How to choose a circuit breaker for a motor? !

Typical circuit breakers use thermal-magnetic tripping, incorporating both thermal tripping and electromagnetic tripping functions. Thermal tripping is achieved by the heat-induced deformation of a bimetallic strip due to overcurrent, which in turn drives the tripping mechanism ； Magnetic tripping is achieved by the instantaneous current from the short circuit in the electromagnetic coil pushing the armature to trigger the trip. When selecting a circuit breaker for motors, it is sufficient to ensure that the magnetic trip current is greater than the starting current. Thermal tripping provides inverse-time protection: the higher the current, the faster the tripping occurs, and vice versa; essentially, thermal tripping is not necessary in most cases ； The magnetic trip mechanism of micro-circuit breaker D-type or molded case motor protection types has a tripping current that is 10–14 times the rated tripping current; it is sufficient to choose a rated tripping current that is slightly higher than the motor’s rated current. If the temperature rise inside the enclosure is high, it may be advisable to select a circuit breaker of a larger capacity, in other words, one of a higher rating.

This post was last edited by blueskyym007 on 2019-6-20 at 12:23. This requires that the setting of the circuit breaker’s quick-break function be such that it can accommodate the motor’s starting current; typically, the quick-break current should be 2 to 2.5 times the motor’s starting current. By using a circuit breaker designed specifically for motors, namely the type D trip device mentioned above, this value can generally be set. If the original poster wants to understand this in more depth, they can refer to Appendix 4, where the coefficient of 2–2.5 is considered mainly from aspects such as peak current, the operating error of the circuit breaker, and margin

The typical starting multiple for cage-type motors is 5–7, while the instantaneous trip value for D-type circuit breakers is 10–10 (15–18 for standard products). Using the most unfavorable conditions, the motor’s starting multiple is set at 7 and the instantaneous trip value for the circuit breaker at 10. Given the motor’s rated current Ib and the circuit breaker’s setting current Ib, the relationship In×10 > Ib×7×2 holds; from this, it follows that In > 1.4Ib. A factor of 1.5 is adopted here. If the instantaneous trip current is set to 2.2 times the maximum effective value of the starting current, then the value would be 1.54 times, so again a factor of 1.5 is used. This factor of 1.5 is based on empirical values, but it isn’t entirely precise. Refer to clause 2.3.5 of GB50055-2011, which states that the instantaneous trip current should be 2–2.5 times the maximum effective value of the starting current