Back to Basics

A contactor can stand on its own as a power control device, or as part of a starter. Contactors are used in applications ranging from the light switch to the most complex, automated industrial equipment. Contactors are used by electrical equipment that is frequently turned off and on (opening and closing the circuit), such as lights, heaters, and motors.

Whatever the application, the function of the contactor is always the same: to make and break all power supply lines running to a Load. Or, as defined by NEMA, to repeatedly establish and interrupt an electrical power circuit.

Knife Blade Switches

The first device used to stop and start electric motors was a simple Knife Blade Switch. This was a lever that would drop a strip of metal onto a Contact to make the electric circuit. In the late 1800’s, “throwing the switch” meant exactly that –someone had to stand next to the knife blade switch and lever it into the closed position.

When industry began to demand more powerful electric motors, the knife blade switch quickly became obsolete and was no longer used. Why?

Figure 2. A Knife Blade Switch Used to Control a Motor

Engineers discovered that the contacts quickly wore out because humans could not open and close the switch fast enough to prevent Arcing. Arcing, a condition where high voltage leaps across the open space as the contacts closed in or pulled away from the switch , corroded the soft copper switches with pits. Dirt and moisture compounded the problem.

More importantly, as motors became larger, the currents to operate them also had to become larger, creating a serious safety concern. It was physically dangerous to handle the switch. Willing knife blade switch operators became harder and harder to find.

Mechanical improvements were made, but with their dangerous operation and short contactor life, knife blade switches remained at a design dead-end. The knife blade switch certainly wasn’t the best solution, but from it, engineers learned what issues needed to be addressed:

Manual Controller

The manual controller was the next step up the evolutionary ladder , offering several important new features:

Double-break contacts open the circuit in two places simultaneously. Dividing the connection over two sets of contacts allows you to work with more current in a smaller space than you get with a single-break contact. In addition, the mechanical linkage more quickly and consistently opens and closes the circuit, sparing the metal from some of the arcing experienced under knife blade switches.

Figure 3. Double-Break Contactor

With a manual controller, the operator presses a button or moves a switch that is integral to the electrical equipment being run. In other words, the button or switch is physically attached to the controller itself, and is not operated remotely.

When an operator activates a manual controller, the Power Circuit engages, carrying the electricity to the load .

The manual contactor was a big improvement over the knife blade switch. Variations of manual contactors are still in use today.

Figure 4. Two Typical Manual Contactors Still in Use Today

Magnetic Contactors

Engineers eventually made a breakthrough with the magnetic contactor.

A magnetic contactor is operated electromechanically without manual intervention. This means that the contactor can be operated remotely, without the need for putting a person in a potentially dangerous location. Magnetic contactors use a small control current to open and close the circuit.

NOTE: From this point forward, the term “contactor” will refer only to an AC magnetic contactor.

Contactor Components

If you sat down and took apart a contactor as shown in Figure 5, you would find the following components: an Electromagnet (E-frame), an Armature, a coil , a spring , and two sets of contacts , one movable set and one stationary set.

Figure 5. Magnetic Contactor Components

How the Contactor Operates

So, how exactly does the contactor open and close? The E-Frame , when energized by the coil , becomes an electromagnet. The armature , a companion to the E-frame, is connected to a set of contacts . The armature is moveable but is held by a spring .

When the coil is energized, the moveable contacts are pulled toward the stationary contacts because the armature is pulled toward the E-frame . Once the two sets of contacts meet, power can flow through the contactor to the load.

When the coil is de-energized, the magnetic field is broken, and the spring forces the two sets of contacts apart.

In Figure 6, we step through the process again, using pictures to help you understand.

Contactors are used when no overload protection is necessary, and at lower levels of electrical current. Applications include lighting circuits, heaters, and transformers.

In summary, contactors operate electromechanically and use a small control current to open and close the circuit . (We will discuss control current in more detail in the section on starters.) The electromechanics do the work, not the human hand, as in a knife blade switch or a manual controller.

Pushbuttons and selector switches, like the ones on this control panel, are used in hundreds of manufacturing industries.

Each button and switch is connected to a contactor, for use in making or breaking an electrical circuit remotely.

Figure 6. How a Magnetic Contactor Operates

Contact Life: Arcing

A major customer concern is the life expectancy of a contactor. It has been said that, “The worst thing you can do to a car is start it.” The same is true for contacts. The more frequently the contacts are opened and closed, the shorter the life of the contactor.

As contacts open and close, an electrical arc is created between them. The arcs produce additional heat, which, if continued, can damage the contact surfaces.

Figure 7. Arcing Causes Corrosion, Pitting and Oxide Build-Up

Eventually, the contacts become blackened with burn marks and pitting made by the electrical arcs. This is not a reason for contact replacement. In fact, this black deposit (Oxide) helps them to make a better “seat” to conduct the electricity. However, contacts do need to be replaced once the surface is badly corroded or worn away.

Contact Life: Bouncing

Applying some logic, you can conclude that the faster the contact closes, the sooner the arc is extinguished, and the longer the life expectancy of the contact . But, modern contactors have been designed to close so quickly and with such energy that the contacts slam against each other and rebound, causing a bouncing action . This is referred to as Contact Bounce. When the contact bounces away, a secondary arc is created. The contacts slam together again and again, each time the bouncing and arcing become less and less.

So, in addition to closing the contacts as fast as possible, you also want the contacts to bounce as little as possible, to reduce secondary arcing and wear.