The Basics of a Sagittarius and Types (2023)

Industrial control basics: Part 1: Contactors

Introduction to Sagittarius

A contactor is an electrical device used to switch a circuit on or off. It is considered a special type of relay. However, the basic difference between relay and contactor is that the contactor is used in higher current applications while theretransmissionIt is used for low power applications. Contactors can be easily mounted on site and are compact in size. In general, these electrical devices have multiple contacts. In most cases, these contacts are normally open and provide operating current to the load when the contactor coil is energized. Contactors are most commonly used to control electric motors.

There are different types of shooters, and each type has its own characteristics, capabilities and uses. Contactors can interrupt power over a wide range of currents from a few amps to thousands of amps and voltages from 24 Vdc to thousands of volts. Furthermore, these fixtures come in a variety of sizes, from portable dimensions to sizes measuring (approximately) a meter or a yard on a side.

The most common application area of ​​the contactor is high current charging. Contactors are known for their ability to handle currents in excess of 5,000 amps and high powers in excess of 100 kW. Strong motor currents arcing when interrupted. These arcs can be reduced and controlled by a contactor.

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contactor components

The following three are critical contactor components:

  1. Coil or electromagnet:This is the most important component of a contactor. The driving force required to close the contacts is provided by the coil or electromagnet of the contactor. The coil or electromagnet and the contacts are protected by an enclosure.
  2. wrapper:Just like switch cabinets used in any other application, the contactors are enclosed to provide isolation and protection from people touching the contacts. The protection box is made of different materials, such as polycarbonate, polyester, nylon 6, bakelite, thermosetting plastics, among others. The open frame contactor usually has an additional enclosure that protects the device from weather, explosion hazards, dust and oil.
  3. Contact:This is another important part of this fixture. The current-carrying task of the contactor is taken over by the contacts. There are different types of contacts in a contactor, namely contact springs, auxiliary contacts and power contacts. Each type of contact plays an individual role.

contactor operation

How a contactor works:Current flowing through the contactor excites the electromagnet. The energized electromagnet creates a magnetic field that causes the contactor core to move the armature. A normally closed (NC) contact completes the circuit between the fixed contacts and the moving contacts. This allows current to flow through these contacts to the load. When current is removed, the coil is deactivated and opens the circuit. Contactor contacts are known for their quick closing and closing.

Different types of contactors

knife blade switch

The knife blade switch was used in the late 19th century. It was probably the first contactor used to control (start or stop) electric motors. The switch consisted of a strip of metal that fell over a contact. This switch had a lever to pull the switch down or push it up. Back then, you had to level the knife blade switch to the closed position by standing next to it.

However, there was a problem with this method of modification. This method caused the contacts to wear out quickly because it was difficult to manually open and close the switch quickly enough to avoid arcing. As a result, soft copper switches corroded, making them even more susceptible to moisture and dirt. Over the years, motors continued to increase in size, creating the need for larger currents to operate these motors. This created a potential physical hazard when operating switches that carried so much current, creating a serious safety issue. Despite several mechanical improvements, the knife switch could not be fully developed due to problems and risks related to dangerous operation and short contact life.

controle manual

As the knife blade switch became potentially dangerous to use, engineers developed a different protection device that offered several features that the knife blade switch lacked. This device was called a hand controller. These features include:

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  • safe to use
  • Unexposed unit that is properly closed
  • physically smaller
  • Single break contacts are replaced by double break contacts

As the name suggests, double break contacts can open the circuit in two places at the same time. So you can work with more power in a smaller space. Double break contacts split the connection to form two sets of contacts.

The hand control switch or button is not remotely controlled and is physically connected to the hand control.

The circuit is connected as soon as an operator activates the hand control. Once activated, it transports energy to the load. Knife switches were soon completely replaced by manual contactors, and several variants of this type of contactor are still in use today.

magnetic contact

The magnetic contactor does not require human intervention and works electromechanically. This is one of the more advanced designs of a contactor that can be remotely operated. Thus, it helps eliminate the risks of manual operation and potential danger to the operator. The magnetic contactor requires only a small control current to open or close the circuit. This is the most common type of contactor used in industrial control applications.

