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DOL Starter (Direct On Line Starter) Diagram
And Working Principle
Last updated April 19, 2024 by Electrical4U
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Key learnings:
💡 » Thermistor: Definition, Uses & How
They Work
• DOL Starter Definition: A DOL starter (Direct On Line
Starter) is a simple electrical device that starts a motor by » Half Wave Rectifier Circuit Diagram
applying full line voltage directly to its terminals. & Working Principle
• Starting Mechanism: By pressing the green start button, » Lenz’s Law of Electromagnetic
the motor is connected to the power supply, drawing a Induction: Definition & Formula
high inrush current to initiate motion.
• Diagram Components: The DOL starter wiring diagram
includes a circuit breaker, contactor, and overload relay,
essential for motor protection and operation.
• Working Principle: The starter directly connects the
motor to a three-phase supply, utilizing a control circuit
energized from two phases to manage start and stop
functions.
• Protection Strategy: Overload relays in DOL starters
safeguard the motor by interrupting the power supply if
the current exceeds safe operational levels.
What is DOL Starter?
A DOL starter (also known as a direct on line starter or across the line
starter) is a method of starting a 3 phase induction motor. In a DOL
Starter, an induction motor is connected directly across its 3-phase
supply, and the DOL starter applies the full line voltage to the motor
terminals.
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Even with direct connection to the power supply, the motor remains
protected. A DOL motor starter includes protective devices and, in
some models, condition monitoring features. Below is a wiring diagram
of a DOL starter:
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Since the DOL starter connects the motor directly to the main supply
line, the motor draws a very high inrush current compared to the full
load current of the motor (up to 5-8 times higher). The value of this
large current decreases as the motor reaches its rated speed.
A direct on line starter can only be used in circumstances when the high
inrush current of the motor does not cause an excessive voltage drop in
the supply circuit. If a high voltage drop needs to be avoided, a star
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delta starter should be used instead. Direct on line starters are
commonly used to start small motors, especially 3 phase squirrel cage
induction motors.
As we know, the equation for armature current in the motor.The value
of back emf (E) depends upon speed (N), i.e. E is directly proportional
to N.
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At start-up, the back electromotive force (E) is zero, resulting in a very
high initial current. Small motors with longer axial lengths and smaller
diameters accelerate quickly due to this effect.
Hence, speed increases and thus the value of armature current
decreases rapidly. Therefore, small rating motors smoothly run when it
is connected directly to a 3-phase supply.
If we connect a large motor directly across 3-phase line, it would not
run smoothly and will be damaged, because it does not get accelerated
as fast as a smaller motor since it has short axial length and larger
diameter more massive rotor. However, for large-rated motors, we can
use an oil-immersed DOL starter.
DOL Starter Wiring Diagram
The wiring diagram for a DOL stater is shown below. A direct online
starter consists of two buttons, a GREEN button for starting and a RED
for stopping purpose of the motor. The DOL starter comprises an
MCCB or circuit breaker, contactor and an overload relay for
protection. These two buttons, i.e. Green and Red or start and stop
buttons control the contacts.
To start the motor, pressing the green button closes the contact,
applying full line voltage to the motor. Contactors may have 3 or 4
poles; the diagram below shows a 4-pole contactor.
It contains three NO (normally open) contacts that connect the motor to
supply lines, and the fourth contact is “hold on contact” (auxiliary
contact) which energizes the contactor coil after the start button is
released.
If any fault occurs, the auxiliary coil gets de-energized, and hence the
starter disconnects the motor from supply mains.
3 Phase Motor Starter with Overload Protection
When a motor draws excessive current to meet the load requirements
such that this load requirement goes beyond the rated limit, this is
known as overload.
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Thermal overload protection safeguards the motor from damage by
interrupting the power when excessive current leads to overheating.
Overload relays help regulate the current to prevent this issue.
But that does not mean that protects the short circuit. Fuse or MCB
used in the system protects the over current. Overload protection opens
a circuit at relatively low currents that are a little higher than the rating
of the motor.
Overload currents are likely to damage if they persist for a long time,
i.e. it will not trip if a high value of current flows for a short period such
as starting of the motor.
We often provide overload protection via an overload relay. Overload
relays may be solid-state devices with adjustable trip setting also called
as the electronic relay or by interacting with related temperature
sensors called as a thermal relay or if only operates for excess current
flow then called as a magnetic relay.
For most motors, the maximum rating of the overload protection device
is 125% of the full load ampere rating.
DOL Starter Working Principle
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The working principle of a DOL starter begins with the connection to
the 3-phase main with the motor. The control circuit is connected to any
two phases and energized from them only.
When we press the start button, the current flows through the contactor
coil (magnetizing coil) and control circuit also.
The current energises the contactor coil and leads to close the contacts,
and hence 3-phase supply becomes available to the motor. The control
circuit for a DOL Starter is shown below.
If we press the stop button, the current through the contact becomes
discontinued, hence supply to the motor will not be available, and the
similar thing will happen when the overload relay operates. Since the
supply of motor breaks, the machine will come to rest.
Even after releasing the start button, the contactor coil (Magnetizing
Coil) continues to receive power from the primary contacts, as shown in
the DOL Starter diagram.
Advantages of DOL Starter
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The advantages of a DOL starter include:
1. Simple and most economical starter.
2. More comfortable to design, operate and control.
3. Provides nearly full starting torque at starting.
4. Easy to understand and troubleshoot.
5. DOL starter connects the supply to the delta winding of the
motor.
Disadvantages of DOL Starter
The disadvantages of a DOL starter include:
1. High starting current (5-8 times of full load current).
2. DOL Starter causes a significant dip in voltage, hence suitable
only for small motors.
3. DOL Starter reduces the lifespan of the machine.
4. Mechanically tough.
5. Unnecessary high starting torque
DOL Starter Applications
The applications of DOL starters are primarily motors where a high
inrush current does not cause excessive voltage drop in the supply
circuit (or where this high voltage drop is acceptable).
Direct on line starters are commonly used to start small water pumps,
conveyor belts, fans, and compressors. In the case of an asynchronous
motor (such as the 3-phase squirrel-cage motor) the motor will draw a
high starting current until it has run up to full speed.
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