1.
Analyze the implementation of a Direct-On-Line (DOL) starter utilizing a contactor
circuit, elucidating the roles of each component and the operational sequence.
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.
DOL starter is a device consist of main conta ctor, protective devices and overload relay
which is used for motor starting operations. It is used for low rating usually below 5HP motors.
Construction of DOL Starter:
A DOL or Direct Online starter has simply two buttons; Green and Red, where the green button is
used for starting and the red is used for stopping the motor. The green button connects the terminals
and closes the circuit while the red button disconnects the terminals and breaks the circuit.
The DOL starter is made of a circuit breaker or MCCB or fuse, an
overload relay and contactor or coil. The circuit breaker is used
for protection against short circuits while the overload relay
protects the motor from overloading. The contactor is used for
starting and stopping the motor where the green and red buttons
are connected. The wiring for the start and stop button is briefly
explained in this article below.
A DOL starter is made of following parts:
Circuit breaker or Fuse:
The circuit breaker or fuse is directly connected to the power mains and it is used for protection
against short circuits. It trips the power supply in case of short circuit to protect the system from
any potential hazards.
Magnetic Contactors:
A magnetic contactor is an electromagnetic switch that operates electromagnetically to switch the
power supplied to the motor. It connects and disconnects multiple contacts conveniently by
providing remote control over the operation.
The magnetic field generated by the coil is used for switching the terminals. The passing
current through the coil magnetizes the iron core that is surrounded by the coil. The
magnetic force pulls on the armature to close or open the contacts.
The magnetic contactors have three NO (Normally open) main contacts used for power
supply to the motor and auxiliary contacts (NO and NC) with a lesser rating used for the
control circuit. The coil is connected to the voltage source through auxiliary contacts. Also, keep
in mind that the coil used for a single-phase and three phase supply vary as the supply voltages are
different.
Overload Relay:
OLR or overload relay is the last part used in the DOL starter and it is used for protection against
overloading of motor. It breaks the current flow when it exceeds a certain limit but it also tolerates
the high starting current. So the OLR is carefully selected in such a way that its tripping current
limit does not fall below the starting current range.
The excessive amount of current flow can damage the insulation of electrical wires as well as the
motor winding. The motor life expectancy decrease and it can short the windings causing a risk of
fire.
A simple fuse or circuit breaker cannot protect the system from overloading because they are used
for overcurrent (short circuit) protection. The OLR has current sensing properties that can
differentiate between the starting and overload current.
Three Phase DOL Starter Wiring Diagram:
This is the basic wiring diagram of a DOL starter
DOL Starter Working Principle
The working principle of a DOL starter begins with the connection to the 3-phase main with the
motor. The DOL starter connects the 3 phase voltage supply i.e. R-phase, Y-phase and B-phase to
the induction motor terminals. 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.
The contactor coil (Magnetizing Coil) gets supply even though we release start button because
when we release start button, it will get supply from the primary contacts as illustrated in the
diagram of the Direct Online Starter.