The life expectancy of a contactor or contact life

The life expectancy of a contactor or its "contact life" is a major concern for the user. It is natural for contacts to open and close more often, the service life of the contactor decreases. When the contacts open and close, an arc occurs which generates additional heat. Continuously generating these arcs can damage the mating surface.

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In addition, arcing causes corrosion and burn marks that eventually blacken the contacts. However, the black or oxide deposit on the contacts makes them even more capable of efficiently conducting electricity. However, if the contacts are badly corroded and worn, they will need to be replaced.

The faster the contact closes, the faster the arc is extinguished. This, in turn, helps extend the life of the contact. Newer versions of shooters are designed to close very quickly and very hard. This causes them to slam into each other and create impact after impact movement. This action is called a contact bounce. The contact vibration phenomenon creates a secondary arc. Not only is it important to close contacts quickly, it's also important to reduce contact rejection. This helps to reduce wear and secondary arcing.

NEMA versus IEC

There are two standards for contactors:.

NEMA (National Electrical Manufacturers Association)

NEMA is the largest trade association for electrical equipment manufacturers in the United States. NEMA encouraged manufacturers to standardize frame sizes so that users could safely specify, purchase, and install electrical components from different manufacturers without a lot of effort or cross-referencing. NEMA contactors are also designed with safety factors greater than design values ​​(oversized) by up to 25%. NEMA is primarily a North American standard.

NEMA contactors for low voltage motors (less than 1000 volts) are rated according to the NEMA rating, which provides a maximum continuous current and power rating for connected induction motors. NEMAstandard contactorSizes are labeled 00, 0, 1, 2, 3 to 9.

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IEC (International Electrotechnical Commission)

IEC is a worldwide standard. IEC contactors are not oversized. They are smaller than NEMA contactors and less expensive. The range of sizes offered by manufacturers is greater than the ten NEMA standards. As such, they are more specific to a given application and are specified when operating conditions are well known. Whereas NEMA can be chosen if operating conditions, eg B. load, are not well defined.

IEC contactors are also "fingerproof". While NEMA requires safety caps on the contactor terminals. Another big difference is that IEC contactors respond more quickly to overloads, while NEMA contactors are more short-circuit proof.

People often mistakenly perceive NEMA contactors as more robust. In fact, this is because its design is oversized.

The following two tables list NEMA and IEC ratings for contactors and starters.


light control

Contactors are often used to centrally control large lighting installations such as office or commercial buildings. To reduce the current consumption in the contactor coils, latching contactors are used, which have two working coils. A momentarily energized coil closes the circuit contacts, which are then mechanically held closed; the second coil opens the contacts.

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electric starter

Contactors can be used as magnetic starters. A magnetic starter is a device designed to power electric motors. It includes a contactor as a critical component, while providing protection against power failure, undervoltage and overload.

examples of motor control


A contactor is a special type of relay used to switch a circuit on or off. They are most commonly used with electric motors and lighting applications. The use of a contactor provides a degree of isolation from the high electrical currents associated with these applications, protecting workers and equipment. IEC contactors are smaller and come in a variety of sizes, while NEMA contactors are larger and rated with safety factors up to 25% greater than design values. IEC is a global standard. NEMA contactors are mainly used in North America, however, more and more companies are using IEC contactors, c3controls specializes in IEC.

The content provided in this whitepaper is for general informational purposes only and is provided with the understanding that the authors and publishers are not rendering any technical or professional service or advice. Engineering practice is guided by site-specific circumstances that are unique to each project. Therefore, any use of this information should only be made in consultation with a qualified and licensed professional who can consider all relevant factors and the desired results. The information in this white paper has been published with reasonable care and attention. However, some of the information in these white papers may be incomplete, inaccurate, or inapplicable to specific circumstances or conditions. We accept no responsibility for any loss, direct or indirect, arising from the use of, reliance on, or reliance on the information contained in this white paper.


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