Design and selection issues for low-voltage electrical appliances
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I’m currently facing a problem of moderate severity. It’s a 380V low-voltage motor with a rated power of 75KW and a rated current of 139.7A. The air switch selected for use was from ABB, model TMAX T3S 250; the rated current of its trip device is 160A, while the instantaneous peak current is 989A. However, the motor causes the air switch to trip during no-load testing, preventing it from being tested properly. I checked the design drawings, and it seems that almost the same type of air switch was chosen in all cases. Those with experience say that the selected air switch is too small; some suggest a factor of 1.2–1.3, while others recommend a factor of 1.5–2. I’m not sure if this is indeed the case. I hope experienced experts can help me figure this out. Thank you!
Additional note: Someone with experience told me that the circuit breaker should be selected with a rating of more than 200A; 250A or 300A options work well as well, which will prevent the breaker from tripping. However, after checking drawings from two different projects with various designs, including those from some of the top design firms in the petrochemical industry, it seems there is no issue with the choice of circuit breakers
The phenomenon you mentioned is likely a short-circuit shutdown; it’s best to check and confirm before taking any action. The circuit breaker selection is correct. The quick-break setting for low-voltage circuit breakers is conventionally 10 times the rated load current. When the motor starts directly, the starting current is 4-7 times the rated current, which the circuit breaker can easily handle.
At first, I also suspected it wasn’t a switch issue, but the motor and cable insulation were fine. Could it be due to the mixing of TN-S and TN-C systems?
Finally, the Huajian LM311 controller used has a function to prevent reverse phase sequence; it cuts off power immediately if the phase sequence is reversed.
What is the relationship between comprehensive protection and circuit breaker tripping?
Try using another identical circuit breaker; if there’s no problem, then it’s a problem with the motor
Use an insulation megger to measure the insulation to ground
I personally think the instantaneous current value is a bit low; 7 times the rated current of 140A gives 980A, which is quite close to the instantaneous tripping current
The instantaneous trip device’s current element failed to withstand the peak current; the rated current is 139.7, the starting current is assumed to be 4, while the peak current is 558. Multiplying this by a reliability factor of 2 gives 1117. Check the setting value for the instantaneous operation of the circuit breaker.