A DOL starter (also known as a direct online starter or a cross-line
starter) is a method of starting a three-phase induction motor. In the
DOL starter, the induction motor is directly connected to its
three-phase power supply, and the DOL starter applies the full-line
voltage to the motor terminals.
Despite this direct connection, it will not cause damage to the motor.
DOL motor starters include protection devices and, in some cases, status
monitoring. The wiring diagram of the DOL starter is as follows:
DOL starter wiring diagram
The wiring diagram of the DOL state machine is shown below. The direct
online starter consists of two buttons, a green button for starting and a
red button for stopping the motor. The DOL starter includes an MCCB or
circuit breaker, contactor and an overload relay for protection. These
two buttons, namely green and red or start and stop buttons, control the
contacts.
To start the motor, we close the contacts by
pressing the green button, and a full-line voltage will appear on the
motor. The contactor can be 3-pole or 4-pole. The contactor given below
is a 4-pole type.
It contains three NO (normally open) contacts to connect the motor to
the power cord, and the fourth contact is a "holding contact" (auxiliary
contact), which energizes the contactor coil after releasing the start
button.
If any failure occurs, the auxiliary coil will be de-energized, so the
starter will disconnect the motor from the power supply.
3-phase motor running with overload protection
When the motor draws excessive current to meet the load requirements such that the load requirements exceed the rated limit, this is known as an overload.
Thermal overload protection is a safety protection
when the motor draws over current or excessive current and causes the
device to overheat. Overload is also a kind of overcurrent. Therefore,
an overload relay is used to limit the amount of current consumed.
But this does not mean to protect against short circuits. The fuse or
MCB used in the system protects against overcurrent. Overload protection
opens the circuit at a relatively low current that is slightly higher
than the rated value of the motor.
If the overload current persists for a long time, it may be damaged,
that is, if a high value current flows in a short period of time (such
as starting a motor), it will not trip.
We often provide overload protection through overload relays. Overload
relays can be solid state devices with adjustable trip settings, also
called electronic relays, or by interacting with related temperature
sensors called thermal relays, or if they only operate on overcurrents,
they are called magnetic relays.
Working principle of DOL launcher
The working principle of the DOL starter starts by connecting the motor
to the three-phase main power supply. The control circuit is connected
to any two phases and powered by them only.
When we press the start button, the current also flows through the
contactor coil (excitation coil) and the control circuit.
The current energizes the contactor coil and causes the contacts to
close, so the motor can use a three-phase power supply.The control
circuit of the DOL starter is shown below.
If we press the stop button, the current through the contacts will be
interrupted, so that power can not be supplied to the motor, and a
similar situation occurs when the overload relay is activated.
The
machine will stop due to the interruption of the motor supply.
Even if we release the start button, the contactor coil (magnetizing
coil) will get power, because when we release the start button, it will
get power from the main contact, as shown in the picture in the diagram of the Direct Online Starter.
