coil. The magnetic force pulls on the armature to close or open the contacts.

The magnetic contactors have three NO (Normally open) main contacts used for power supply to the motor and auxiliary contacts (NO and NC) with a lesser rating used for the control circuit. The coil is connected to the voltage source through auxiliary contacts. Also, keep in mind that the coil used for a single-phase and three-phase supply varies as the supply voltages are different.

Overload Relay:

OLR or overload relay is the last part used in the DOL starter and it is used for protection against overloading of motor. It breaks the current flow when it exceeds a certain limit but it also tolerates the high starting current. So the OLR is carefully selected in such a way that its tripping current limit does not fall below the starting current range.

The excessive amount of current flow can damage the insulation of electrical wires as well as the motor winding. The motor life expectancy decreases and it can shorten the windings causing a risk of fire.

A simple fuse or circuit breaker cannot protect the system from overloading because they are used for overcurrent (short circuit) protection. The OLR has current sensing properties that can differentiate between the starting and overload current.

Related Post: Main Difference between Contactor and Starter

DOL Starter Wiring Diagram:

  The wiring of 3 phase and single phase slightly differs from each other. 

DOL Starter for Motors – Direct Online Starter Diagram, Working, Types & Applications

The induction motor draws a huge amount of current at startup. This starting current can damage the motor windings. In order to avoid any damage, we use different techniques to reduce the starting current using Motor Starter. These techniques depend on the motor ratings and the load connected to the motor. Apart from this, the motor starter also protects the motor from overloading and overcurrent.

The Direct Online or DOL starter employs full voltage or across the line starting technique where the motor is directly connected to full voltage through MCCB or circuit breaker and relays for overload protection. This is why such a starter is used with induction motors rated below 5 hp.

What is Direct Online (DOL) Starter?

DOL Starter (Direct Online Starter) is also known as “across the line starter”. DOL starter is a device consisting of main contactor, protective devices and overload relay which is used for motor starting operations. It is used for low rating usually below 5HP motors.

In direct online starter method of motor starting, the motor stator windings is directly connected to the main supply where the DOL protect the motor circuit from high inrush current which may damage the overall circuit as the initial current is much higher than the full rated current .

Protection Offered by DOL Starter:

The Motor starters not only provide the safe starting current but also provide protection to keep the motor safe during operation. It is clear that the DOL starter provides the full line voltage but it does provide the following protection:

Overcurrent Protection:

The condition that causes the flow of a faulty current in a large amount mostly due to a short circuit or ground fault is called overcurrent.

The overcurrent condition can cause damage to the motor, power lines and can be a hazard for operators. Such an amount of current is too dangerous for a brief moment.

In the DOL starter, we use a circuit breaker or fuses for protection against overcurrent. They open the circuit and breaks the current flow in an instant until the problem in the system is resolved. The fuse or circuit breaker is carefully selected with its rating kept in mind. Because we do not want the fuse to break but to tolerate the starting current as well as the heavy load current. The overcurrent breaker’s rating is kept a bit higher than the rated starting current of the motor.

Related Post: What is Soft Starter? Its Working, Diagram and Applications

Overload Protection:

The condition where the load connected to the motor increases beyond its limit and the motor draws an excessive amount of current is called overload condition. During overload, the current flow is beyond the safe limits which damage the wires as well as the motor windings. It melts the windings and may cause fire hazards.

In order to protect the motor from overloading, we use an overload relay that trips the power supply and protects the system from overheating. The overload relay monitors the current and breaks the current flow when it exceeds a certain limit for a period of time. The tripping mechanism may vary and depends on the application of the motor.

Following are a few types of overload relays used for motor protection:

Thermal Overload Relay: This type of overload relay works on the principle of expansion due to the heat generated by the current flow. A bimetallic strip is used with different thermal expansion to break or make the circuit based on the temperature.

Magnetic overload relay: such relays work on the principle of the magnetic field generated by the current flow through a coil. An excessive current drawn by the motor (that is a predetermined amount) generates enough magnetic field to trips the contact terminals and breaks the current supply.

Electronic Overload Relay: Electronic relay is a solid-state device without any movable parts or contacts. It uses current sensors to monitor the motor current and has an adjustable setting that allows the tripping at a wide range of current ratings.

Related Post: Why We Need to Install a Starter with a Motor?

Construction of DOL Starter:

A DOL or Direct Online starter has simply two buttons; Green and Red, where the green button is used for starting and the red button is used for stopping the motor. The green button connects the terminals and closes the circuit while the red button disconnects the terminals and breaks the circuit.

The DOL starter is made of a circuit breaker or MCCB or fuse, an overload relay and contactor or coil. The circuit breaker is used for protection against short circuits while the overload relay protects the motor from overloading. The contactor is used for starting and stopping the motor where the green and red buttons are connected. The wiring for the start and stop button is briefly explained in this article below.

Parts of DOL Starter:

A DOL starter is made of following parts:

The circuit breaker or fuse is directly connected to the power mains and it is used for protection against short circuits. It trips the power supply in case of short circuit to protect the system from any potential hazards.

Magnetic Contactors:

A magnetic contactor is an electromagnetic switch that operates electromagnetically to switch the power supplied to the motor. It connects and disconnects multiple contacts conveniently by providing remote control over the operation.

The magnetic field generated by the coil is used for switching the terminals. The current passing through the coil magnetizes the iron core that is surrounded by the coil. The magnetic force pulls on the armature to close or open the contacts.

The magnetic contactors have three NO (Normally open) main contacts used for power supply to the motor and auxiliary contacts (NO and NC) with a lesser rating used for the control circuit. The coil is connected to the voltage source through auxiliary contacts. Also, keep in mind that the coil used for a single-phase and three-phase supply varies as the supply voltages are different.

Overload Relay:

OLR or overload relay is the last part used in the DOL starter and it is used for protection against overloading of motor. It breaks the current flow when it exceeds a certain limit but it also tolerates the high starting current. So the OLR is carefully selected in such a way that its tripping current limit does not fall below the starting current range.

The excessive amount of current flow can damage the insulation of electrical wires as well as the motor winding. The motor life expectancy decreases and it can shorten the windings causing a risk of fire.

A simple fuse or circuit breaker cannot protect the system from overloading because they are used for overcurrent (short circuit) protection. The OLR has current sensing properties that can differentiate between the starting and overload current.

Related Post: Main Difference between Contactor and Starter

DOL Starter Wiring Diagram:

  The wiring of 3 phase and single phase slightly differs from each other.