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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.

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    \$\begingroup\$ I believe that starting current inrush is higher than locked rotor for the first half cycle or several half cycles. Can the fluke capture the first half-cycle? I suspect that the magnetic trip is sensitive to the first half cycle. \$\endgroup\$ – Charles Cowie Jan 10 '17 at 19:01
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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

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  • \$\begingroup\$ I don't think the fluke clamp meter could capture anything that fast. Final setting that worked was about 15x FLA, 1900A. From what ive read now it depends on the curve of the breaker. \$\endgroup\$ – DrTarr Jan 11 '17 at 1:33
  • \$\begingroup\$ I would really worry that there is an underlying problem that is making the current imbalanced causing the high peak inrush current. I think you should very carefully check the current balance during normal running. \$\endgroup\$ – Charles Cowie Jan 11 '17 at 2:15
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Across the line starting of motors over 100 HP is very stressfull to electrical distribution systems. Listen carefully and you will probably hear the conductors jump in the conduit as the motor is started across the line. Adding a softstart is a good economical way to soften the high starting loads on your overcurrent device and electrical distribution system. A VFD is even better if you can afford it.

http://www.schneider-electric.com/en/product-subcategory/2940-soft-starters/

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  • \$\begingroup\$ For sure, initially we thought there was a real trip condition but not the case. \$\endgroup\$ – DrTarr Jan 11 '17 at 1:35
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15x FLA is not at all unusual with modern energy efficient motors. This is not STARTING current, this is MAGNETIC INRUSH current that flows only for the first cycle (at most) as the magnetic flux is created in the motor windings and core. For that first cycle, there is no mutual induction yet, which is what accounts for impedance. So initially, the only thing impeding the flow of current in each coil is the very low resistance of the copper wire itself; it is in essence a "short circuit" for that instant.

As motor mfrs were required to improve energy efficiencies in the 1990s, one way they reduced I^2t losses in the motor was to reduce the coil winding resistances. That resulted in much higher instantaneous magnetic inrush currents. THAT is why the NEC allowed for the increased values starting a few years back. But many older textbooks and even some newer on-line resources have not adequately kept up with that change in design philosophy, leaving people confused as to why older setting rules seem to no longer work.

Bottom line, don't worry about it.

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