Across the Line Magnetic Starter is applied to motors whose locked rotor current is only a few times more than the full load current of the motor.
- The terms: energize, activate, operate, run, turn-on, switch on, and start; are sometimes used interchangeably in the proceeding explanation of control circuit operation. The terms are to indicate that a machine or an equipment commence to function and is now taking electrical energy from the source.
- The terms: de-energize, stops, de-activate, turn-off, switch off, cut-off, and released are often used interchangeably. Those terms are used to indicate that a machine or an equipment stops from functioning and taking energy from the source.
The “ Across Line Magnetic Starter ” is the simplest method of controlling 3-phase motor using a magnetic contactor,
( see figure below ).
Schematic diagram of the control and power circuits of the “Across The Line Magnetic Starter”.
Control Operation
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Referring to Figure above, after the main circuit breaker CB is
closed, the IDLE pilot lamp indicator is *energized (turn on) indicating that
the motor is at stand still condition.
Magnetic contactor C, RUN and FAULT pilot lamp indicator are still
in the de-energized (turn off) state. When the START push button is pressed,
magnetic contactor C is *energized.
With contactor C energized, contacts C(1-2, 3-4, and 5-6) will be
closed and they connect the motor M to the 3-phase supply lines (L1, L2, and
L3).The motor will run (rotate).Contacts (11-12) will open and make the IDLE
pilot lamp de-energized. Contacts (13-14) will be closed and maintain contactor
C at energized state even if the push button switch is released**.
When the STOP push button is pressed, the current through the
magnetic contactor C is cut off**.Magnetic contactor C is
de-energized**.Contacts C (1-2, 3-4, 5-6) will open and they will de-energized
motor M.Contacts C (11-12) will closed and it will
make the IDLE lamp energized* again
to indicate that the motor goes back to its stand still condition.Contacts C (13-14) will open to release
contactor C from its maintained energize state. The RUN lamp is also
de-energized.
When the motor operates* at more than its
rated capacity (overloaded), it will draw an overload current above its rated
current. The thermally operated device O.L. (1-2, 3-4, 5-6) set to operate at
the rated current of the motor will overheat.
This thermally operated device will expand
and push a mechanism that will open the normally closed contacts O.L. (95-96
and will close the normally open contact O.L. (95-98). Opening contacts O.L. (95-96) will cut
off the current through the contactor C.
This is similar as if the STOP push button
is pressed. Closing the normally open contacts O.L. (95-98) will make the
BUZZER to alarm and alert an operator that something goes wrong with the
operation. The FAULT lamp is also energized* and will be lighted. The lighted
FAULT lamp will inform the operator which motor from a group of motors is at
fault.
The circuit involve in starting and stopping starts from line L1,
contact I of contactor C, contacts (95-96) of overload relay O.L., contacts
(1-2) of stop push button, contacts (3-4) of start push button or contacts
(13-14) of contactor C, contact 3 of contactor C and the line L2.
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