How a Direct Online Starter Works
A Direct online starter is a method of starting small electric motors, typically those rated 4kW and below.
It consists of two circuits: the power circuit and the control circuit. The power circuit provides power to the electric motor, and the control circuit, controls the operation of the electric motor.
Power and Control Circuits
- L - Live
- NO - Normally open
- NC - Normally closed
Key
Components of a Direct Online Starter
As shown in the previous diagram, the direct on line starter consists of a triple-pole circuit breaker, a contactor, an overload relay, an electric motor, a single-pole circuit breaker, a start button, and a stop button.
A start button is used to start the electric motor and a stop button is used to stop the electric motor. A contactor is a device that closes the electric motor power circuit when the start button is pressed, or opens the electric motor power circuit when stop button is pressed.
Overload relay is a device that cuts the power supply to the electric motor when the current flowing through the circuit exceeds the rated value.
Triple-pole and single-pole circuit breakers are devices that trip where there is a short circuit or overload in their circuits.
Since the triple-pole circuit breaker in this circuit works with an overload relay, it does not have an overlaod protection component, while the single-pole circuit breaker hear is designed to work alone, it includes both overload and short-circuit protection.
How a Direct Online Starter Works
On a direct online starter, assuming the circuit breakers are in the ON position, when the start button is pressed, current is allowed to flow through the contactor coil.
The flow of current through the contactor coil causes the contactor power contacts to close, resulting in power reaching the electric motor and the electric motor begins to run.
The 13/14 contactor contact that is in parallel with the start button serves as a hold-in contact. The 13/14 contact is a normally open contact; it is put in the closed position when the contactor coil is energized.
Since the start button is a normally open contact, soon after pressing it, it returns to its normally open position. Current will then continue to flow to the energized contactor coil through the 13/14 contact.
To stop the electric motor, the stop button is pressed; this cuts the supply of current to the contactor coil, causing the contactor power contacts to open and the electric motor to stop running.
Why is a Direct Online Starter only used for Small Electric Motors?
The starter starts an electric motor by directly applying full line voltage to the electric motor terminals, and this poses a significant risk to larger electric motors. Since large electric motors are typically designed to run in delta configuration, directly connecting them to full line voltage in delta would cause excessive current flow during startup, potentially leading to destruction of the electric motor. Therefore, other starting methods like the star-delta starter are necessary, where the electric motor is started in star configuration and then switched to delta to run.Advantages and Disadvantages of a Direct Online Starter
Advantages
The circuit is simple, easy to understand ,and to install.
It needs few components, therefore it is less expensive.
The circuit is easy to troubleshoot.
It offers protection to electric motors by automatically switching off the electric motor in the event of a fault.
It provides a safe environment to the people working with the electric motor.
Disadvantages
Starting current is very high.
It is limited in use, only applicable to small electric motors.