Most motor applications require the
use of remote control devices to start and stop the motor. Magnetic contactors,
similar to the ones shown below, are commonly used to provide this function.
Contactors are also used to control distribution of power in lighting and
heating circuits.
Basic Contactor
Operation
Magnetic contactors operate utilizing
electromagnetic principles. A simple electromagnet can be fashioned by winding
a wire around a soft iron core. When a DC voltage is applied to the wire, the
iron becomes magnetic. When the DC voltage is removed from the wire, the iron
returns to its nonmagnetic state. This principle is used to operate magnetic contactors.
The following illustration shows the
interior of a basic contactor. There are two circuits involved in the operation
of a contactor. The control circuit and the power circuit. The control circuit
is connected to the coil of an electromagnet, and the power circuit is connected to the stationary
contacts.
The operation of this electromagnet is
similar to the operation of the electromagnet we made by wrapping wire around a
soft iron core. When power is supplied to the coil from the control circuit, a
magnetic field is produced magnetizing the electromagnet. The magnetic field
attracts the armature to the magnet, which in turn closes the contacts. With
the contacts closed, current flows through the power circuit from the line to the
load. When the electromagnet’s coil is de-energized, the magnetic field
collapses and the movable contacts open under spring pressure. Current no
longer flows through the power circuit.
The following schematic shows the
electromagnetic coil of a contactor connected to the control circuit through a
switch (SW1). The contacts of the contactor are connected in the power circuit
to the AC line and a three-phase motor. When SW1 is closed, the electromagnetic
coil is energized, closing the “M” contacts and applying power to the motor.
Opening SW1 de energizes the coil and the “M” contacts open, removing power
from the motor.
Overload Relay
Contactors are used to control power
in a variety of applications. When applied in motor-control applications,
contactors can only start and stop motors. Contactors cannot sense when the motor
is being loaded beyond its rated conditions. They provide no overload
protection. Most motor applications require overload protection. However, some
smaller-rated motors have overload protection built into the motor (such as a household garbage
disposal). Overload relays, similar to the one shown below, provide this
protection. The operating principle, using heaters and bimetal strips, is
similar to the overloads discussed previously.
Motor Starter
Contactors and overload relays are
separate control devices. When a contactor is combined with an overload relay,
it is called a motor starter.
Motor Starter in a Control
Circuit
The following diagram shows the
electrical relationship of the contactor and overload relay. The contactor,
highlighted with the darker grey, includes the electromagnetic coil, the main
motor contacts, and the auxiliary contacts. The overload relay, highlighted by
the lighter grey, includes the “OL” heaters and overload contacts. The
contactor and the overload relay have additional contacts, referred to as
auxiliary contacts, for use inthe control circuit. In this circuit a
normally closed “OL” contact has been placed in series with the “M” contactor
coil and L2. A normally open “M” auxiliary contact (“Ma”) has been placed in parallel
with the “Start” pushbutton.
