DOL Starter diagram and working principle

 A DOL starter (also known as a direct online starter or a cross-line starter) is a method of starting a three-phase induction motor. In the DOL starter, the induction motor is directly connected to its three-phase power supply, and the DOL starter applies the full-line voltage to the motor terminals. 

Despite this direct connection, it will not cause damage to the motor. DOL motor starters include protection devices and, in some cases, status monitoring. The wiring diagram of the DOL starter is as follows:


 DOL starter wiring diagram  

The wiring diagram of the DOL state machine is shown below. The direct online starter consists of two buttons, a green button for starting and a red button for stopping the motor. The DOL starter includes an MCCB or circuit breaker, contactor and an overload relay for protection. These two buttons, namely green and red or start and stop buttons, control the contacts.


To start the motor, we close the contacts by pressing the green button, and a full-line voltage will appear on the motor. The contactor can be 3-pole or 4-pole. The contactor given below is a 4-pole type. It contains three NO (normally open) contacts to connect the motor to the power cord, and the fourth contact is a "holding contact" (auxiliary contact), which energizes the contactor coil after releasing the start button. If any failure occurs, the auxiliary coil will be de-energized, so the starter will disconnect the motor from the power supply.

 3-phase motor running with overload protection

When the motor draws excessive current to meet the load requirements such that the load requirements exceed the rated limit, this is known as an overload.

Thermal overload protection is a safety protection when the motor draws over current or excessive current and causes the device to overheat. Overload is also a kind of overcurrent. Therefore, an overload relay is used to limit the amount of current consumed. But this does not mean to protect against short circuits. The fuse or MCB used in the system protects against overcurrent. Overload protection opens the circuit at a relatively low current that is slightly higher than the rated value of the motor. If the overload current persists for a long time, it may be damaged, that is, if a high value current flows in a short period of time (such as starting a motor), it will not trip. 

We often provide overload protection through overload relays. Overload relays can be solid state devices with adjustable trip settings, also called electronic relays, or by interacting with related temperature sensors called thermal relays, or if they only operate on overcurrents, they are called magnetic relays.


Working principle of DOL launcher  

The working principle of the DOL starter starts by connecting the motor to the three-phase main power supply. The control circuit is connected to any two phases and powered by them only. 

When we press the start button, the current also flows through the contactor coil (excitation coil) and the control circuit.

 The current energizes the contactor coil and causes the contacts to close, so the motor can use a three-phase power supply.The control circuit of the DOL starter is shown below.

 If we press the stop button, the current through the contacts will be interrupted, so that power can not be supplied to the motor, and a similar situation occurs when the overload relay is activated. 

The machine will stop due to the interruption of the motor supply. Even if we release the start button, the contactor coil (magnetizing coil) will get power, because when we release the start button, it will get power from the main contact, as shown in the picture  in the diagram of the Direct Online Starter.

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