When starting high-power motors, is it not recommended to use dedicated circuit breakers? !
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I see that there are very few circuit breakers designed specifically for Delexi motors of 30KW and above. Is it true that for high-power motors, it is more advisable to use an inverter for starting? !
It’s simply impossible to use an inverter if the process does not require speed control. It’s not the case that just because the power is high one should use an inverter – then how many inverters would be needed? Soft starters, star-delta starters, autotransformers, and other methods can be used for starting. The latter two are hardly used these days; soft starters are the ones that are employed instead.
I’m not sure what you mean by a dedicated circuit breaker Is it the built-in thermal protection of the circuit breaker? In general motor protection circuits, the circuit breaker isn’t much different from ordinary ones; the only difference is the multiple of the setting current, which allows the motor to overcome the starting current during startup. Starting has little to do with the frequency converter; as long as the transformer capacity and the other loads on the busbar allow it, a motor of any capacity can be started directly.
If the transformer capacity is sufficient and frequency conversion is not required by the process, motors of 200–300 KW can be started directly. Molded case circuit breakers can be used; for higher capacities, we may opt for vacuum contactors to reduce the failure rate of these contactors. I’ve heard that for molded-case circuit breakers, it’s necessary to choose one with a capacity three to four times that of the motor; otherwise, tripping occurs easily during startup. This applies to brands like Delta, as well as counterfeit and low-quality products. We have never encountered the normally open type or those from Schneider. Or, process operation errors, such as starting under load or even locked-rotor starting, can also cause a start-up trip; in such cases, it is not good if no trip occurs.