22kW Motor With Star-Delta Starting: Selection Of Air Circuit Breaker And AC Contactor

Step 1: Calculate the rated current of the motor

Method 1: Quick estimation

Estimate by taking 2 amperes per kilowatt: 22kW × 2 = 44A The operating current of the 22kW motor is approximately 44A.

Method 2: Formula calculation

Operating Current = Power ÷ (Voltage × 1.732 × Power Factor) = 22000 ÷ (380 × 1.732 × 0.8) = 42A The operating current of the motor is 42A.

The results from the two calculation methods are very close, so the quick estimation method can be used for fast calculation of motor operating current.

Note: Motors are inductive loads. The power factor must be taken into account for current calculation; the power factor generally ranges from 0.8 to 0.9. Use 0.8 for calculation if the exact value is unknown.

Step 2: Select the air circuit breaker

Air circuit breakers are categorized into four series: Type A, B, C and D. Type A and B are rarely used, while Type C and D are the most common. Type C is for household electricity, and Type D is for power loads. Therefore, a Type D air circuit breaker shall be equipped for the motors.

The rated current of the air circuit breaker for the 22kW motor shall be 1.2 to 1.5 times the motor operating current, which gives a range of 52A ~ 66A. A D63 air circuit breaker is recommended.

Step 3: Select the AC contactor

The rated current of the AC contactor shall be 1.5 times the motor operating current: 44A × 1.5 = 66A Choose an AC contactor with a rated current higher than and closest to 66A, i.e., a 65A contactor.

Conclusion

Based on the above calculations, a 3-phase Type D63 air circuit breaker and a 65A AC contactor shall be matched for the 22kW motor.