Function of Contacts, Relays, and Switches in Control Circuits

Electrical Contacts in Control Circuits

Contacts are designed to provide feedback, signaling, and interlocking for contactors and work processes. They are generally used in services involving inductive loads and low current flows. They are categorized into two main types:

• Instantaneous Contacts: These provide immediate feedback, signaling, and interlocking for contactors to perform work processes. These contacts come in two types:
  • Normally Open (NO): Also known as a work contact, this auxiliary contact closes the circuit when its operating coil is energized.
  • Normally Closed (NC): Also known as a rest contact, this auxiliary contact opens the circuit when its operating coil is energized.

Dual-Effect Push Buttons

This component has two contacts: one normally open and one normally closed. When operated, the normally closed contact opens, which can interrupt one circuit while the normally open contact closes to complete another, hence the name dual-effect.

Auxiliary Relays for Maneuvering

Used in combination with contactors, these relays help establish control circuits of varying complexity. They primarily consist of an electromagnet, typically fed by an alternating current, and a series of open and closed contacts.

Types of Time-Delay Relays

Time-delay relays are devices that make or break contact connections after a specified time has passed since their electromagnet is energized or de-energized. Common types include:

• Clock-Based: The delay is achieved by a clock mechanism that actuates a contact when the coil is connected or disconnected.
• Pneumatic: These relays achieve delays, from fractions of a second to several minutes, through the movement of a membrane controlled by air entry.
• Thermal: The action is delayed by a bimetallic strip that, when heated, triggers a contact.
• Motor-Driven: These operate using a clockwork motor powered by an electromagnetic clutch.
• Electronic: These are based on electronic circuits, often involving the charging or discharging of capacitors and resistors, and may be part of an integrated circuit assembly.

Remote Control Devices

Control Switches

Similar to push buttons, these are equipped with contacts for instant action. They are characterized by maintaining their selected position until manually operated again.

Limit Switches

Limit switches are devices designed to control a machine's component based on its position. Their primary application is for safety features, such as stopping motion or reversing the direction of travel, thereby protecting equipment and personnel.