Contactors Questions and Answers

What is a Contactor?

contactorA contactor is an electrical component that is similar to a relay in many regards, but that is typically used in much larger scale applications and, thus, has many features that distinguish it from a standard relay. In most cases, a contactor is designed specifically with high current usage in mind.

Another distinguishing factor that separates contactors from relays is the fact that relays can typically be employed in either of two configurations: normally open or normally closed. Most contactors are designed to be used in only the normally open position. There are variations, but this is the norm. Circuit breakers, different from both, are normally closed, but open up when dangerous conditions on the circuit to which they are affixed manifest.

Contactors are not always, but oftentimes are, quite a bit larger than relays in terms of their physical size. Small contactors are typically around the size of a person's hand. Contactors can get much larger than that, however, and are sometimes large enough to require heavy equipment to move from place to place and install.

Contactors, at the root of it, are essentially switches that can be controlled remotely. They can be designed to operate at voltages similar to the control devices that control their state or they can be designed to operate at voltages similar to the loads that they control. Because of their widespread usage in that capacity, it is safe to generally categorize contactors as industrial grade equipment that is used in circuitry that is usually much higher voltage and higher current than what most household uses could allow.

Because of the operating conditions in which contactors are used, there are vastly different considerations that go into their manufacture than go into the manufacture of their smaller cousins, relays. Most contactors, for instance, do have some sort of an arc suppression system included in their design that greatly increases their lifespan. The type of arc suppression system chosen can depend upon whether the contactor is operating on a DC current or an AC current.

Contactors are also defined by the type of switching they are provided with, described as poles and throws. This allows them to function like any other switch, sometimes changing the state of multiple circuits and sometimes only changing the state of one particular circuit.

Contactors are relatively simple devices but they are components of some of the most sophisticated machinery in the world. They provide an effective means of controlling very large-scale machinery remotely and in an automated fashion.


What are Contactors used for?

Contactors are typically used for very heavy duty industrial scale applications and their ratings systems reflect as much. They come in different classifications under IEC 60947-4-1. These classifications are expressed as AC numbers, from AC-1 to AC-4. Other classification systems are also used.

The applications of contactors include starting motors, providing control over furnaces, controlling the speed of very large motors and other large-scale applications.

Contactors not employed as magnetic blowouts are commonly utilized in devices on contactors that actually physically move the arc away from the contacts as part of the means of controlling arcing and the damage that it causes.

The amount of current to which a contactor is exposed, the materials of which it's made, the arc suppression technology utilized to protect it and other factors all go into determining the maximum lifespan of a contactor. Mechanically, however, these devices are extremely sound and, when they are provided with proper protection and utilized under appropriate conditions, they can last a very long time.

What are Contactor Timers?

Contactor timers are oftentimes used in lighting systems, allowing the lights to be turned on at specific times and turned off at others. They are devices that control the function of a contactor based on some sort of a timing device, whether it is a clock or a simple countdown timer.

Contactor timers don't necessarily have to operate under extremely high currents. In fact, many of these are simple household devices that are intended for the consumer market. Timing, however, is such an integral part of industry that timers come in all sizes and designed to work with all levels of load and current.

What are Auxiliary Contacts?

Auxiliary contacts may be part of the design of a contactor or may be added to a contactor by way of a separate component that is attached or removed as a module. Auxiliary contacts oftentimes function to provide control information to a logic controller that can signal devices to turn on or off, or to do so in specific orders, based on that input, in most applications.

The auxiliary contacts are usually mechanically affixed to the main contacts and they change state as the state of the main contacts changes. They are also used as hardware control units. On pushbutton controllers, they oftentimes are incorporated in the design of start buttons.

What do you mean by pole and throw configuration of a contactor?

Pole and throw are used in describing the number of circuits that a contactor can be hooked to and the number of different positions that the contactor can be in. The simplest type of switch is a single pole, single throw switch, which controls the on or off status of one circuit.

