What is DOL Starter?
In a DOL starter, the three phase induction motor is connected directly to the 3-phase power supply. This means that the full line voltage is applied to the motor terminals. However, the motor is protected from damage by several protection devices that are included in the DOL starter.
One such protection device is a thermal overload relay. This device monitors the current flowing through the motor and trips if the current exceeds a certain level. This can be caused by the motor being overloaded or running at an excessively high temperature.
Another protection device that is often included in a DOL starter is a short circuit protection device. This device is designed to protect the motor and the starter itself from damage due to a short circuit in the power supply or in the motor windings.
In addition to protection devices, some DOL starters also include condition monitoring devices. These devices allow the user to monitor the condition of the motor and the starter, and to detect any potential problems before they result in a failure.
The high inrush current of a motor at start up is due to the fact that the motor is starting from a standstill, and it takes a large amount of current to get the rotor moving and up to its rated speed. As the motor reaches its rated speed, the back emf (electromotive force) generated by the rotor increases, which reduces the current flowing through the armature.
The reason that small motors can be started using a DOL starter, while large motors cannot, is due to the relative size and mass of the rotor. Small motors have a more considerable axial length and a small diameter, which means that they can be accelerated more quickly to their rated speed. As a result, the inrush current of small motors decreases more rapidly as the motor reaches its rated speed, and they are able to run smoothly when connected directly to the power supply.
In contrast, large motors have a shorter axial length and a larger diameter, which means that they are more massive and take longer to reach their rated speed. As a result, the inrush current of large motors decreases more slowly, and if a DOL starter is used, the motor may be damaged due to the high inrush current.
To start large motors, a starter that can control the inrush current more effectively is needed. One option is a star delta starter, which uses a set of contactors to connect the motor to the power supply in a way that reduces the inrush current. Another option is an oil-immersed DOL starter, which uses oil to cool the motor and reduce the risk of damage from the high inrush current.
Working of DOL Starter:
- The motor is connected across the full supply voltage using a starter, which consists of a coil-operated contactor (C) controlled by start and stop push buttons (S1 and S2). The start button (S1) is held open by a spring.
- To start the motor, the START pushbutton (S1) is pressed. This energizes the contactor coil (C) from the two line conductors (L1 and L2).
- As a result, the three main contacts (M) and the auxiliary contact (A) are closed. The terminals (a and b) are short-circuited, and the motor is connected to the supply mains.
- The S1 button moves back under the spring action as soon as the pressure is released, but the contactor coil (C) remains energized through (a and b). This means that the main contacts (M) remain closed, and the motor continues to get supplies. The auxiliary contact (A) is known as the Hold-On-Contact because it keeps the contactor coil (C) energized and the main contacts (M) closed.
- To stop the motor, the STOP pushbutton (S2) is pressed. This disconnects the supply through the contactor coil (C). As the coil is de-energized, the main contacts (M) and the auxiliary contact (A) are opened, and the supply to the motor is disconnected, stopping the motor.
Parts of DOL Starter:
It consists of several components that work together to control the flow of electricity to the motor.
- Contactor: A contactor is an electromechanical switch that is used to control the flow of electricity to the motor. It has a coil that is energized and de-energized to open and close the contacts, respectively. The contactor has three normally open (NO) contacts that connect the motor to the power supply, and one auxiliary contact that acts as a hold-on contact when the start push button is released. This helps to maintain the energization of the contactor coil.
- Overload protection: Overload protection is provided to prevent the motor from overheating and damaging the winding insulation. It consists of a thermal overload relay that has a bimetallic strip that bends due to overheating caused by overcurrent. When this happens, the relay disconnects the motor from the power supply and prevents further damage to the motor.
- Miniature circuit breaker (MCB): An MCB is used to protect the motor from short circuit conditions and to avoid damage to the motor winding. It is a type of electrical switch that automatically opens the circuit when the current exceeds a certain level.
- Push buttons: There are two push buttons in a DOL starter circuit - a green start button and a red stop button. The start button is normally open, and pressing it applies the power supply to the motor through the contactor. The stop button is normally closed and pressing it de-energizes the contactor coil and opens the contacts, disconnecting the motor from the power supply and stopping it from rotating.
- Start push button: The start push button is a type of electrical switch that is used to start a motor. It is normally open, which means that the circuit is open and no electricity is flowing when the button is not pressed. When the start button is pressed, the circuit is closed and electricity flows to the motor through the contactor. The contactor then energizes its coil, closing the contacts and connecting the motor to the power supply.
- Stop push button: The stop push button is a type of electrical switch that is used to stop a motor that is already running. It is normally closed, which means that the circuit is closed and electricity is flowing when the button is not pressed. When the stop button is pressed, the circuit is opened and the electricity flow is interrupted. This causes the contactor coil to de-energize and the contacts to open, disconnecting the motor from the power supply and stopping it from rotating.
Advantages:
- DOL starters are the cheapest motor starters, as they have the simplest design and require minimal maintenance.
- DOL starters provide 100% starting torque, which means the motor can reach its full speed from a standstill without any slipping.
- DOL starters can be used for motors with a power rating up to 5 horsepower (HP), depending on the insulation class of the motor.
Disadvantages:
- DOL starters have a high starting current, typically 7 to 8 times the rated current of the motor. This high current can stress the motor windings and cause damage over time.
- DOL starters are not suitable for high-rating motors due to the high starting current and torque.
- The high starting current of a DOL starter can cause a voltage dip on the power system, which can affect other equipment connected to the same power source.
- The high starting torque of a DOL starter can cause mechanical stress on the motor and the load it is driving, potentially leading to damage or malfunction.