Direct OnLine Starter or DOL Motor Starter

Direct On-Line motor starter is the most common and economic motor starting technique. Widely used technique in industry. Let's see the principle, advantages & disadvantages...

Among all the motor starters DOL STARTER is very popular and most commonly used technique.

The power circuit is drawn below.

Fig 1: POWER CIRCUIT DIAGRAM OF DOL STARTER/
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The power circuit diagram of a Direct On Line or DOL starter is very simple,

B stands for circuit breaker, which are MCCB, SFU, or MPCB.

MCCB means Moulded Case Circuit Breaker,

SFU stands for Switch Fuse Unit and MPCB for Motor Protection Circuit Breaker.

MPCB is used motor rated upto 30 kW.

CT with Ameter Selector Switch is given for current measurement.

The next component is the Contactor. This contactor is controlled by energizing or de energizing its coil named as R.

Now question may come why we need this contactor when we have the circuit breaker. Basically in this configuration the circuit breakers are MCCB, SFU or MPCB. They can trip the circuit automatically during any short circuit. Also MCCB, MPCB can be tripped from remote end (away from switchboard) by using a shunt trip coil to the breakers but they can not be switched on by Electrical signal. These can be switched on by moving the grip only. So for remote operation, interlock with the process system automatic switching on off is required, which is achieved by contactor.



Bimetallic thermal over load relay is given for overload protection.

Rating of these switchgear component viz. MCCB/SFU/MPCB, contactor and thermal over load relays for a particular motor is selected as per Type 2 co ordination chart, which can be easily found in the internet.

In general motor upto 90 kW are fed by this arrangement however motor rated 110 kW and above has air circuit breaker with motor protection relay. Motor protection relay covers all the protection required, and air circuit breaker can be controlled by Electrical signal, so there contactor or bimetallic relays are not used.

We have seen the power circuit diagram, now we shall focus on the control. The control circuit diagram is shown below.

Fig 2: POWER AND CONTROL CIRCUIT DIAGRAM OF DOL STARTER/
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Before starting the description please note that NO & NC stand for normally open and normally closed contact respectively. These states are applicable when the corresponding coil is at de -energized state, suppose we say NO contact of a relay coil A, means when A is de-energized then the contact is open, but when the coil A will be energized it shall be closed, similarly an NC contact will open when the coil is energized.

Now let us see the components.

R-Contactor coil. Energizing which will convert NC contacts to open and NO contacts to close and vice versa. Energizing it the main contact shall be closed to run the motor and de energizing it shall cause motor to stop.

Emergency Stop P.B- Emergency stop push button switch, it shall be installed near to the motor. It is a stay put type button that means when pressed it shall be locked at the new position.

Start P.B- Push button switch for motor starting. It is a spring return type switch, means after pressing it shall return to its previous position. It is NO type switch.

Stop P.B- Push button switch for motor stop. It is also spring return type. It is NC type switch.

Both start and stop push buttons are located near to the motor in a panel called "Local Push Button Station"

Loc./Rem. Sel. Sw.- Local Remote selector switch. Local mans LPBS viz. near to the motor, and Remote means control station say DCS or PLC. Generally motor operation is done from either of these two.

So for starting the motor first have to close the circuit breaker, then auxiliary contact of the CB will be closed allowing circuit continuity.

O/L relay contact shall be normally closed. That means it the relay has not picked uo it will be closed, when overload occurs the relay contact will open and control supply will be disconnected.

Emergency stop push button shall be connected as shown, it shall be connected such that pressing it shall de energize the contactor coil R to stop the motor irrespective of the operation selection that is Local or remote.
See the connection diagram, if you press the Emergency Stop push button, it will de energize the coil R irrespective of the L/R selection.

Now local or remote selection to be done means it has to be selected if the motor shall be operated from LPBS or DCS.

Local operation:
Say the motor has been selected to be operated from local.
For starting the motor start push button have to be pressed. Once pressed the coil R shall be energised and all its NO contacts will get closed. So the main contact R2, R2; R3,R4; R5, R6 will be closed to start the motor. However since the start push button is a spring return type switch it will not remain closed and shall back to open position causing de energization of the coil R, hence another contact R7, R8 to be connected across the start push button, once the coil R has been energized, this R7 R8 contact will keep the circuit continuous.

For stop the motor stop push button to be pressed, once pressed it will de energize the coil R, causing main contacts and R7, R8 to get opened. So the motor supply will be disconnected and the motor shall stop.

Remote Operation:

If the L/R selector is selected to remote then the motor shall be controlled from DCS /PLC. In general from remote the Operation is automatic. Means bssed on the field data or process requirement the DCS or PLC processor determies if the motor to start or stop. The contact as shown is closed or open as per the program installed in the DCS or PLC.

So this is the basic of DOL starting of motor. Being economic and simple it is widely used in industry but it has some disadvanges.

Disadvantages:

1. Starting current is not restricted so the motor takes upto 6 to 7 times of full load current as starting current.

2. Starting current cause huge system voltage dip.

3. Higher cable size is required to maintain motor terminal voltage suitable for motor starting.

However these difficulties are taken care suitably in industrial design engineering, hence this method is widely used.

In this Article we have shown the basic circuit and principle. however in actual two more things to be considered.

A. PROVISION FOR ATLEAST ONE FUTURE INTERLOCK (ONE OPEN TERMINAL) TO BE PROVIDED IN SERIES AFTER THE EMERGENCY STOP PUSH BUTTON, WHICH SHALL EVENTUALLY BE SHORTED TO ALLOW FLOW OF POWER IF IT IS NOT IN USE.

B. WE HAVE SHOWN ON/OFF WITH SAME CONTACT. HOWEVER ANOTHER NC CONTACT OF THE MAIN CONTACTOR COIL(R) SHALL BE USED FOR OFF INDICATION.

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