CONTACTORS
Contactors or relays ( 1) are required in automatic controls to transmit varying conditions in one circuit to influence the operation of other devices in the same or another electrical circuit. Relays have been designed to respond to one or more of the following conditions: voltage current current direction power direction phase angle phase failure over-voltage overcurrent differential current volt-amperes power factor impedance temperature under-voltage undercurrent power (watts) frequency phase rotation speed.
Magnetic switches are widely used in controllers because they can be used with remote control, and are economical and safe.
A relay or contactor usually has a coil which can be energized to close or open contacts in an electrical circuit. The coil and contacts of a relay are represented by symbols on the circuit diagram or schematic of a controller. Symbols commonly used to represent contactor elements
Schematic symbols for contactor elements
If the control coil is connected in series in the motor power circuit, the heavy line symbol shown at the left of figure 2 is used. If the coil is connected in parallel (shunt), the light line symbol .
A series coil has a large current carrying conductor with few turns designed to carry large currents. A shunt coil has a small wire size with many turns; it carries small currents. It is possible for a series coil and a shunt coil to have the same ampere turns, resulting in similar magnetic results .
Contacts which are open when the coil is deenergized are known as normally open contacts and are indicated by two short parallel lines. Contacts which are closed when the coil is deenergized are called normally closed contacts and are indicated by a slant line drawn across the parallel lines.
To minimize heavy arcing which burns the contacts, a dc contactor usually is equipped with a blowout coil and an arc chute. ill 3 shows a magnetic contactor which is provided with a blowout coil and an arc chute.
Magnetic blow out coils magnetically move the arc away from the contacts: arc barriers; power contacts. When a heavy current is broken by the contacts of the contactor, an arc occurs. ill 4 illustrates the behavior of the arc as it's quickly extinguished by the electromagnetic and thermal action of the magnetic blowout coil and arc chute
Behavior of arc with correctly designed blowout: ARCING HORN; ARC CHUTE; STATIONARY TIP
TYPICAL CONTROL CIRCUIT is an elementary control circuit with start and stop buttons and a sealing circuit. The following sequence describes the operation of the circuit. The typical control circuit uses an electromagnetic coil to move sets of contacts. The contacts move to open and close the power circuit to the motor, and also open and close contacts in the control circuit. The control contacts provide a sealing circuit in parallel to the start momentary contacts. This parallel circuit's referred to as the sealing circuit. It seals a current path around the normally open start button contacts. The circuit operation is as follows :
1. When the start button is pressed to close contacts 2-3, current flows from L1 through normally closed contacts 1 - 2 of the stop button, through the closed contacts 2 - 3 of the start button, and through coil M to Line L2.
2. The current in coil M causes the contact M to close. Thus, the sealing circuit around contacts 2-3 of the start button closes. The start button may now be released, and even though the spring of the pushbutton opens contacts 2-3, coil M remains energized and holds contacts M closed to maintain a sealing circuit around the normally open contacts 2-3 of the start button. Coil M, being energized, also closes M contacts in the power circuit to the motor (not shown.
3. If the stop button is momentarily pressed, the circuit's interrupted at contacts 1 - 2 and coil M is deenergized. Contacts M then open and coil M cannot be energized until the start button again closes contacts 2 - 3