I know this is an older thread, but came across it as am using a 3-phase overload on a single-phase motor, and the quote caught my attention:
Also: They say to set the 3-phase trip at the motor FLA, but then that the devices are de-rated for single-phase. If the trip curve is lower, then how should they be set? Still at the same current? Again, it seems from a heating standpoint that there should be no difference.
Why is this exactly? I'm looking at using an ABB MS132 motor starter for a 7.5hp single-phase motor (FLA ~21A). As described above, ABB gives de-ratings for their starters when used for single-phase. To see if the de-rating has to do with the shifted phases, I calculated and plotted instantaneous power over time in all contacts using the contact resistance given in their catalog. This is assuming a single-phase is looped 3 times through the overload, and in the normal 3-phase case. The instantaneous power over time fluctuates significantly in the single phase case compared to the 3-phase (assuming all power is dissipated into a single element), but integrating to get the energy put into the trip element gives the same result in both cases (assuming same current). So why would single-phase trip earlier?The trip curve is slightly lower so that it will trip at a lower current value than if all 3 phases are present.
Also: They say to set the 3-phase trip at the motor FLA, but then that the devices are de-rated for single-phase. If the trip curve is lower, then how should they be set? Still at the same current? Again, it seems from a heating standpoint that there should be no difference.
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