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In this article we will cover some basics about the electrical device called a contactor, without going into details. We will try to explain clearly what it is, we will talk about the different types of contactors, about the way they work its applications, and the most important features of this device. Without trying to copy Wikipedia we will give a brief overview, including images, addressed to non-specialists, with the main purpose to improve the general technical knowledge of the readers of our humble blog.


The Contactor is an on-off electrical device used for frequent remote switching of power circuits in normal working mode. The closing and opening of the contacts of the contactor is most often done by an electromagnetic actuator field.


The Contactor consists of main contacts, auxiliary contacts, a coil, magnetic cores, a cage and an arc-extinguishing system. The coil and the magnetic core, consisting of a fixed Ш -shaped part and a movable part (armature), together with their fastening elements, form the electromagnetic system of the contactor. The contactor has a main, power circuit and an auxiliary, control circuit. There are also contactors for small current and in this case there are no arc-extinguishing cameras or auxiliary contacts.

Structure of a three-phase contactor.
Structure of a three-phase contactor.

1. Coil  2. Spring  3. Movable part (armature) 4. Contact system

Operating principle

When  the control circuit is closed, the current flows through the coil of the contactor and creates electromagnetic field in and around the magnetic core, which is directed in such a way so that it can attract the armature. The armature is connected with the movable power contacts which close, and thus the current starts flowing through the power circuit. In this way  can be implemented remote control of electric motors or other devices which have higher voltage and current than those of the control loop. The contactor in general has no mechanical means of holding /cannot hold/ support the power contacts in switched-on position in case the control voltage is switched off, therefore in the absence of control voltage the contacts return in opened position. In order to keep the power contacts in switched-on position, various types of mechanical retaining stoppers are implemented.


Contactors are used for a remote frequent switching of powerful consumers. Contactors follow the principle of the relay, but the values with which they operate are much higher than those of conventional electromagnetic relays and can extend from several amperes to hundreds of amperes and several kV voltage. In combination with a thermal relay the contactors are used to protect the electrical system from overvoltage. DC contactors have various applications such as in electric and diesel locomotives, electric trains, trams, trolleybuses, elevators, lifts, etc. Alongside, they are suitable for compensation of reactive power and switching of large permanent powers.

Contactors types

The type of the contactor depends on:
– the current type – Altering Current contactors and Direct Current contactors (direct current contactors are designed for switching DC circuits and are usually triggered by DC solenoid , AC contactors are designed for switching AC circuits and their electromagnets can be both AC and DC)
– the poles number – from 1 to 8 poles
– the main circuit rated current – from 1.5 to 4800A
– the main circuit rated voltage – from 27 up to 2000 VDC, from 110 to 1600 VAC with frequency 50 , 60, 500 , 1000, 2400 , 8000, 10 000 Hz;
– the frequency
– the working conditions , etc.


How to select a contactor

When selecting a contactor it is important to consider the following main characteristics:
1. The current and voltage of the power circuit, power of the consumer and the of load type (active or reactive), the presence of input current.
2. The operational circuit current and voltage, type and size.
3. Working mode, frequency, presence of repeated short loads.
4. Number of poles
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