What are Magnetic Contactors and Relays? What is their role in the Automation industry?

Working Principle

Magnetic contactors operate on the principle of electromagnetic attraction. When an electric current flows through the magnetic field coil located in the center leg of the coil core, it creates a magnetic field. This magnetic field generates a force that overcomes the spring force, causing the steel core to move down, closing the circuit. This state is referred to as the “ON” position.

There are two sets of contactors that determine the operating condition:

• Normally Closed (NC) Contactor: Opens the contact point circuit when activated.
• Normally Open (NO) Contactor: Closes the contact point circuit when activated.

When no current is flowing through the coil, the magnetic field dissipates, and the contactors return to their default states.

Major Structure of Magnetic Contactors

The basic elements of magnetic contactors are as follows:

1. Iron Core: Divided into two parts:
   • Fixed Core: Becomes an electromagnet when the coil is energized.
   • Moving Core: Slides into the fixed core when the coil is energized, closing the circuit.
2. Coil: The coil is powered to pull the main contacts close, with auxiliary contacts using the coil’s power to operate.
3. Contacts: Magnetic contactors have two types of contacts:
   • Main Contact: Used in the power circuit to connect the electrical system to the load. These contacts are designed to handle high current capacities.
   • Auxiliary Contact: Used in the control circuit as secondary switching. These contacts can be normally open (NO) or normally closed (NC) and are designed for lower current capacities.

Types of Magnetic Contactors

Magnetic contactors are divided mainly into two categories:

1. AC Contactors: Used with AC power, they are further classified based on their application:
   • AC 1 Magnetic Contactor: Suitable for resistive loads with a power factor between 0.95 and 1, such as heaters and electrical furnaces.
   • AC 2 Magnetic Contactor: Designed for slip-ring motors and high-torque applications.
   • AC 3 Magnetic Contactor: Ideal for starting and stopping squirrel cage motors, used in elevators and lifts.
   • AC 4 Magnetic Contactor: Suitable for frequent starting and stopping, used in cranes and other rapid start/stop applications.
2. DC Contactors: Used with DC power, they are categorized as:
   • DC-1 Magnetic Contactor: Suitable for inductive and slightly non-inductive loads like resistance furnaces.
   • DC-2 Magnetic Contactor: Used for shunt motors and dynamic braking.
   • DC-5 Magnetic Contactor: Designed for series motors and applications involving dynamic braking.

Advantages of Magnetic Contactors

Magnetic contactors offer several advantages over other switches:

1. High Safety: They provide increased safety for operators.
2. Ease of Control: They simplify the control of electrical circuits.
3. Economical: They are cost-effective compared to manual controls.
4. Remote Control: Magnetic contactors can be controlled automatically or remotely, enhancing operational flexibility.

Applications of Magnetic Contactors

Magnetic contactors are widely used in various applications, including:

1. Magnetic Motor Starters: These are electrically operated switches that include motor overload protection. They are similar to contactors but with added overload relays to protect motors from excessive current flow.
2. Lighting Control Contactors: Used for remote or local switching of large lighting loads, such as mercury, fluorescent, tungsten, or LED lights. They enable centralized ON/OFF control of lighting systems.