Introduction to DOL Starter (Direct On Line Starter)

Hello, readers welcome to the new post. In this post, we will discuss the Introduction to DOL Starter (Direct On Line Starter). The induction motor takes a larger amount of current at the start. This high current is harmful to the windings of the motor.  There are different methods used to save windings from the effect of starting current. These methods are chosen on the basis of motor ratings and the load attached to the motor. The motor starter circuitry also protects the motor from the effect of overcurrent and overloading conditions.

There is Direct Online or DOL started are used full volts about the line starting methods where the motor is directly attached to complete volts configuration through MCCB, Circuit breaker, and relay protection. In this post, we will have a detailed look at the working, features, and other parameters of DOL. So let’s get started Introduction to DOL Starter (Direct On Line Starter)

Introduction to DOL Starter (Direct On Line Starter)

• The DOl or Direct Online Starter also called across-line Starter is a method used for starting 3 Phase induction motor. In this starter technique, an induction motor is attached to the three-phase supply and the DOL starter provides the full line voltage to the motors connections

• As this is a direct connection there is no damage for the motor. The DOL motor starter comes with protection devices
• As the DOL starter make the connection of the motor with the main supply line motors get the high inrush current than the full load current of the motor is about five to eight time larger.
• The value of this current reduces when the motor gets the rated speed. The direct online start can be used where the high inrush current of the motor does not produce a high voltage loss in the supply circuitry.
• To avoid the high voltage loss there is a need for a star delta starter. Direct online starters are normally used for starting small motors like 3-phase squirrel cage induction motors.
• The equation of armature cure in the motor is

Ia=(V-E)/Ra

• The value of back emf or E in this equation depends on speed and has a direct relation with speed N.
• At the start of the motor value of E or back, emf is 0 which causes a high starting current. In small rating motor rotors length of the rotor is good and the dia is more miniature which helps to rotate it fastly.
• So with the increment of speed amateur current decreases fastly. So small rating motors move smoothly for a three-phase supply connection
• If we attach larger-size motors to the three-phase line directly, it will not operate smoothly like small-size motors and get damaged since it does not rotate fastly as small-size motors do. A larger motor has a small axial length and large dia than a larger motor.
• So larger size motor uses oil immersed DOL starter

DOL Starter Construction

• The DOL or direct online starter uses two buttons Green and Red, where the green button is used for starting and the red for stopping the motor.
• The green button connects the terminal and closes the circuitry when red button disconnects the terminals and breaks the circuitry
• The DOL starter is created through a circuit breaker fuse, or MCCB, overload relay, and contractors. The circuit breaker provides protection for short circuitry overload relay to protect the motor from overloading.
• The contactor is used for starting and motor stopping from the point where the green and red buttons are attached.
• Parts of the DOL starter are discussed here

Fuse or Circuitry Breaker

• The circuit breaker or fuse is connected to the main power supply and it is used to protect short circuitry. It trips the power supply for short-circuit protection

Magnetic Contactors

• The magnetic contactor is an electromagnetic switch that functions electromagnetically to switch on the power supply of the motor.
• It connects and disconnects many contacts easily by controlling the operation remotely.
• The magnetic field produced by the coil is used for switching terminals. The current flowing through the coil magnetizes the iron core which is surrounded by a coil
• The magnetic force pulls the armature to close or open the contacts
• The magnetic contactors have three NO or normally open contacts that are used for power supply to motor and auxiliary contacts. NO or NC with less rating used for control circuitry.

Overload Relay

• The overload relay is connected at end of the DOL starter and used to protect overlading of the motor. It stops the flow of current when it is larger than the limited value with a tolerance of a high starting current.
• The overload relay used a must-have feature that tripping the current limit does not less than the starting current range
• The excess current flow damage the insulation of wires and motor winding. The motor operating life decreases and it can decrease the winding risk to cause fire
• The normal fuse or circuit breaker cannot protect the system from overloading since they are used for overcurrent protection

DOL Starter Wiring Diagram

• In the below figure, we can see the DOL starter circuit. The direct online starter has two buttons, Green colored caused for starting and RED color used for motor sop as we discussed above.
• It also has a circuit breaker, contactor, and overload relay.  Red and green buttons start and stop control of contacts
• For starting motors close the contact by pressing the Green button and voltage will be given to the motor. A COntactor is of three poles or four poles in this circuit we have four pole
• It consists of three NO contacts which connect the motor with the supply lines, and 4th contact is held on the contact that energizes the contactor coil after the start button is not pressed
• If there is any fault exists auxiliary coil gets de-energized and the starter disconnects the motor from the main supply

3-Phase Motor Starter Overload Protection

• When the motor uses excessive current according to the load needs the load requirements to pass the rated limit are called overload
• Thermal overload protection is used when the motor gets a high value of current and results in overheating of the machine.
• Overload is also called over-current. So overload relay used to restrict the current to a limited value and not limit the short-circuit current
• A fuse or MCB is used to protect over current. Overload protection opens the circuit at less current values that are high than the rating of the motor
• Overload current damage the machine if it remains for a long time. Overlaodprotecion is given through use of an overload relay.
• Overload really can be solid state component having an adjustable trip setting called an electronic relay or through interacting with a temperature sensor known as thermal relay or work for extra current flow then known as a magnetic relay
• In some mtors highest rating of overload protection instrument is 125% of full load ampere rating

DOL Starter Working Principle

 

• The working of the DOL starter is initiated through the connection of three phase supply with the motor. The control circuitry is attached to any 2 phases and energized them
• When the start button is pressed current passes through the contactor coil and the control circuit also
• The current energizes the contactor coil and closes the contacts and three phase supply goes to the motor
• The control circuitry of DOL we can see above figure
• If we push the stop button curent through contacts will not flow so the motor not operate and this also happens for overload relay operation. as the motor has no supply so it will stop rotating
• The contactor coil gets supplied even start button is open since the start button was released it will get supplied through the primary contact

DOL Advantages

• This circuit can make delta winding of the motor
• Its desing is simple so the troubleshooting process is easy
• Its control circuit is easy to operate
• It has one hundred percent stating torque
• It is a less expensive type of starter
• It has simple desing

DOL Disadvantages

• It has no control over starting current and torque
• Its high starting torque produces mechanical stress that reduces the operating life of the motor
• Its starting torque is sometimes not needed
• High inrush current curent results in voltage dip in the power supply line that can damage the devices connected in a parallel configuration
• Strating curernt can damgethe motor

DOL Features

• It has no control for starting and torque
• It is used for low-power-rated motor
• It has a simple control structure
• It causes a voltage dip in the power supply lines
• It has a high value of starting torque
• Its starting curent is high which is needed for motor starting

DOL Applications

• It is used for small water pumps, conveyor belts, and compressors
• It is used for projects where starting curent not results in high dips in line voltage
• It is used for applications where staring curent not have an effect in the windings of motors
• these starts are the best option for low-rating motors

That is all about the DOL Starter (Direct On Line Starter) all details has explained. If you have any questions ask them here