DOL Starters Explained: Key Features, Working & Benefits

There are many methods to start/run the Induction motor; Through DOL starter, Star-delta starter, Variable frequency drive(VFD) & Auto transformer starter.

Motor starting; is not the only function of the starter. Along with the starting, Starters provide protection from overload, phase failure, phase imbalance & prevention from high initial starting current. All the starters don’t provide all the protections except VFD.

All the starting methods mentioned above run the Induction motor in a different way. All are having a different working principle which makes each starting method has its own merits & demerits. For example, In DOL starter, Initial starting current can’t be controlled. But In Star-delta starter, VFD and autotransformer starter, starting current can be controlled easily. That’s why DOL starters are used for small rating motors only.

In this post, We will briefly Discuss Direct Online Starters ; also known as D.O.L.

An image of front cover of DOL Starter MK-1 by L&T — DOL Starter MK-1 by L&T

DOL (DIRECT ONLINE STARTER)

“Direct online starters connects the motor’s terminals directly with the power supply, Without any change in motor’s winding connection, Without any change in voltage & frequency that’s why known as Direct On Line Starter.”

DOL starter consists of thermal overload relay, a contactor and on-off push buttons assembly.

A thermal overload relay is provided for short circuit protection, overload protection & phase failure imbalance. These thermal overload relay are available in different ampere ranges according to the motor rating.
Contactor current rating is also depends upon the motor rating. Probably higher than the motor’s rated current so that it easily bear the fault current. & its coil voltage is selectable which depend upon the control circuit voltage.

HOW DOES DOL STARTER WORKS ?

One important thing about the DOL starter is; To START & STOP the motor, ON&OFF push buttons are only used for once. If motor needed to be started, Start button is needed to be pressed only once. While to stop the motor, Stop button is needed to be pressed only once. There is no need to hold the button in pressed position, Only a one single press.

Apart from this, Starter get disconnected when power supply goes off. We need to press the start button again to start the motor. These all the things are happening due to the ‘latch circuit’. Next, Latch circuit is been discussed.

LATCH CIRCUIT

Latch circuit is represented through a ladder logic below,

In a ladder diagram : I0(NO), I1(NC) are inputs While Q0 is output(Contactor’s coil).

When I0 get pressed, supply get available to Q0. Parallelly, q0 (NO contact) connected parallelly across the I1 input also changes its state & become a closed contact. Which result Supply to Q0 get hold through q0(No contact) even if the I0 (NO contact) get released.

Supply to Q0 breaks only when; Either series I1 (NC contact) get break or power supply get cut off.

This circuit is known As ‘HOLD circuit’ Which is used in DOL Starter.

I0 is no contact of start button.
I1 is nc contact of stop button.
Q0 is coil of contactor.

LAYOUT OF DOL STARTER

Every electrical panel has two wirings : ‘Power wiring’ & ‘control wiring’. Power wiring is heavy wiring at which load current flows. While control wiring is light wiring at which low current flows. Same in a way, DOL starter also consists of power wiring & control wiring. Both the wirings are discussed below :

Wiring diagram of DOL Starter| Power Diagram is marked with Blue While control diagram is marked with Green

Power wiring(dol starter)

In wiring diagram above, Power supply is represented in blue color.

3phase Power supply (VLL=415) is passing through thermal overload relay first. Then these 3 lines are connected to the contactor & motor terminals are connected on another side of the contactor. Motor get power supply when contactor get energized. Power supply to the motor is controlled by controlling the coil voltage of the contactor. Power to coil voltage represents the control wiring which is discussed next.

Control Wiring(dol starter)

In wiring diagram above, Control wiring is represented in green color.

Since the contactor’s coil voltage is 415 Volts that’s why red phase is directly given to the coil. Whole control circuit is working through the second phase which is blue.

Blue phase passes through NO contact(START). Then it passes through both the NC contacts; NC contact of stop button & NC contact of thermal overload relay.

1 extra NO auxiliary contact of contactor which is used for HOLD; is parallelly connected with NO contact(START).

When start button; which is type of NO contact; get pressed. contactor get energized & get hold. Power supply to motor get disconnected if Stop button(NC) get pressed or thermal overload relay’s (NC) get trip.

In this post, DOL Starter explanation is theoretical. If you are curious about the practical DOL Starter which are widely used in industries, you can go through the posts DOL Starter By L&T & ABB made DOL Starter which are entirely based on practical working.

For better understanding, a single line control diagram of DOL starter is shown. In this, NC contact of thermal overload relay, NC contact of stop button, NO contact of start button – all are connected in series. Contactor’s one side is connected neutral while phase connected through a series connection. For holding or a latch circuit, NO contact of contactor is connected parallel to NO contact of start button.

SUMMARY

The post discusses various methods for starting and running of induction motors, including DOL (Direct Online) starters, star-delta starters, variable frequency drives (VFD), and auto transformers. While all starters serve to initiate motor function, their protective features vary, with VFDs offering the most comprehensive protection against overload and phase failures.

DOL starters draw high initial currents and are suitable for small motors only. The mechanics of DOL starters involve a thermal overload relay and contactor working with a latch circuit. Usage of on-off buttons for operation, allowing for simple start and stop processes. The importance of power and control wiring of DOL starter also highlighted.

Hope! Post is beneficial for you. Your suggestions and queries are always welcome. Please do comments.

Tagged Induction motor starters

Understanding DOL Starters: Key Features and Benefits

DOL (DIRECT ONLINE STARTER)