Industrial Control Basics: Part 1 – Contactors
A contactor is an electrical device which is used for switching an electrical circuit on or off. It is considered to be a special type of relay. However, the basic difference between the relay and contactor is that the contactor is used in applications with higher current carrying capacity, whereas the relay is used for lower current applications. Contactors can be field mounted easily and are compact in size. Generally, these electrical devices feature multiple contacts. These contacts are in most cases normally open and provide operating power to the load when the contactor coil is energized. Contactors are most commonly used for controlling electric motors.
There are various types of contactors, and each type has its own set of features, capabilities, and applications. Contactors can break current over a wide range of currents, from a few amperes to thousands of amperes, and voltages from 24 VDC to thousands of volts. In addition, these electrical devices come in varying sizes, from hand-held dimensions to sizes measuring a meter or yard on one side (approximately).
The most common application area of the contactor is high-current load. Contactors are known for their capability to handle currents of over 5000 amperes and high power over 100 kW. Heavy motor currents produce arcs when being interrupted. These arcs can be reduced and controlled using a contactor.
The following three are crucial components of the contactor:
- Coil or Electromagnet: This is the most crucial component of a contactor. The driving force that is required to close the contacts is provided by the coil or electromagnet of the contactor. The coil or electromagnet and contacts are protected by an enclosure.
- Enclosure: Just like the enclosures used in any other application, contactors also feature an enclosure, which provides insulation and protection from personnel touching the contacts. The protective enclosure is made from different materials, such as polycarbonate, polyester, Nylon 6, Bakelite, thermosetting plastics, and others. Generally, the open-frame contactor features an additional enclosure, which protects the device from bad weather, hazards of explosion, dust, and oil.
- Contacts: This is yet another important component of this electrical device. The current carrying task of the contactor is done by the contacts. There are different types of contacts in a contactor namely, contact springs, auxiliary contacts, and power contacts. Each type of contact has an individual role to play.
How the Contactor Operates
Operating Principle of a Contactor:The current passing through the contactor excites the electromagnet. The excited electromagnet produces a magnetic field, causing the contactor core to move the armature. A normally closed (NC) contact completes the circuit between the fixed contacts and the moving contacts. This permits the current to pass through these contacts to the load. When current is removed, the coil is de-energized and opens the circuit. The contacts of the contactors are known for their rapid open and close action.
Different Types of Contactor Devices
Knife Blade Switch
The knife blade switch was used earlier in the late 1800’s. It was probably the first ever contactor that was used to control (start or stop) electric motors. The switch consisted of a metal strip, which would drop onto a contact. This switch had a lever for pulling the switch down or pushing it up. Back then, one had to level the knife blade switch into the closed position by standing next to it.
However, there was a problem with this method of switching. This method caused the contacts to wear out quickly, since it was difficult to manually open and close the switch fast enough to avoid arcing. As a result of this, the soft copper switches underwent corrosion, which further made them vulnerable to moisture and dirt. Over the years, the size of the motors increased which further created the need for larger currents to operate them. This created potential physical danger to operate such high current carrying switches, thus leading to a serious safety concern. In spite of doing several mechanical improvements, the knife blade switch couldn’t be fully developed due to the pertaining problems and risks of dangerous operation and short life of the contacts.
Since the knife blade switch became potentially dangerous to use, engineers came up with another contactor device, which offered a number of features that were missing in the knife blade switch. This device was referred to as a manual controller. These features included:
- Safe to operate
- Non-exposed unit, which is properly encased
- Physically smaller size
- Single break contacts replaced with double break contacts
As their name implies, double break contacts can open the circuit in two places at the same time. Thus, even in smaller space, it allows you to work with more current. Double break contacts divide the connection in such a way that it forms two sets of contacts.
The switch or button of the manual controller is not operated remotely and is attached to the controller physically.
The power circuit is engaged once the manual controller is activated by an operator. Once activated, it carries the electricity to the load. Soon, manual contactors replaced knife blade switches completely, and even today different variations of these types of contactors are being used.
The magnetic contactor does not require human intervention and operates electromechanically. This is one of the most advanced designs of a contactor, which can be operated remotely. Thus, it helps eliminate the risks involved in operating it manually and putting operating personnel in potential danger. Only a small amount of control current is required by the magnetic contactor 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 one of the biggest concerns of a user. It is natural that the contacts are being opened and closed more frequently, the life of the contactor will decrease. The opening and closing of the contacts create an electric arc, which generates additional heat. The continued production of these arcs can damage the contact surface.
Furthermore, the electrical arcs cause pitting and burn marks, which eventually blacken the contacts. However, the black deposit or oxide on the contacts make them even more capable of conducting electricity efficiently. Nevertheless, when the contacts get worn out and corroded to a large extent, then it is necessary to replace them.
Thus, the faster the contact closes, the quicker the arc extinguishes. This in turn helps to increase the life of the contact. The latest contactor versions are designed in such a way that they close very quickly and energetically. This causes them to slam against each other and produce a bouncing action as they rebound. This action is known as contact bounce. The contact bounce phenomenon creates a secondary arc. It is not only important to close the contacts quickly, but also to reduce the contact bouncing. This helps reduce wear and secondary arcing.
NEMA vs IEC
NEMA (National Electrical Manufacturers Association)
NEMA is the largest trade association of electrical equipment manufacturers in the United States. NEMA encouraged manufacturers to standardize on frame sizes to allow users to confidently specify, purchase, and install electrical components from different manufacturers without a lot of hassle and cross-referencing. NEMA contactors also are designed with safety factors that go beyond design ratings (oversized), up to as much as 25%. NEMA is primarily a North American Standard.
NEMA contactors for low-voltage motors (less than 1,000 volts) are rated according to NEMA size, which gives a maximum continuous current rating and a rating by horsepower for attached induction motors. NEMA standard contactor sizes are designated 00, 0, 1, 2, 3 to 9.
IEC (International Electrotechnical Commission)
IEC is a global standard. IEC contactors are not oversized. They are smaller than NEMA contactors and less expensive. The range of sizes offered by manufacturers is more numerous than the ten NEMA standards. As such, they are more specific to a given application and are specified when the operating conditions are well understood. Whereas, NEMA may be chosen when operating conditions, such as load are not well defined.
IEC contactors are also “finger safe.” Whereas NEMA requires safety covers on contactor terminals. Another key difference is IEC contactors react faster to overloads, NEMA contactors are better at withstanding short circuits.
People often mistakenly perceive NEMA contactors as more robust. In reality, this is due to their design being oversized.
The two tables below detail how NEMA and IEC size contactors and starters.
Contactors are often used to provide central control of large lighting installations, such as an office building or retail building. To reduce power consumption in the contactor coils, latching contactors are used, which have two operating coils. One coil, momentarily energized, closes the power circuit contacts, which are then mechanically held closed; the second coil opens the contacts.
Electric Motor Starter
Contactors can be used as a magnetic starter. A magnetic starter is a device designed to provide power to electric motors. It includes a contactor as an essential component, while also providing power-cutoff, under-voltage, and overload protection.
Examples of Motor Control
A contactor is a special type of relay used for switching an electrical circuit on or off. They are most commonly used with electric motors and lighting applications. The use of a contactor provides a level of isolation away from the high electric currents associated with those applications, protecting workers and equipment. IEC Contactors are smaller and are offered in a wide variety of sizes, whereas NEMA contactors are larger and designed with safety factors that go beyond design ratings by up to as much as 25%. IEC is a Global standard. NEMA contactors are primarily used in North America however, more companies are adopting IEC contactors, c3controls specializes in IEC.
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