I have separate interrupting devices in an MCC bucket. One is a magnetic trip-overload 250A breaker, where the lowest trip setting is 1100A. The other is an electronic device that has 3 CT's and gives me options of trip class, ground fault, phase imbalance, etc…
A 90kW, 3 phase 480V uncoupled motor is tripping the magnetic overload. Initially I thought phase imbalance (hence other post) but I realized that would only appear in the electronic trip. With a fluke meter we recorded peak currrents of 200A, 422A, and 448A, none of which were close to the minimum 1100A instantaneous trip. However, an old timer explained that the magnetic trips are extremely sensitive and have to be such much higher. Normally when I think of a trip value I think of 6x rated current for full-locked rotor value, but for magnetic trips it appears this is not the case. We bumped the settings up to 1700A and it resolved the issue.
In this situation, is there any type of rule of thumb for choosing a successful magnetic trip setting (example, 10-15x FLA?), or at least a start point, for a 3 phase motor of this size?
It seems after incrementing to 1700A, and then 1900A, the tripping condition still exists intermittently. The breaker tripping is a Siemens FXD6-A (FXD63A250 C Series). I checked current while running and I had less than 10% imbalance, looked pretty normal, running less than FLA (114, 106, 106). Right now I'm looking for the trip curve.
Best Answer
The USA National Electrical Code Table 430.52 and Section 430.54 say that the the maximum setting for an instantaneous-trip breaker shall be no higher than 800% of FLA for motors other than NEMA design B high efficiency motors and no higher than 1100% for NEMA design B high efficiency motors. If that is not high enough to start the motor, the setting can be increased to 1300% or 1700% for the respective motor designs. I did not look for guidance regarding IEC motors.
I believe that starting current inrush is higher than locked rotor for the first half cycle or several half cycles and that magnetic circuit breakers can trip with one half-cycle of current above the trip rating.
I found a paper that showed a trace of motor starting current with the first half cycle peak with an RMS value of 7.4 X FLA. After that, the starting current was within 10% of 5.3 X FLA until it began to decline to normal.
M. J. Melfi and S. D. Umans, "Squirrel-Cage Induction Motors: Understanding Starting Transients," in IEEE Industry Applications Magazine, vol. 18, no. 6, pp. 28-36, Nov.-Dec. 2012