Electromagnetic Contactors, Control Relays and Timers for Circuits

Circuit Constitution and Switching Maneuver

Circuit constitution maneuver: Equipment are made to perform the connection and interruption of electric power from the network to the receivers. Main features include:

• Make a manual or automatic remote connection, with the help of a command element, of a small section of conductors using a switching device.
• Interrupt high currents both immediately and at the final opening of the receiver or electric machine.
• Functioning: ensure continuous or intermittent operation.
• Be able to delay or bring forward an action on the receiver.

Power Circuit

The power circuit is responsible for connecting or disconnecting a receiver from the supply under the action performed by the control circuit. A key element in the power circuit is the contactor.

Control Circuit

The control circuit is responsible for performing the actions to activate and deactivate remotely, temporize on CP (control panel) and provide the basic elements of control such as relays, timers and control devices.

Contactors

Contactors are devices capable of interrupting the power supply to a receiver or installation and can be operated remotely. They generally have two stable positions: energized (closed) or at rest (open).

Classification of Contactors

Classification according to actuation method:

• Electromagnetic: actuation through an electromagnet or coil (most commonly used).
• Electromechanical: mechanically actuated devices.
• Pneumatic: driven by compressed gas or air pressure (gas-powered pressure).
• Hydraulic: driven by liquid pressure.

Most Used Contactors

Most commonly used are electromagnetic contactors.

Parts of a Contactor

• Main contacts: conductors that carry the load current; they are usually open at rest for normally open contacts.
• Auxiliary contacts: arranged for open and close operations, mechanically coupled to the main contacts; they can be normally open or normally closed at rest.
• Coil: the winding that generates the magnetic field when energized.
• Armature: the movable part of the contactor that moves under magnetic attraction.
• Core (yoke): the fixed magnetic path that guides the magnetic flux produced by the coil.
• Spring: provides the return force to bring contacts back to their rest position when the coil is de-energized.

Operation of an Electromagnetic Contactor

When a current flows through the coil of the contactor, it closes the magnetic circuit, establishing magnetic flux through the core and yoke. This flux produces an attractive force that moves the main contacts and changes the state of the auxiliary contacts. When the coil current disappears, the magnetic flux collapses and the springs return the contacts to their resting state.

Contactor Selection Criteria

Selection should consider:

• Nominal voltage.
• Current in amperes consumed by the service or receiver.
• Nature and utilization of the receiver (type of load).
• Breaking capacity: the ability to interrupt fault or overload currents, which depends on service levels.

Practical steps for selection:

1. Determine the service current of the receiver.
2. Categorize the receiver (motor, resistive load, inductive load, etc.).
3. Obtain other relevant service currents and select the contactor with adequate rated current and breaking capacity for the service category.

Control Relays

Control relays are used for handling very low currents or sequential cycles and are also called auxiliary contactors. Their constitution is similar to that of the main contactor, but:

• Main contacts may not have the same current carrying capacity as power contactors.
• Auxiliary contacts can be instantaneous or timed.
• They are designed so that small currents can operate control circuits but cannot operate power circuits directly.
• Operating principles are identical to contactors, but relays are designed to support fewer maneuvers and lower currents.

Timers

Timers are control elements responsible for creating delays and maintaining the opening or closing of a contact during a scheduled time set by the user from a control signal.

Classification: pneumatic and electronic.

Types:

• Delay on make (time to close) — the contact closes after a set time from the activation command.
• Delay on break (time to open) — the contact remains in its activated position for the scheduled time and then changes state after deactivation.
• Some timed contacts change their state instantly after the scheduled delay, depending on timer design.

Note: Timers can be pneumatic (using air pressure) or electronic; selection depends on required timing accuracy, environmental conditions, and compatibility with the control circuit.

All information above preserves original content while correcting spelling, grammar and capitalization for clarity.