Fuse selection for motor DOL starter - motor transient inrush current
Fuse selection for motor DOL starter - motor transient inrush current
(OP)
Do low-voltage short-circuit protection devices such as semi-conductor fuses and short-circuit relays with no harmonic filtering of dc offset and without intentional time delay respond to the transient asymmetrical rms motor inrush current ?
Can motor starting power-factor be determined from motor test data such as 'Locked Rotor Test at 50Hz' values of motor amps, volts, and input watts ?
My calculations in the attached excel file <45kW-Mtr-Fuse-Size-Calcs.xls> assume yes for my above questions.
The time-current curves in the attached excel file were created with PTW/CAPTOR software which enables the motor transient inrush to be plotted by entering the ratio of asymmetrical rms inrush current to locked rotor symmetrical rms current.
Can motor starting power-factor be determined from motor test data such as 'Locked Rotor Test at 50Hz' values of motor amps, volts, and input watts ?
My calculations in the attached excel file <45kW-Mtr-Fuse-Size-Calcs.xls> assume yes for my above questions.
The time-current curves in the attached excel file were created with PTW/CAPTOR software which enables the motor transient inrush to be plotted by entering the ratio of asymmetrical rms inrush current to locked rotor symmetrical rms current.





RE: Fuse selection for motor DOL starter - motor transient inrush current
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RE: Fuse selection for motor DOL starter - motor transient inrush current
RE: Fuse selection for motor DOL starter - motor transient inrush current
Yup.
RE: Fuse selection for motor DOL starter - motor transient inrush current
The juxtaposition of the terms assymetric rms motor inrush current confuses me.
If I get a chance later I'll look at the spreadsheet to see if I can figure out the question.
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Fuse selection for motor DOL starter - motor transient inrush current
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Fuse selection for motor DOL starter - motor transient inrush current
We could probably be of more help if we knew what you are actually trying to accomplish.
RE: Fuse selection for motor DOL starter - motor transient inrush current
A bypass contactor enables DOL ( direct on line ) starting.
Fast acting semi-conductor fuses ( type shown in the tcc in excel file )protect the softstarter, however during DOL start the fuses sometimes blow. The tcc shows that my calculated asymmetrical rms current of 13 x motor FLC 1/2 cycle after motor DOL start cuts the pre-arc curve ( pink ) of the semi-conductor fuse.
The semi-conductor fuse is a 160A Ferraz Shawmut cat No. 6.9 URD 000 PV 160 ( shown pink in the tcc ).
The 'motor circuit protector' breaker is a 500A Schneider magnetic only type NS160SX_MA150 ( shown blue in the tcc )
I posted the original question to ask for confirmation of : 1) my calculation of motor transient inrush asymmetrical rms current 1/2 cycle after motor start, and
2) do standard 'motor circuit protector' magnetic only breakers respond to asymmetrical rms current.
RE: Fuse selection for motor DOL starter - motor transient inrush current
As to Question 2 - for a standard Motor Circuit Protector, it will respond to the dc offset current, hence the allowable NEC settings.