Direct online starter (DOL) L&T Type- MK-1 |

STARTERS

Starter play an important part for Electrical Motor. It not only starts the motor but it provides protection from overload conditions.
(NOTE: Motor word in this post; used only for Induction motor)

It is not possible to limit the starting current in DOL starter. That’s why DOL is used for small rating motors only. This post is overall explanation about MK-1 DOL Starter made by L&T in which its “parts”, “Circuit diagram” & “Working” is been discussed.

 

DOL STARTER TYPE-MK1 BY L&T

” DOL stands for ‘DIRECT ONLINE‘. In DOL starters, Motor get directly connected to power supply without any change in voltage, frequency & winding connections, That’s why starter is been called Direct On Line. “
MK-1 DOL Starters are most widely used DOL starter in Industry for small rating Induction motors. Best part of this MK-1 starter is;  Its Every smallest single part is easily replaceable. An image of front cover of MK-1 type DOL starter is shown below :

Front Cover of L&T DOL Starter Type-MK1
Front cover of DOL Starter Type-MK1 by L&T

PARTS INSIDE THE STARTER

Like any other DOL starter, It also consists of thermal overload relay, Contactor & push button unit. We are going to discuss all these parts one by one, next !

1. Thermal Overload Relay

Thermal overload relay is a protection device which protects the motor from overload conditions.
Motor’s rated current depend upon the power rating of the motor that’s why for different power rating, thermal overload relay comes in different ampere ranges; such as 1.5-2.5Amps, 2.5-4Amps, 4-6.5Amps, 6-10Amps, 9-14Amps.
An image of thermal overload relay MK-1 by L&T is shown next. The relay ranges from 9 to 14 Amps Which is the perfect range for 5.5KW 3 phase Induction motor.

At the Top of relay; for input connections; 3phase Input terminals are connected.
On the other hand, at opposite side, 3phase Output+1 blue phase for HOLD is been provided.

Collage of Thermal Overload Relay (Type-Mk 1) BY L&T
Thermal Overload Relay; Type-Mk 1 MADE BY L&T

Marking of Input & Output terminals of thermal overload relay
Input & Output terminals of thermal overload relay| L&T

 

JUMPER wire (Red) & HOLD wire(Blue) in Mk-1 overload Relay by L&T
JUMPER wire (Red) & HOLD wire(Blue)in Mk-1 overload Relay| L&T
Thermal overload relay consists of NC & NO contacts in which NC contact is used for stop function whereas NO contact is used for start function.
To connect the NC point with NO point RED jumper wire is being used. While BLUE wire is used in hold circuit.
Thermal overload relay provides the protection for both overload and short circuit. But the mechanism have different working mechanism.
Thermal overload relay consists of Bimetallic strips fixed on all the three phases.
Whenever current exceeds the limit in overload condition; Bending movement of Bimetallic strips produces the trigger in NC terminal of the relay.
The wire which carries the load current, wrapped around these bimetallic strips. In short circuit scenario these coil act like an electromagnet and moment is utilized to initiate the trip circuit.

Thermal Bimetallic strips inside thermal overload relay
Thermal Bimetallic strips in thermal overload relay| L&T

2. Contactor

Contactor is a type of electromechanical assembly. In this starter’s contactor, Contactor connects motor terminals & output terminals of thermal overload relay. Contactor consists of plunger, Electromagnetic coil & power-motor terminals.

Inner mechanism of MK-1 DOL starter by L&T
MK1 Starter’s Contactor | L&T
Plunger contains 4 copper strips which are fixed with spring mechanism.
These all the 4 strips are having a 2 sides which connects both motor terminals &  thermal overload relay output terminals. (shown in an image Below:)

Copper strips of contactor which connects motor terminal with Overload relay output
Copper strips which connects motor terminal with thermal overload relay output| Mk-1 DOL starter by L&T

Collage images of MK-1 Starter's contactor which shows its working
MK-1 Starter’s contactor| L&T
 

Electromagnetic Coil

Electromagnetic coil is a type of Solenoid coil which lift the plunger upward when an electromagnetic coil is get energized.
Coil voltage available in 230VAC & 415VAC. 
In this post, the coil voltage is 415VAC.

MK-1 starter's Electromagnetic coil & Contactor's plungers arrangement
MK-1 starter’s Electromagnetic coil | L&T

Push Button Set

A pair of push button are fitted on the cover of the starter; Just in front of the thermal overload relay.
The pair of push buttons in which STOP button pushes the NC . While START button pushes NO contact of thermal overload relay.

Collage of MK_1 DOL starter which shows Start & Stop Push Button positioning Infront of thermal overload relay's NC & NO contact.
Start & Stop Push Button of MK-1 Starter | L&T
 

CIRCUIT DIAGRAM OF DOL STARTER

Like any other Electrical panel, This starter also contains two wirings; one is ‘control wiring’ and another one is ‘power wiring’. Power wiring is heavy wiring through which heavy load current flows. On the other hand, Control wiring is light wiring through which low control current flows.
 

POWER WIRING

Power wiring shown(below) in Red, yellow& Blue colors Which represents the 3 phases. Which starts from the top of thermal overload relay to an end at motor terminals.