Altering the number of poles and throws allows contactors to be utilized to control very sophisticated circuitry. In some situations, the poles and throws on a switch are wired in unconventional ways to provide another functionality to the circuit.

What do you mean by normal state configuration of a contactor?

contactor configurationThe normal configuration of a contactor describes the disposition of the contacts when current is not applied to them. Most contactors are normally open, meaning that they close in the presence of current. There are variations, but this configuration is one of the defining characteristics of contactors and one of the things that separates them from standard relays.

The application of this is easy to understand, using a common example. In start buttons for machinery, contactors can be provided with a current when the button is depressed, and will remain closed until that current is removed. Industrial start up buttons are among the most common applications for contactors in regards to hardware controls.

What is the coil voltage of a contactor?

Coil voltage is one of the ratings under which contactors are sold. The voltage on a contactor can be alternating current or direct current. The coil voltage may be chosen to accommodate lower voltage devices or higher voltage devices, with contactors sometimes having the same voltage as the load that they are controlling.

The coil voltage of a contactor is one of the ratings that makes it appropriate or inappropriate for specific applications. The coil voltage has to be selected with the circuit being controlled in mind and in accordance with the rating of the device, going over which may cause malfunctions or premature wearing of the contactor.

On lower voltage contactors, air break suppressors and vacuum suppressors are oftentimes employed to control arcing.

What is the contact current rating?

The contact current rating is the maximum amount of current that the contacts can be utilized under without sustaining damage. The damage may be incremental damage that is acquired over time or it may be immediate failure, but operating contactors under currents that are higher than those for which they are rated can significantly reduce the lifespan of the device

Current behaves differently in different types of circuits. In an alternating current circuit, the current is alternately high and low. In a direct current circuit, the current may actually be very high and sustained at that level. This has a significant influence on arcing and how it is controlled.

What is the contact voltage rating?

The voltage rating of a contactor describes the voltage at which it is designed to function properly. Going over this rating can cause issues with the components and possible failures. Voltage ratings are sometimes referred to simply as "ratings", but all literature about contactors will specify.

Contactors are, to a great degree, defined by their typically high contact voltage ratings. These devices can handle voltages well into the high voltage range. The contact voltage rating oftentimes plays a part in the type of arc suppression system utilized on the device. For instance, high voltage contactors sometimes utilize inert gases and vacuums to control arc. Some of them used charges of compressed air. These types of suppression systems are usually found on contactors rated to work at over 1000 V.

What are Magnetic Blowouts?

magnetic blowoutMagnetic blowouts are arc suppression devices. These devices suppress arc by lengthening and moving it. These devices work with rare earth magnets, in most cases, which are integrated right into the device. The rare earth magnets produce a magnetic field, which extinguishes the arc and pushes it outwards.

Magnetic fields are particularly effective where inductive loads are concerned.

Magnetic blowouts are extremely useful designs for circuitry that carries direct-current. Direct-current tends to arc with a very high sustained current, which can damage the contacts on the contactor instantly or, with repeated exposure, over time. Alternating current has periods when the current in the arc is low, which makes it much easier to extinguish the current and prevent damage than it is on a direct-current circuit.

Magnetic blowouts are only one type of arc suppression system. Other types of arc suppression systems are selected based upon the application of the contactor, the amount of voltage it is supposed to carry and other criteria. In some cases, the danger of arcing is high enough that the contactor may actually be mounted at a safe distance from other circuitry and separated from the rest of the circuit by a breaker.

Are current contacts polarity sensitive?

On units with magnetic blowouts, the contacts are polarity sensitive. On such units, the higher potential terminal is marked with the +. On contacts that are not equipped with blowout magnets, they are not polarity sensitive.

How do you determine the currents drawn by contactor coils?

The formula for determining the currents drawn by contractor coils is VA/V.

What does trip free mean?

Trip free is a term used to describe a breaker circuit. In such a breaker circuit, if there is a fault while the pushbutton is holding the breaker in the on position, the breaker will still trip. It will remain tripped, even if the on button is held in.