How does a Direct Online starter work?
A Direct online starter is a method for 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 operations of the electric motor.
Power and control circuits.
- L - Live
- NO - Normally open
- NC - Normally close
Key
Components of a direct online starter
A Direct Online consists of:
1. Start button - a normally open contact button that is pressed to run the electric motor .
2. Stop button - a normally close contact button that is pressed to stop the electric motor.
3. Circuit breakers - are either manually operated to switch off the circuit for maintenance or they automatically trip, breaking the circuit in the event of a fault.
4. Overload relay - overload relay breaks or switches off the circuit in event of overload that causes overheating of the electric motor windings.
Overload is a condition in which current flowing in a circuit exceeds the amount required, leading to overheating of the electric motor windings which result in burning of the electric motor. Some overloads may not cause tripping of circuit breakers, so there is a need of an overload relay.
5. Contactor - is a device that when current flow through its coil, closes the contacts of the power circuit for electric motor to run and vice versa.
6. Electric motor - is a device that converts electrical energy into mechanical energy.
How does a Direct Online Starter work?
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 close position when the contactor coil is energized.
Since the start button is a normal open contact, soon after pressing it, it retains to its normal 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 posses a siginificant risk to larger electric motors. Since large electric motors are typically designed to run in delta confuguration, 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 confuguration 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 less expensive.
The circuit is easy to trace a fault.
It offers protection to electric motors by automatic switching off the electric motor in event of fault.
It provides a safe invironment to people working with the electric motor.
Disadvantages.
Starting current is very high.
It is limited in use, only applicable to small electric motors.