What size of air switch should be installed for a three-phase device?

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What size of air switch should be installed for a three-phase device? The selection of the size of the air switch for a three-phase device mainly depends on factors such as the power of the device, the rated current, and the starting current. The following are the specific calculation and selection methods:

### Calculation of the rated current of the three-phase device

According to the three-phase power formula \(P = \sqrt{3}UI\cos\varphi\), the formula for calculating the rated current \(I\) can be derived as \(I=\frac{P}{\sqrt{3}U\cos\varphi}\), where:

- \(P\) is the power of the device, in watts (W).

- \(U\) is the line voltage, which is usually 380V in a three-phase four-wire system.

- \(\cos\varphi\) is the power factor. Generally, the power factor of inductive loads such as three-phase motors is between 0.75 and 0.9, and the power factor of resistive loads (such as electric heaters) can be regarded as 1.

For example, if a three-phase motor has a power of 10kW and a power factor of 0.85, its rated current \(I=\frac{10000}{\sqrt{3}\times380\times0.85}\approx17.9A\).

### Consideration of the starting current

- For devices such as motors, the starting current is usually several times the rated current. Generally, the starting current of a cage-type asynchronous motor is 5 to 7 times the rated current, and the starting current of a wound-rotor asynchronous motor is 2 to 2.5 times the rated current.

- If the above 10kW motor is a cage-type asynchronous motor, calculated with the starting current being 6 times the rated current, then the starting current \(I_{starting}=17.9\times6 = 107.4A\).

### Selection of the air switch

- **Rated current of the air switch**: It should generally be greater than the rated current of the device, and at the same time, it is necessary to consider being able to withstand the starting current of the device. For the above motor, the rated current of the air switch can be selected as 25A or 32A. In this way, it can work reliably during the normal operation of the device and will not be tripped by mistake during starting.

- **Tripping characteristics of the air switch**: The appropriate tripping characteristics should be selected according to the characteristics of the device. For example, for inductive loads such as motors, an air switch with appropriate overload protection and short-circuit protection characteristics should be selected. Generally, an air switch with a C-type or D-type tripping curve is selected. The C-type tripping curve is suitable for general loads, and the instantaneous tripping current is 5 to 10 times the rated current; the D-type tripping curve is suitable for power loads, and the instantaneous tripping current is 10 to 20 times the rated current. For motors, it is usually more appropriate to select the D-type.

In practical applications, factors such as the length of the line, line losses, and ambient temperature that affect the selection of the air switch also need to be considered. If you are not sure, it is best to refer to the technical manual of the device or consult a professional electrical engineer.