DOL starter and direct online starters are commonly used to start small induction motors without causing an excessive voltage drop in the supply unit. In this article, we will discuss the working principle, advantages, disadvantages, and applications of these starters.
DOL starter is the simplest and cheapest starting method used to start an induction motor.
When the three-phase induction motor started it draw a very high starting current so to reduce this starting current we needed a reduced voltage starter that can start an electrical motor safely without damaging the winding.
DOL Starters circuit wiring is easy and cheap as compared to other motor starters. DOL short form of direct online starter and can be used up to 5 HP or 3.7kw motor.
It is used for 3-phase and single-phase induction motors. The three-phase dol starter power diagram and control circuit is shown in the figure.
A small motor can be started by directly plugging into an electric switch. A larger HP motor required a specialized switching unit called a motor starter.
When we energized the DOL motor starter, it immediately connects motor terminals directly to the supply.
DOL Starter applies the full line voltage induction motor and it is the simplest type of motor starter.
DOL motor starter circuit also contains protection devices like MCB, overload relay, fuse unit.
Small DOL starters are operated manually but large size motors use electromechanical contactors to switch the motor circuit.
DOL starters are used where the high inrush starting current of the motor does not cause an excessive voltage drop in the supply circuit.
DOL starters starting current is 6-7 Times greater than the full load current. to reduce this heavy current starting current larger motors have reduced voltage starters like soft starters.
DOL Starter is sometimes used to start small water pumps, compressors, fans, and conveyor belts.
In case asynchronous motor like 3 phase squirrel cage motor the motor will draw a high starting current until it has run up to full speed.
Reverse forward dol starter can run the induction motor in either direction such starter contains two DOL circuits one for forwarding rotation and the other for anticlockwise rotation.
For 3 phase motor, It can be achieved by interchanging any two phases.
The disadvantage of a direct online starter is it gives a high starting current because it connects the motor directly to the supply.
DOL Starter Control Circuit Diagram
DOL starter control circuit consists of a start push button generally green, a stop push button red, a power contactor, a bimetallic overload relay, and indication lamps.
The control circuit wiring diagram is shown in the below diagram
When the start button is pressed it energized the contactor coil and contactor contacts get latched hence supply gets connected to the motor through a power contactor.
If we pressed the stop button, it disconnects the supply to the contactor coil and hence it de-energizes the contactor coil and opens latched contact and the supply gets disconnected from an induction motor.
The coil gets supply even though we release the start button because the supply will flow continuously through Contractors NO as it provides holding to starter.
DOL Starter Power Circuit Wiring Diagram
The power circuit consists of MCB, a Main contactor, and a thermal overload relay connected in series with an induction motor. power circuit wiring of the starter is simple as shown in the above circuit diagram where L1, L2 and L3 are three phases.
DOL Starter Working Principle
Dol starter working principle is simple
When we pressed the start button on the DOL starter, the contactor coil received supply through the control circuit,
Hence the contactor coil gets energized and it latches contacts of the contactor due to electromagnetic action.
3 phase supply gets applied to the induction motor and the induction motor starts running.
After releasing the start push button, the supply to the contactor coil continued through the holding provided to the main contactor of the DOL starter, for this contactor NO contact is used as holding contact.
For stopping the motor, we have to press the stop push button on the DOL motor starter, this action disconnects the supply of the contactor coil, and the contactor coil gets de-energized. hence it releases the contactor latched pole, and the motor gets disconnected from the supply circuit.
Parts of Direct Online starter
Overload relay (OLR)
The bimetallic overload relay in DOL starter is serve a critical task, it protects the induction motor against overload.
Electric motors need overcurrent protection to prevent damage from over-loading the motor, or to protect against short circuits in connecting cables or internal faults in the motor windings. for this purpose overload relay is used in the DOL starter circuit.
The overload sensing devices are a form of the heat-operated relay where a coil heats a bimetallic strip, releasing a spring to operate auxiliary contacts.
These auxiliary contacts are in series with the coil. If the overload senses excess current in the load, the coil is de-energized.
This thermal protection operates relatively slowly allowing the motor to draw higher starting currents before the protection relay will trip. Where the overload relay is exposed to the same environment as the motor.
The other common overload protection system uses an electromagnet coil in series with the motor circuit that directly operates contacts. This is similar to a control relay but requires a rather high fault current to operate the contacts.
Contactor
A contactor is a heavy-duty relay with higher current ratings, used for switching electric motors. Continuous current ratings for common contactors range from 10 amps to several hundred amps.
High-current contacts are made with alloys containing silver. The unavoidable arcing causes the contacts to oxidize; however, silver oxide is still a good conductor. Contactors with overload protection devices are often used to start motors using DOL starter.
Contactors come in many forms with varying capacities and features. Unlike a circuit breaker, a contactor is not intended to interrupt a short circuit current.
Contactors range from those having a breaking current of several amperes to thousands of amperes and 24 V DC to many kilovolts.
The physical size of contactors ranges from a device small enough to pick up with one hand, to large devices approximately a meter (yard) on aside.
Contactors can be noisy when they operate (switch on or off), so they may be unfit for use where noise is a chief concern. In such cases, solid-state relays are preferred.
Stop button NC (normally closed type)
Stop push button used in DOL starter circuit is normally closed type and used to stop the motor
Start button NO (normally open type)
Start button is normally open type and used to start motor
Advantages of Direct Online Starter
- Most Economical and Cheapest Starter
- Simply connect, operate and maintain
- Simple Control Circuit
- It provides 100% torque at the time of starting.
Disadvantages of DOL Starter
- It does not reduce the starting current of the motor.
- High Starting Current
- Thermal Stress on the motor, thereby reducing motor life.
- High starting Torque
- DOL starter used up to 5HP motor
Applications
- The DOL starters are used to start small motors having low power ratings.
- Where the inrush starting current does not cause damage to motor winding and supply dip.
- DOL starters are used to start small water pumps, conveyor belts, fans, and compressors
Conclusion:
DOL starter and direct online starter are simple and cheap starting methods used to start small induction motors. The working principle, advantages, disadvantages, and applications of these starters are discussed in this article. Understanding these starters’ characteristics can help you choose the right one for your specific application.
Frequently Ask Question
A DOL (Direct On-Line) starter is an electromechanical switch used to start and stop three-phase induction motors. It works by connecting the motor directly to the power supply, causing the motor to draw a high starting current. This high starting current can cause voltage drops in the system, so DOL starters are typically used for smaller motors.
The advantages of using a DOL starter include its simplicity, reliability, and cost-effectiveness. DOL starters require minimal maintenance, and their simple design makes them easy to install and operate.
The main disadvantage of using a DOL starter is its high starting current, which can cause voltage drops in the system. This can lead to decreased efficiency and potential damage to the motor. Additionally, DOL starters cannot be used with larger motors or in systems with limited power capacity.
