What is the calculation formula for contactor selection?

A contactor is a device that can quickly cut off the AC and DC main circuits and can frequently connect and disconnect high current control (up to 800A) circuits. Therefore, it is often used in motors as control objects. It can also be used to control power loads such as factory equipment, electric heaters, machine tools, and various power units. The contactor can not only connect and disconnect the circuit, but also has a low voltage release protection function. The contactor has a large control capacity and is suitable for frequent operation and remote control. It is one of the important components in the automatic control system.

The calculation formula for contactor selection includes the following aspects:

1. Current load capacity calculation

According to the requirements of the contactor load capacity, the current load capacity of the controlled equipment is calculated, generally using the following formula:

I_load = P_load / U_load

Among them, I_load is the load current, P_load is the load power, and U_load is the load voltage.

2. Input signal voltage calculation

According to the voltage requirements of the contactor control input signal, the voltage of the input signal is calculated. Generally, the following formula is used:

U_in = U_coil × k

Among them, U_in is the input signal voltage, U_coil is the working voltage of the contactor, and k is the proportional coefficient of the input signal voltage to the working voltage.

3. Calculation of contactor power loss

During operation, the contactor will have a certain power loss. According to the rated current and voltage of the contactor, the power loss of the contactor is calculated. Generally, the following formula is used:

P_loss = I_nom × I_nom × R_on

Among them, P_loss is the contactor power loss, I_nom is the rated current value of the contactor, and R_on is the resistance value of the contactor.

4. Calculation of switching time

The switching time of the contactor needs to be calculated according to the actual application situation, generally using the following formula:

t = L × (R_on + R_load) / U_coil

Among them, t is the switching time of the contactor, L is the self-inductance of the control coil, R_load is the load resistance, and U_coil is the operating voltage of the contactor.

It should be noted that the selection calculation formulas for different types of contactors may be slightly different, and the specific selection should be made based on actual conditions.

The service life of the contactor depends on its quality, the use environment, the load current during use, the frequency of use and other factors. Generally speaking, high-quality contactors can ensure a longer service life. At the same time, under low load current and low frequency conditions, the service life of the contactor will also be longer. The service life of the contactor is generally marked in the product manual, and users can choose the appropriate contactor according to actual conditions. During use, regular cleaning and maintenance can extend the service life of the contactor.