Power wiring marking in MK-1 DOL Starter
POWER WIRING |MK-1 DOL Starter by L&T
 

CONTROL WIRING

Control wiring is the wiring where very minor current flows which is used to control the power wiring.
In this control wiring, Red phase is directly connected to one of the terminal of coil. Another terminal of the coil; is been energized with Blue phase. Blue phase which passes through the NO contact of relay & than through a NC contact.

Control wiring highlighted in MK_1 DOL starter
Control Wiring |MK-1 DOL Starter by L&T

WORKING OF DOL STARTER TYPE-MK1 BY L&T

image shows the clamp meter measuring Input supply at terminals of MK1 Starter by L&T
Input supply at MK1 Starter by L&T
 

3 phase input supply is connected at the top of Thermal overload relay. Since one side of the coil is directly connected to red phase. When start(NO ) button get pressed; coil gets blue phase & coil get energized.

Coil becomes Electromagnet which attracts the plunger upward which results 4 copper strips fitted on the plunger connects the output of thermal overload relay & motor terminals. Supply get available at motor terminals & Motor starts rotating.
Since NO button is pressed only once after that NO button is being released But coil still remains energized. This all happening due the ‘HOLD Circuit’.
 

An image shows a clamp meter measuring voltage at output terminals of MK_1 DOL Starter
Output supply at DOL Starter type-MK1 by L&T

 

HOLD CIRCUIT

“Hold circuit is also known as ‘latch circuit’. As name implies, This circuit get hold when it get a signal only once. “
Since Red phase is directly connected to the coil. We are controlling the whole circuit by just controlling the Blue phase. When we press start(NO) button of thermal overload relay, Coil get blue phase & get energized. In the mean time, mechanical plunger get lifted up & forth terminal of plunger also produces the blue phase directly to the coil Which results coil remains energized.
Hold circuit only breaks When we press stop button; the NC of thermal overload relay; contact get break & coil get de-energized.

TRIP CIRCUIT

The tripping circuit is also uses the same NC contact of thermal overload relay which is used by stop button. Since, power current is passing through the bimetallic strips of  thermal overload relay; When current exceeds the limit;  Bimetallic strip produces bending movement which is used to trip the NC of thermal overload relay.

SUMMARY

  • In DOL Starter, Motor get directly connected to power supply without any change in voltage, frequency & winding connections, That’s why starter is called Direct On Line.
  • MK-1 Starter by L&T is a type of DOL Starter.
  • MK-1 DOL Starters are most widely used starter in Industry used for small rating induction motors. Best part of this MK-1 starter is;  Its every smallest single part is easily replaceable.
  • Like any other DOL starter, It also consists of thermal overload relay, Contactor & push button assembly.
  • Thermal overload relay is a protection device which protect the motor in overload conditions.
  • Contactor is a type of Electromechanical Element which connects power supply with motor terminals when its electromagnet get energized.
  • Push button arrangement is used in a function of ON & OFF.

Frequently Asked Questions (FAQ) – Direct Online Starter (DOL) | L&T MK-1

Q1. What is a Direct Online Starter (DOL)?
A Direct Online Starter (DOL) is a motor starting method where the motor is connected directly to the power supply at full voltage using a contactor and overload relay.

Q2. What is L&T MK-1 DOL Starter?
The L&T MK-1 DOL Starter is a ready-to-use motor starter manufactured by Larsen & Toubro (L&T). It comes with a contactor, thermal overload relay, start/stop push buttons, and enclosure.

Q3. How does an L&T MK-1 DOL starter work?
When the START button is pressed, the contactor energizes and supplies full voltage to the motor. Pressing the STOP button or an overload condition de-energizes the contactor and stops the motor.

Q4. What components are used in L&T MK-1 DOL Starter?
The main components include a contactor, thermal overload relay, start and stop push buttons, enclosure, and internal wiring terminals.

Q5. What is the voltage rating of L&T MK-1 DOL Starter?
L&T MK-1 DOL starters are generally designed for 415V AC, three-phase power supply. 230V AC control coil is also available.

Q6. What motor HP range is supported by L&T MK-1 DOL Starter?
L&T MK-1 DOL starters are commonly used for motors up to 5 HP.

Q7. Where is L&T MK-1 DOL Starter commonly used?
It is used in water pumps, air compressors, conveyors, agricultural motors, and small industrial machines.

Q8. What are the advantages of L&T MK-1 DOL Starter?
It offers simple construction, easy installation, low cost, high starting torque, and reliable L&T build quality.

Q9. What are the disadvantages of DOL Starter?
DOL starters draw high starting current and are not suitable for large-capacity motors or weak power supply systems.

Q10. Is L&T MK-1 DOL Starter suitable for domestic use?
Yes, it can be used for domestic pump motors if the motor rating matches the starter specification.

Q11. What protection does L&T MK-1 DOL Starter provide?
It provides overload protection through a thermal overload relay and short-circuit protection when used with an MCB or MCCB.

Q12. What is the difference between DOL starter and Star-Delta starter?
A DOL starter applies full voltage directly to the motor and is used for small motors, while a star-delta starter reduces starting current and is used for larger motors.

Q13. How to select the correct L&T MK-1 DOL Starter?
Selection should be based on motor HP, full load current (FLC), supply voltage, and application type.

Q14. Is L&T MK-1 DOL Starter durable?
Yes, L&T MK-1 DOL starters are known for their durability, reliability, and long service life.

Hope, This post has something helpful. 
For any query or suggestion, Please do comments !
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