Control Circuits in Electrical Systems: Contactors, Relays, and Timers
Control Circuits
Constitution of Control Circuits: This system, comprised of electrical appliances and elements, manages the connection and interruption of power from the network to receivers. Its main functions include:
- Enabling manual or automatic control from a distance using small-section drivers and command elements.
- Interrupting high currents during both the closing and opening of the receiver.
- Ensuring continuous intermittent operation.
- Controlling the timing of actions on the receptor.
A control circuit consists of a power circuit (CP) and a control circuit (CM). The CP connects or disconnects a host based on the action taken by the CM. The key element in the CM is the contactor. The CM activates/deactivates the CP and manages timing functions. Basic control elements include relays, timers, and control devices.
Contactors
A contactor is a device capable of interrupting the electric current of a receiver. It can be operated remotely and has two stable operating positions (rest and activated).
Classification
- Electromagnetic: Operated by an electromagnet or coil.
- Electromechanical: Powered by mechanical means.
- Pneumatic: Powered by gas pressure.
- Hydraulic: Operated by liquid pressure.
Constitution of an Electromagnetic Contactor
- Main Contacts: Open and close the CP.
- Auxiliary Contacts: Open and close the CM.
- Coil: Generates a force of attraction when current flows through it.
- Armature: The moving part that activates the main and auxiliary contacts.
- Core: The fixed part that closes the magnetic flux produced by the coil.
- Spring: Returns contacts to their rest position.
Operation
When current flows through the coil, it creates a magnetic flux through the core and armature. This flux generates a force that closes the main contacts and activates the auxiliary contacts. When the current stops, the spring returns the contactor to its resting state.
Selection Criteria
- Nominal operating voltage.
- Current consumption (amperes).
- Receiver type and usage category.
- Required breaking capacity (depends on service category and operating current).
Selection Steps
- Determine the receiver’s current consumption based on its service.
- Determine the service category based on the receiver type.
- Determine the required breaking capacity based on the service category, which will determine the contactor size.
Caliber and Breaking Capacity
- Caliber: The current a contactor can withstand for 8 hours without overheating.
- Breaking Capacity: The maximum current a contactor can interrupt without damage.
Control Relays
Control relays operate within the CM, performing tasks like validating auxiliary contact combinations and handling external commands. They control very low currents and are used in sequential or combinational cycles, often in sequential electrical systems. They are also known as auxiliary contactors or automation relays.
Constitution
Similar to a contactor, but without main contacts. All contacts are auxiliary and can be instant or timed. Designed for small currents.
Operation
Identical to a contactor, but with a smaller magnetic circuit, reducing losses and increasing robustness.
Timers
Timers delay or maintain the opening/closing of a contact for a programmed time based on a command signal.
Types
- Pneumatic Timer: Consists of an auxiliary contactor, auxiliary contact block, and a mechanically coupled pneumatic timer.
- Solid-State Timer: Uses an electronic circuit to control contact timing based on current flow.
Timer Functions
- On-Delay Timer: Delays the closure or opening of a contact after activation.
- Off-Delay Timer: Maintains contact position for a set time after deactivation.
- On/Off-Delay Timer: Delays closure/opening upon activation and maintains the position for a set time after deactivation.
Reactance
Raises voltage while maintaining current. Partial parallel voltages are equal and, in turn, equal to the total voltage (Vt). Total current is the sum of the individual currents.