Direct On Line Starter or DOL Motor Starter
Direct On-Line motor starter is the most common and economical motor starting technique. Widely used technique in the industry. Let’s see the principle, advantages & disadvantages…
Among all the motor starters DOL STARTER is a very popular and most commonly used technique.
The power circuit is drawn below.
The power circuit diagram of a Direct On Line or DOL starter is very simple,
B stands for circuit breakers, 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 a used motor rated up to 30 kW.
CT with Ammeter 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 R.
Now the 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 and MPCB can be tripped from the remote end (away from the switchboard) by using a shunt trip coil to the breakers but they can not be switched on by an electrical signal. These can be switched on by moving the grip only. So for remote operation, interlock with the processing system automatic switching on and off is required, which is achieved by the contractor.
A bimetallic thermal overload relay is given for overload protection.
Rating of this switchgear component viz. MCCB/SFU/MPCB, contactor, and thermal overload relays for a particular motor are selected as per the Type 2 coordination chart, which can be easily found on the internet.
In general motor up to 90 kW is fed by this arrangement however motor rated 110 kW and above has an air circuit breaker with a motor protection relay. Motor protection relay covers all the protection required, and the air circuit breaker can be controlled by an Electrical signal, so their 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
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 a de-energized state, suppose we say NO contact with relay coil A, which means when A is de-energized then the contact is open, but when 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 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 the motor to stop.
Emergency Stop P.B.– The emergency stop push button switch, shall be installed near 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 the button switch for the motor to start. It is a spring return type switch, which means after pressing it shall return to its previous position. It is a NO type switch.
Stop P.B.– Push the button switch for motor stop. It is also a spring return type. It is NC type switch.
Both start and stop push buttons are located near the motor in a panel called “Local Push Button Station”
Loc./Rem. Sel. Sw.– Local Remote selector switch. Local man 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 the auxiliary contact of the CB will be closed allowing circuit continuity.
O/L relay contacts shall be normally closed. That means if the relay has not picked up it will be closed, when an overload occurs the relay contact will open and the control supply will be disconnected.
The 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 whether 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 locally.
For starting the motor start push button have to be pressed. Once pressed the coil R shall be energized and all its NO contacts will get closed. So the main contact R1, R2; R3, R4; R5, and 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 the 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.
To stop the motor stop the push button to be pressed, once pressed it will de-energize coil R, causing the main contacts and R7, R8 to get open. 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. This means based on the field data or process requirement the DCS or PLC processor determines if the motor starts or stops. The contact as shown is closed or open as per the program installed in the DCS or PLC.
So this is the basis of the DOL starting of the motor. Being economic and simple it is widely used in industry but it has some disadvantages.
Disadvantages:
1. Starting current is not restricted to the motor takes up to 6 to 7 times of full load current as starting current.
2. Starting current causes a 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 of 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 AT LEAST ONE FUTURE INTERLOCK (ONE OPEN TERMINAL) TO BE PROVIDED IN SERIES AFTER THE EMERGENCY STOP PUSH BUTTON, WHICH SHALL EVENTUALLY BE SHORTED TO ALLOW THE FLOW OF POWER IF IT IS NOT IN USE.
B. WE HAVE SHOWN ON/OFF WITH THE SAME CONTACT. HOWEVER, ANOTHER NC CONTACT OF THE MAIN CONTACTOR COIL(R) SHALL BE USED FOR OFF INDICATION.
Welcome to my website! I’m Aanchal Gupta, an expert in Electrical Technology, and I’m excited to share my knowledge and insights with you. With a strong educational background and practical experience, I aim to provide valuable information and solutions related to the field of electrical engineering.
I hold a Bachelor of Engineering (BE) degree in Electrical Engineering, which has equipped me with a solid foundation in the principles and applications of electrical technology. Throughout my academic journey, I focused on developing a deep understanding of various electrical systems, circuits, and power distribution networks.