The DOL starter, also known as the Direct On-Line starter, is a widely used method for starting a 3-phase induction motor. It provides a simple and cost-effective solution for initiating motor operations. In this comprehensive guide, we will explore the working principle, control diagram, advantages, and disadvantages of the DOL starter.
Table of Contents
What is DOL Starter?
Direct On-Line is the simplest and cheapest method used for starting a three-phase motor. Dol is used for up to 5 HP motors. Dol Starter is more popular due to the low price.
DOL starter connects the motor directly to the supply without a reduction in supply voltage and applies full line voltage to 3 phase motor. this is the main disadvantage of dol starter.
A small HP motor can be started simply by plugging into the power supply. But a large HP motor required a special unit called a motor starter.
Direct online starters for 3-phase induction motors contain parts like a miniature circuit breaker (MCB) for motor protection purposes, overload relay, power contactor, fuse unit, start button, and stop button.
Small size dol starter is operated manually but for larger HP motors, dol motor starter are operated using an electromechanical contactor (relay) to switch the motor ON and OFF.
DOL Motor Starter can be used if a high starting current cannot cause an excessive voltage drop in the motor circuit.
D O L starter is used for starting the small motor, compressor, small water pumps, and industrial conveyor belts. Fans, air curtains, exhaust fan, blower, etc.
A reversing starter can connect the motor for rotation in either direction. Such a starter contains two DOL circuits—one for clockwise operation and the other for counter-clockwise operation, with mechanical and electrical interlocks to prevent simultaneous closure.
For three phase motors, this is achieved by changing the wires connecting any two phases. Single phase AC motors and direct-current motors require additional devices for reversing rotation
In the case of three phases, the asynchronous motor draws a very high starting current up till achieving full speed. This starting current is up to 6 to 7 times of full load current.
To reduce this starting current larger motor has starting methods like star delta starter or variable frequency drive.to reduce voltage dips on the power supply.
The demerit of the dol starter is that gives the highest possible current to the motor. Which is 6 to 7 times the rated current of the motor.
Components of DOL Starter
To understand the DOL starter’s functioning, let’s explore its key components, including protection devices, control devices, and monitoring and auxiliary devices.
Protection Devices
The DOL starter incorporates two essential protection devices: the miniature circuit breaker (MCB) and the overload relay (OLR).
Miniature Circuit Breaker (MCB)
The MCB is an electromechanical device that automatically switches off the circuit in case of irregularities or malfunctions. It senses overcurrent caused by short circuits or other faults and protects the electrical circuit from damage.
Overload Relay (OLR)
Most of motor winding failure occurs due to overload. Motor overload tends to the heating of motor winding that results in the weakening of winding insulation. A minor overload does not cause a motor failure immediately but it will eventually shorten the expected lifetime.
To prevent this situation thermal overload relay is provided into a direct online starter circuit.
Thermal overload relay has a bimetallic strip that bends because of overheating caused by overcurrent.
This disconnects the motor from the supply and prevents further motor winding damage and insulation failure.
Control Devices
Contactor
A contactor is a heavy-duty relay with a high current rating, used for powering up an electrical motor. The current rating for contactors varies from 10 amps to several hundred amps.
The primary control device in a DOL starter is the contactor, an electrical device used to make or break a contact with a load. The contactor acts as a switch for controlling the electrical circuit, allowing the motor to be turned on or off.
Overload protection is provided along with the contractors to start the motor. The contactor is not used to interrupt a short circuit current, Unlike the circuit breaker used. Contactor size varies from small to large for high-current appliances.
The contactor has 3 NO contact which connects a motor to the power supply and one additional contact refer as Auxiliary contact, which acts as hold on contact when start the push-button release. To maintain energization of the contactor coil.
If sometimes power fails abruptly due to this contactor coil getting de-energized, it opens the contactor terminal, and the motor gets disconnected from the supply.
Monitoring and Auxiliary Devices
Push buttons
There are two push buttons used in the dol motor starter, one is a green color for the start button and the other is red for the stop push button.
Switching of power supply carried out by using electromechanical contactor which has 3 to 4 pole
Start push button
this is normally an open-type push button, to start the motor, this push button needs to be pressed, and the supply gets applied to the motor through a power contactor.
