Trystan Casey
Holding contacts are an essential component of electrical circuits, serving as protective devices that safeguard the circuit from faults. They are used in conjunction with relay switches, which are mechanical devices employed in lower current applications. When the start button is released, the holding contact provides a current path to the relay coil, ensuring the circuit remains closed. Conversely, the stop wire is in series to break the relay circuit and halt the motor. Understanding the functionality of holding contacts is crucial for maintaining the efficiency and safety of electrical systems, particularly in industrial control applications where contactor failure due to welding or sticking contacts can occur.
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What You'll Learn
Holding contacts in relay circuits
Holding contacts are an essential component of relay circuits, playing a critical role in maintaining the circuit's functionality and protecting devices from faults. A relay is a mechanical device used in conjunction with electrical protective devices to shield them from any kind of fault.
In a relay circuit, holding contacts serve the vital function of providing a current path to the relay coil as the start button is released. This means that the holding contacts ensure a continuous supply of power to the relay, allowing it to remain energised and operational. The holding contacts are positioned in parallel with the start button, while the stop wire is in series, enabling it to break the relay circuit and stop the motor when activated.
The term "holding" in the context of electrical circuits refers to the contact's ability to maintain its position as long as power is available to the associated coil. When power is lost, the contact opens, interrupting the circuit. This characteristic of holding contacts is crucial in relay circuits, as it ensures that the relay remains active as long as necessary, and can be deactivated when desired.
Holding relays are commonly used to "'hold in" magnetic motor starters, allowing for continuous operation. To achieve this, a momentary contact (MC) pushbutton is used to energize a small multi-pole control relay. Subsequently, a normally open (NO) contact on the small relay maintains power to its coil, while another NO contact energizes a larger motor starter coil. As a result, the small relay continues to power the larger relay, and the motor keeps running.
In addition to their role in maintaining power, holding contacts in relay circuits also contribute to safety. By providing a closed path, holding contacts prevent unintended activation of the relay, ensuring that the circuit remains stable and controlled. This safety feature is particularly important in applications where unintended activation could lead to hazardous situations. Overall, holding contacts in relay circuits are essential for both functionality and safety, making them a critical component in various electrical systems.
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Holding relays
A relay is an electrically operated switch that can be used to control a circuit independently. The most common form of electromechanical relay consists of an energizing coil called the "primary circuit" wound around a permeable iron core. The iron core has a fixed portion called the yoke and a movable spring-loaded part called the armature. The armature completes the magnetic field circuit by closing the air gap between the fixed electrical coil and the movable armature. When the relay is de-energized, the armature is held in place by a spring, creating an air gap in the magnetic circuit.
Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults. In modern electric power systems, these functions are performed by digital instruments called protective relays or safety relays. Latching relays only require a single pulse of control power to operate the switch persistently, and magnetic latching relays are useful when interrupted power should not affect the controlled circuits.
A ratchet relay is a type of relay that uses a ratchet mechanism to hold the contacts closed after the coil is momentarily energized. This type of relay is found in certain cars for headlamp dipping and other functions requiring alternating operations on each switch actuation. A stepping relay is a specialized kind of multi-way latching relay designed for early automatic telephone exchanges.
Holding contacts in a relay circuit are used to provide a current path to the relay coil as the start button is released. They are in parallel with the start, and the stop wire is in series to break the relay circuit and stop the motor. Thus, holding contacts are used to provide a closed path.
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Latching relays
The unique structure of a latching relay enables it to be activated with a much smaller current than many other types of switching devices. Latching relays do not require a continuous application of current to hold their position once actuated. This is achieved when an electromagnetic current, generated by the electrical input from the first device attached to the relay, causes the contacts in the switch to open or close. This action either completes or breaks the circuit the relay is positioned on, enabling or blocking the transmission of the electrical signal to the rest of the circuit(s).
Magnetic latching relays use permanent magnets to maintain their position after being actuated. They can be either single or dual-coil operated. Mechanical latching relays, on the other hand, use a ratchet and pawl mechanism to maintain the position of the contacts. The relay is set or reset by moving the pawl with a pulse to the coil.
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Magnetic contactors
The working principle of a magnetic contactor is based on electromagnetism. When a current passes through the electromagnetic coil, a magnetic field is created, causing the armature within the contactor to move, either opening or closing the electrical contacts. This movement establishes or interrupts the circuit, allowing or stopping the flow of power. The contactor's design determines whether it is normally open (NO) or normally closed (NC). In a NO contactor, exciting the coil with voltage pushes the contacts together, completing the circuit and allowing power to flow. In contrast, an NC contactor has a complete circuit when de-energised, and the circuit is interrupted when current is supplied to the electromagnet.
The structure of a magnetic contactor is relatively simple, consisting of a terminal bar, moving and fixed contacts, a coil terminal, an arc parachute, and shaded coils. The choice of magnetic contactor depends on technical parameters such as the type of load, rated current/load power, operating voltage, control voltage, and the number of poles. Magnetic contactors are available in different sizes and capacities, making them versatile for various applications, from simple motor control to fully automated systems.
It is important to note that magnetic contactors are not protective devices like circuit breakers. While they can handle high voltage and current applications, they are susceptible to faults such as contact wear and coil burnout if excessive current passes through them. Therefore, protection devices such as overload relays and fuses must be used in conjunction with magnetic contactors to safeguard the system effectively.
There are different types of magnetic contactors, including AC and DC contactors. AC magnetic contactors are used in AC electrical load systems, while DC contactors are designed for DC systems, commonly found in automobile systems, photovoltaic structures, and electric vehicles. Miniature magnetic contactors are also available for use in areas with limited space, such as control panels, small machinery, and home automation systems.
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Double break contacts
A contactor is an electrical device used for switching an electrical circuit on or off. It is a special type of relay with a higher current-carrying capacity. Contactors can be easily field-mounted and are compact in design. They are typically used for controlling electric motors and can be used to break currents ranging from a few amperes to thousands of amperes.
The switch or button of the manual controller in a double break contact is physically attached to the controller and is not operated remotely. Once the manual controller is activated, the power circuit is engaged, and electricity is carried to the load. Today, variations of double break contacts are still commonly used, although magnetic contactors, which can be operated remotely, are now available.
Magnetic contactors do not require human intervention and operate electromechanically. They are considered one of the most advanced designs of contactors, helping to eliminate the risks associated with manual operation and improving safety for operating personnel. Contactors are equipped with several key safety features to protect both the electrical system and its users. For example, they are designed to automatically interrupt the electrical circuit if power is lost, preventing accidental electrical flow and reducing the risk of electrical hazards.
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Frequently asked questions
A holding contact in an electrical circuit is a type of relay that is used to provide a current path to the relay coil as the start button is released. The holding contact stays in position only for as long as power is available to the associated coil.
A holding contact is energised by the amount of current flowing through its coils, as determined by an outside load, typically a starter motor. As the load decreases, so does the energy in the coil and the relay opens.
A holding contact stays in position only for as long as power is available to the associated coil. On the other hand, a latching contact is put into position by the action of one coil and is held in place by a mechanical locking device.
