When a 22kW motor is starting normally via frequency conversion, the selection of the protective air circuit breaker can be calculated as follows:
First, a rough calculation based on empirical values.
1. Calculated relative to empirical power values.
(1) For motors or electrical equipment with a power of 3KW or above: Rated current value = 2 × power of electrical equipment.
(2) For motors or electrical equipment with a power of less than 3KW: Rated current value = 2.5 × power of electrical equipment.
2. Calculated according to the relative value of voltage level.
(1) The electrical equipment uses 380V voltage, 2A per kilowatt.
(2) The electrical equipment uses 660V voltage, with 1.2A per kilowatt.
(3) The electrical equipment uses 3000V voltage, 4 kW 1A.
(4) The electrical equipment uses 6000V voltage, 8 kW 1A.
II. Calculation using the formula:
General calculation formula for three-phase motors:
P = 1.732 × IU × power factor × efficiency.
Third, now that we understand the calculation method, let's calculate the current.
1. Empirical calculation: 22 × 2 = 44 currents
2. In the formula calculation, P = 1.732 × IU × power factor × efficiency (both power factor and efficiency are taken as 0.9), U = 380, P = 22000. Substituting into the formula, we can calculate the current I = 41.2 currents.
Third, the selection criteria for circuit breakers: Generally speaking, the rated current operating value I of the circuit breaker should be 1.2 to 1.5 times the calculated current in order to better exert the protective function of the circuit breaker.
Therefore, we can conclude that:
1. Rough calculation based on experience: air switch I = 1.3 × 44 = 57.2A.
2. According to the calculation formula, the rated current of the air switch is I = 1.3 × 41.2 = 53.5A.
IV. Several suggestions for selecting circuit breakers:
1. Generally, it is best to choose a type D air circuit breaker with a rated operating current of 63A.
2. Referring to the inverter's operating instructions, the air switch installed on the inverter is generally selected to be 1.5-2 times the rated operating current, so selecting a 100A air switch is quite normal.
3. Install only air circuit breakers on the frequency converter. It is not recommended to use residual current circuit breakers, as they may trip frequently due to leakage.
4. The frequency converter itself has a very complete protection mechanism for motors and electrical equipment, so it generally does not need to add leakage protection devices.
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