Electric motor controls. Article:

How does a Direct Online starter works.


What is a Direct Online starter.

A Direct online starter is a method of starting electric motors usually of small size of less than 4kw.

It consist of two circuits, the power and control circuit. The power circuit provide power to the motor and the control circuit controls the operations of the motor.

Power Circuit.

Direct online starter power circuit

Control Circuit

Direct online starter control circuit

How does a Direct Online Starter works.

A Direct Online consist of:

1.Start button - a normal open contact button which is pressed for the electric motor to run. 2.Stop button - a normal close contact button which 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 off, breaking the circuit in event of a fault. 4.Overload relay - they break off the circuit in event of overload which may cause overheating of the electric motor windings.

Overload is condition in which current flowing in a circuit exceeds the amount required. Some overloads may not cause tripping of circuit breakers, so they is a need of an overload relay.

5.Contactor - is a device which when current flow through its coil, closes the contacts of power circuit for electric motor to run and vice versa. 6.Electric motor - is a device which convert electrical energy into mechanical energy.

With circuit breakers in ON position, to start the electric motor, the start button is pressed which anables current to flow through the contactor coil.

The flow of current through the contactor coil cause the contactor power contacts to close resulting in power reaching the electric motor and the electric motor begin to run.

The 13/14 contactor contact which is in parallel with the start button is for hold in. The 13/14 contact is a normal 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 this 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 stop running.

Why starting electric motors using a Direct Online Starter.

The starter anables operators to efficiently switch on and off the electric motor using start and stop buttons. It enables automatic switching off of the electric motor in event of fault thus giving protection to electric motor and operators. It prevent automatic starting of electric motor in event of stopage. The power and control circuit operate usually at different levels of voltage. The control circuit uses low voltage as it is usually connected to single phase while the Power circuit uses high voltage as it is connected to three phase, this provide more safety to operators as they will be directly on control circuit.

Why three phase motors are connected in Star or Delta.

In three phase electric motors, star or delta connection are required to complete the circuit of the electric motor since current only flow when the circuit is complete.

Three phase electric motors has three seperate coils or windings which need to be connected together inorder to complete the circuit for current to flow through the electric motor, and this is accomplished by either star or delta connection.

In single phase electric motors they is no need of star or delta connection since the electric motor is designed to run on one live conductor, the neutral wire will complete the circuit making a return paths for current back to the source.

Advantages and disadvantages of Direct Online Starter.


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 provide a safe invironment to people working with the electric motor.


Starting current is very high. It is limited in use, only applicable to small electric motors.

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