How to select a contactor according to the motor power

How to select a contactor according to the motor power. When selecting a contactor according

 to the motor power, the following aspects should be mainly considered:

Calculate the rated current of the motor

- For a three-phase AC motor, the formula for calculating its rated current is: \(I = \frac{P}{\sqrt{3}×

U×cosφ×η}\), where \(P\) is the motor power (unit: \(W\)), \(U\) is the rated voltage (unit: \(V\)), 

\(cosφ\) is the power factor, and \(η\) is the motor efficiency. Generally, the power factor of the 

motor is between \(0.75 - 0.9\), and the motor efficiency is between \(0.8 - 0.95\). If it is a single-phase 

AC motor, the formula for calculating the rated current is: \(I = \frac{P}{U×cosφ×η}\).

Select the rated current of the contactor

- The rated current of the contactor should be greater than or equal to the rated current of the motor. 

Under normal circumstances, the rated current of the contactor can be selected as \(1.2 - 1.5\) times

 the rated current of the motor. For example, if the rated current of a motor is \(20A\), then the rated 

current of the contactor to be selected should be between \(20×1.2 = 24A\) and \(20×1.5 = 30A\), and 

a contactor with a rated current of \(32A\) can be selected.

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- When the motor starts, brakes, or rotates forward and backward frequently, due to the large starting 

current, the rated current of the contactor should be appropriately increased, and it can be selected as 

\(1.5 - 2.5\) times the rated current of the motor.

Consider the operating environment of the contactor

- If the operating environment temperature is high, for example, exceeding \(40℃\), the rated current of

 the contactor should be derated for use. Generally, for every \(10℃\) increase in the ambient temperature, 

the rated current of the contactor should be reduced by \(8\% - 10\%\).

- If the contactor is installed in a dusty, humid, or corrosive gas environment, a contactor with the

 corresponding protection level should be selected to ensure its normal operation and service life.

 Determine the type of the contactor

- Select the corresponding contactor according to the voltage level of the control circuit, such as AC \(220V\),

 \(380V\) or DC \(24V\), etc.

- Select the contactor according to the load nature of the motor, such as resistive load, inductive load 

(the motor belongs to an inductive load), etc. For inductive loads, a contactor with strong arc extinguishing 

ability should be selected.

Check the making and breaking capacity of the contactor

- The making capacity of the contactor should be greater than the starting current of the motor, and the 

breaking capacity should be greater than the maximum short-circuit current that may occur during the 

operation of the motor. Generally, the technical parameters of the contactor will give the indicators of its

 making and breaking capacity, which can be checked according to the relevant parameters of the motor.

 If the starting current of the motor is large or the short-circuit current is large, a contactor with strong 

making and breaking capacity should be selected.