Stop push button
This is NC type of push button we need to press to stop the motor from running condition. after pressing the stop button contactor coil gets de-energized with the opening of contacts. Due to this switching operation motor gets disconnected from the supply and the motor stops rotating.
Indication lamps
Indication lamps are used in the DOL starter circuit to indicate the status of the motor.
Generally, three types of indication lamps are used in direct online starter green for motor off indication, red indication lamp to indicate the motor is running and third assigned to trip.
DOL Starter Control Diagram
The control circuit diagram consists of components like the Main power contactor, start button, stop push button, and overload relay connected in series this circuit is called the control circuit of the dol motor starter. All controlling of starters are controlled by this circuit.
These connections enable the control circuit to energize the contactor coil, allowing the motor to start or stop as desired.
The normally closed auxiliary contact of the overload relay is connected in series with the contactor coil. This auxiliary contact protects the motor from overcurrent. If an overcurrent is detected, the overload relay trips, opening its auxiliary contact and de-energizing the contactor coil.
Working Principle of DOL Starter
1) To start the motor, the start push button need to pressed. After that, the main power contactor coil gets energized due to electromechanical action and this latch contactor pole.it applies full line voltage to the motor terminals. and the motor starts running. The motor will draw a very high inrush current for a short time.
2) As the motor accelerates and heads toward full speed, the current begins to drop but it will not drop significantly till the motor achieves full speed, about 85% of synchronous speed.
3) The motor load will affect the time taken for the motor to accelerate to full speed and therefore the duration of the high starting current, but it will not affect the magnitude of the starting current.
4) If the motor torque developed by the DOL Motor starter is less for load, then the motor must be replaced by the motor which can develop high starting torque.
5)To protect the motor from overcurrent, the DOL starter utilizes the normally closed auxiliary contact of the overload relay. If an overcurrent occurs, the overload relay trips, opening its auxiliary contact and de-energizing the contactor coil. This action interrupts the power supply to the motor, preventing potential damage.
6)Once the contactor is energized, it remains closed even after releasing the start push button. This is due to holding contact provided (contact 13-14 of power contactor -NO as shown in contro diagram), ensuring continuous power supply to the motor.
Advantages of DOL Starter
The DOL starter offers several advantages, making it a preferred choice in various applications:
- Quick start: starter provides instant motor start-up
- Cost-effective: Compared to other starters like star-delta and soft starters, the Direct online motor starter is the Cheapest.
- Easy implementation: DOL is the simplest motor starter to design and maintain.
- Simple troubleshooting: due to the simple wiring connection troubleshooting is simple
- High starting torque: 100% torque during motor start-up.
- D O L starter used up to 5HP motor depending upon insulation class of motor.
Disadvantages
Despite its advantages, the DOL starter has some limitations that should be considered:
- High starting current: typically 7 to 8 time rated current applied at starting
- Potential motor damage: The high starting torque may subject the motor winding to excessive stress.
- Size limitations: The DOL starter is used for motors with ratings up to 5 HP,not suitable for high rating motors.
Applications of DOL Starter
- The DOL starter finds application in various scenarios where the high inrush current of the motor does not pose any detrimental effects. Some common applications include:
- D O L starters are mainly used to start smaller rating motors up to 5HP
- It finds applications, where the starting current does not cause a huge voltage dip in the system
- used for applications that required high initial torque
- They are commonly used in pumps, fans, small motors, compressors, small water pumps, and industrial conveyor belts. Fans, air curtains, exhaust fan, blower etc.
- These applications benefit from simplicity and cost-effectiveness, as they do not require complex control systems or precise speed regulation.
Comparison Between DOL Starter and Star Delta Starter
| Criteria | DOL Starter | Star-Delta Starter |
| Motor Rating | This starter is used up to 5HP motor | It can be used 5 HP to 20 HP motor |
| Starting Current Reduction | Starting currently is so high | The starting current is reduced by 1/3 |
| Cost | Economical | Costly |
| Connection Method | Direct connection to motor for starting | Star connection for starting, delta for running |
| Advantages | Quick start, cost-effective, simple design | Reduced starting current |
| Disadvantages | High starting current, potential damage | Higher cost, complex wiring |
