35
Because instantaneous trip circuit breakers are so fast acting, the NEC actually
allows trip level settings up to 1300% of motor FLA if it can be shown to be
required by engineering evaluation. When a motor is started across the AC line
it can draw a very large peak current for the first half-cycle which can cause
nuisance tripping in instantaneous trip breakers set at 700%. GE Motors &
Industrial Systems has conducted analyses which show that certain models can
require this 1300% trip level to avoid nuisance tripping on across-the-line starts.
Generally the trip level will need to be higher for premium efficiency motors than
for standard efficiency ones.
In panels not requiring bypass the AF-600 FP drive essentially “soft-starts” the
motor by increasing the output voltage and frequency. Therefore high motor
inrush currents will not occur and should not be an issue. However, the rating
plugs for the circuit breakers in the AF-600 FP panels have been chosen to
allow trip level settings up to 13 times the current ratings of the panel. If the
trip level is set at 13 times the panel rated current and the circuit breaker still
trips it is probably due to the fact that the drive has a large DC capacitor bank
which requires charging upon applying AC power. The drive inherently contains
a charge limiting circuit but in a very small number of circumstances, the inrush
may cause the circuit breaker to trip even at a setting of 13 time the panel
rating. If this is the case, possible corrective actions are:
1)
Make sure there is not an actual short causing the trip.
2)
Try starting several times. The inrush level is a function of the point on the
AC line when the breaker is applied. Try enough times and you’ll probably
apply power at a favorable point.
3)
Try another rating plug. Perhaps the tolerances on the one included with
your breakers are unfavorable.
4)
Add an AC Line Reactor to the input of the drive, which increases input
impedance and decreases the input current.
DEH40608
– Page 9
AN 11-2021 Rev 05, 12/15/2011
