In the simplest definition, the contactor is an electrical switching device used to turn an electrical circuit on and off.

The contactor is a special type of relay, but there is a basic difference between the contactor and the relay, which is that the contactor is mostly used in devices with higher flow tolerance capacity, while the relay is used for devices with lower current and the contactor has an interconnected structure and can be easily installed.


Types of contactors:

Contactor (DC Current):


Disconnection of ohmi loads, non-selfie loads or with poor selfie feature, electric heater


Setting up the DC motor shunt stimulation without the number of times of disconnection, disconnection of the motor when working


Setting up a DC motor shunt stimulation with a high number of disconnections at low time intervals, brake circuit


Set up dc motor excitation series without high frequency of disconnection, disconnection of motor feeding when working


Set up dc motor excitation series with high frequency of disconnection at low time intervals, brake circuit


Auxiliary contactor, lacks power blades and lacks magnetic coupling. These contactors are installed on the main contactors and used only in the steering circuit.

Contactor (AC current):


Setting up a three-phase asynchronous shelf rotor engine, with opposite current brakes, changing the direction of the engine rotation, with high number of disconnections at low distances


Auxiliary contactor, lacks power blades and lacks magnetic coupling. These contactors are installed on the main contactors and used only in the steering circuit.


Contactor used for medium voltage.


Contactor used for capacitive bank.


For pure lighting or ohmy loads


For non-motorized loads (e.g. transformers)


For motor loads without brakes and winding rotors


For motor loads with brakes or cage rotors

Types of voltage characteristics of contactors:

Ue rated voltage:

The nominal voltage indicates the maximum allowable voltage of contactor blades in the nominal current. Of course, nominal voltage is one of the effective parameters in contactor selection, proportional to the needs of the


industry used and the load-feeding voltage. For three-phase loads, this voltage indicates the voltage of the line and for single phase loads indicates the phase voltage.

Ui insulation voltage:

The insulating voltage of the contactor indicates the maximum permissible voltage tolerable by the contactor.

If this maximum voltage is applied to the contactor at line times in the system, it will not cause any failure in the insulation of contactors and other contactor components.


Errors such as asymmetric short circuit that increase voltage in healthy phases or higher voltage feeder connection to contactor feeder can cause voltage increase error.

Uc Bobin’s nominal voltage:

The nominal voltage of the bobin indicates the permissible voltage required to stimulate the contactor bobin.

When choosing a contactor according to the conditions of the controlled process and various requirements such as safety of voltage of the steering circuit and control section is selected.

This voltage represents the nominal voltage of the required contactors.

As mentioned earlier, bobin contactors are produced in two types of AC and DC with rated voltages of 380V, 220V, 110V, 48V, 24V.

Types of flow characteristics in contactors:

Name stream:

A named flow is a stream that can pass through the contactor permanently according to the type of load and is regularly disconnected by the contactor without any damage to the contactor blades and the disconnection


Ith2 Permanent Current:

A permanent current, which is greater than the contactor’s named current, is a current that under normal conditions can pass through contactor blades permanently without damaging the contactor.

Ith1 Weekly Stream:

A weekly flow is a stream that can pass through contactor blades for a maximum of one week.

Ith eight-hour stream:

An eight-hour flow is a current that can pass through contactor blades for a maximum of eight-hour shifts.

Is impact short circuit current:

In the time of short circuit before the protective devices attempt to cut off the fault current, contactors, like other equipment such as cables, bass bars, etc.

, should be able to remove the moment currents that pass through the contactor blades during short circuit without any damage such as welding contactors to platinum or due to mechanical stresses and dynamic stresses.

Do not change form due to the short circuit of the blades. The maximum current at the moment of short circuit is called the short circuit current of the impact and displays it with Is.

Also, the maximum effective amount of short circuit current that the contactor can withstand is called low time flow or one second flow.

Advantages of contactors over manual keys:

1- The consumer is remotely controlled.

2- The consumer is controlled from the location.

3- The speed of disconnection and disconnection of the key is high and its depreciation is low.

4- Their effective life is greater.

5- It is possible to design automatic steering circuit for different stages of consumer needs.

6- They are safer in terms of protection and have more appropriate and complete protection.

7- When the power outage of the consumer circuit is also interrupted and needs to restart, thereby preventing the risks of sudden power plugging of the device.

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