The process of starting an induction motor can be accomplished in various ways. The current drawn by these motors is high when they are first started. In the absence of a current check, this can cause some damage. The starting process is smooth and efficient with the use of specialized starters.
Direct Online (DOL) starters, also known as across-the-line starters is one important type of starter. This guide is all about DOL starters in detail.
Construction of DOL starter
Start the motor with the green switch and stop it with the red switch on a DOL starter. In addition to the contactor, overload relay, and MCCB or circuit breaker, this starter is equipped with a contractor. There are two switches on the motor that control the contacts. Pressing the green switch shuts the contact, and the induction motor receives full line voltage.
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Three-pole and four-pole contactors are the most common types. Four-pole contractors, for instance, have three open contacts and auxiliary or hold-on contact. Auxiliary contacts can enhance the contactor coil after the start button has been switched by connecting the induction motor to the supply line through the NO contacts. Holds on the contactor are deactivated in the event of an error. As a result, a DOL starter allows the induction motor to be disconnected from the main power supply.
Working of DOL starter
Three-phase power supplies are used to power DOL starters, which are connected to the three-phase power supplies at their main terminals.
A remote conAs a result of this current, the coils of the contactor (A1-A2) become energized, which closes the contactor’s contacts, and the motor is connected via a three-phase supply.
When the DOL starter’s start button is released, the control circuit still provides power via Hold-On-Contact even though the pushbutton is released. After pressing the stop button, the current path through the contactor coils (A1-A2) breaks, causing the contacts to drop out, preventing the overload relay coil from functioning and shutting down the motor.
As load current directly affects the heating effect of the motor, thermal overload protection relays are activated when the motor heats up.
In addition to a high load current, a high heating effect also occurs. The bimetallic strip inside the spring-loaded control contact expands and trips out during high temperatures. The thermal coil is heated when the load current is too great. The relay operates at a different speed depending on how much current is applied. It is recommended that the motor’s current rating be 3-5 times its rated current.
If you wish to use remote control-based switching, consider the following scenario. The remote “OFF” pushbuttons must be paired with the remote “OFF” pushbuttons of the starter (you must also link the remote “ON” pushbuttons with the “ON” pushbuttons of the starter).
Pros of DOL starter
- DOL starters are the most economical and cheapest way to start your engine.
- Easily understandable and troubleshoot able.
- Direct online starters provide 100% torque when they are started.
- Only one set of cables is needed between the motor and the starter.
- Currently, it connects the power supply to the motor’s delta winding.
- The compact size of DOL allows it to occupy very little space
- As a result, it provides 100% starting torque.
- The supply is connected to the motor’s delta winding through this starter.
Cons of DOL starter
- Excessive starting torque, even when the load does not require, puts much mechanical stress on the machine’s mechanical systems.
- In addition, DOL starters are only suitable for motors with a capacity of less than 10kW.
- DOL starters have the disadvantage of providing the highest starting current possible.
- The Direct On-Line starters have very high starting currents – six to eight times what they require during full load.
- A motor’s lifespan decreases due to the enormous thermal stress it undergoes.
- Due to too much inrush current, the voltage drops significantly in the electrical installation, affecting customers on the same lines, making it only suitable for small motors.
DOL starters are used for what purposes?
DOL (Direct Online) starters are widely used in the electrical industry to start and control the speed of induction motors. DOL starters are suitable for small to medium-sized motors. They are also considered the most basic and economical type of motor starter. DOL starters provide an instant start to the motor and keep it running until it is manually turned off. These starters are mainly used in applications where the motor requires a high starting torque and the starting current is relatively low. DOL starters are particularly useful in industrial applications, such as pumps, compressors, conveyor belts, and fans.
The purpose of a DOL starter is to protect the motor from damage by providing over-current protection. They also protect the electrical circuit from overloading and short circuits. The starter includes a contactor responsible for switching the motor on and off. The control circuit activates the contactor, which includes a start button, stop button, and overload relay. The overload relay is designed to sense when the motor is operating at an abnormal current level and disconnect the circuit if necessary to prevent any damage. DOL starters are crucial components of induction motor control systems, providing a simple and effective means of starting and stopping the motor. They protect the motor and the electrical circuit, making them an essential part of many industrial applications.
Starters with high inrush currents are commonly used on motors that don’t cause excessive voltage drops in the supply circuit (or are tolerant of such voltage drops).
Air compressors are commonly started using direct online starters, along with water pumps, conveyor belts, fans, and compressors. When an asynchronous motor starts up (such as a squirrel cage motor), it draws a considerable a current until the full speed has been reached.
Applications of DOL Starter
DOL starters are primarily used on motors that don’t suffer from excessive voltage drops in the supply circuit due to their high inrush current (or where that voltage drop is acceptable). It is common for direct online starters to start small water pumps, conveyor belts, fans, and compressors. Asynchronous motors (such as squirrel-cage motors) will draw a high starting current until they have reached their maximum speed.
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