For short, a contactor is an encapsulated relay on steroids.
Just as the encapsulated relay, a contactor has:
- a coil,
- a set of contacts and
- a test button.
However, a contactor is built for heavy duty applications.
You can read about the encapsulated relay here: About the encapsulated relay.
Knowing the contactor
A contactor should be easy to identify. You should see coil terminals, auxiliary contacts (NO or NC or a combination of these), power contacts and a small level working as a test button. Depending on the manufacturer and model, these features can be arranged in different ways.
Most models are design for rail mounting inside a cabinet. However, more expensive models may have other features such as small pilot lights or self-coupling to other components such as overload relays, to say an example.
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| Images of sample contactors |
The coil
The contactor coil can be easily identified by the A1 and A2 terminals. You should remember that depending on the model, the coil may be for DC or AC and design for a given voltage too.
In the contactor, the coil does the same work as the encapsulated relay: when it is energized a set of contacts is open or closed.
The contacts
In the contactor, two types of contacts appear,
- auxiliary contacts and
- power contacts.
Auxiliary contacts are meant for turning on/of devices of low voltage such as: pilot lights or alarms, for example. On the other hand, power contacts are used for powering equipment requiring high voltaje and representing a potential danger for an operator, such as: motors or electrical resistances, to name some.
In a contactor, the number of contacts are,obviously, specified according to the application of the component. It is common practice that auxiliary contacts are NO but one NO and one NC or only NC contacts can be looked for. You can identify the auxiliary contacts in the contactor by NO or NC legend.
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| Contacts in a contactor. Manufacturer: WEG, Model: CWB9 |
On the other hand, power contacts are always NO since these are meant for energizing an equipment. You can identify the power contacts by the letters L1, L2, etc. and T1, T2, etc.. The letters L and T stand for: line and terminal; while the numbers indicate correspondence between each terminal, this is: L1 and T1 are a NO contact, and so on. Also, all NO power contacts are independent so that L1 and T2 do not form a contact.
Specification of a contactor
Specifying a contactor is usually reduced to determine the number of NO/NC auxiliary and NO power contacts. Therefore, you should, first, determine what is to be turn on/off by the auxiliary contacts so that a ladder diagram needs to be well established before anything else.
The power contacts are related to the power diagram or circuit. You need to know if the equipment is single or triple phase or something else so that the number of poles (NO contacts) can be determined.
Rating
Contactor rating usually refers to the NO power contacts since these are the ones to be subjected to high voltage and current. These contacts need to be such that can withstand a given maximum voltage and current intensity.
Again, you need to know the electrical features of the equipment to plug into the contactor so that you can safely use the proper option.
Sometimes, rating is also related to the selection of the coil. However, the coil is selected according to ladder diagram and features of other components in the same circuit.
Other stuff of interest
- Understanding the law of Boyle
- Measuring atmospheric pressure
- Some examples of temperature instruments
- Minor losses - Formulas
- What is a process variable?
- What are the most important process variables?
- Time dependence of process variables
- A list of process variables
- Composition variables for mixtures
Ildebrando.
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