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Fuse selection for motor DOL starter - motor transient inrush current

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.  

RE: Fuse selection for motor DOL starter - motor transient inrush current

Quote:

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 watss
Yup. PF = Watts /( sqrt3 * LRC * VLL)

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RE: Fuse selection for motor DOL starter - motor transient inrush current

(OP)
Thanks for your response Electricpete.  On sheet <Toshiba 45kW motor> I used motor test data to calculated the asymmetrical rms motor inrush current.

RE: Fuse selection for motor DOL starter - motor transient inrush current

Quote:

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  ?  

Yup.   

RE: Fuse selection for motor DOL starter - motor transient inrush current

My apologies Eddy, I didn't read closely... just looked for a easy question and answered it. I haven't studied your spreadsheet.

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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RE: Fuse selection for motor DOL starter - motor transient inrush current

I guess the terms are not so confusing.  There are certainly some relays out there that try to remove the effects of the decaying dc component.  I dont' know about fuses.

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RE: Fuse selection for motor DOL starter - motor transient inrush current

The responsiveness of fuses and standard instantaneous trip units to dc offset is reflected in the NEC requirements for motor short circuit protection.  They will certainly respond to dc current.  For solid-state devices, it is more complicated.  Most digital motor protection relays such as the GE 469 can be configured to filter out the dc offset, allowing a lower setting on the instantaneous trip (assuming the instantaneous trip can be safely used).

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

(OP)
The application is a Toshiba 'High Efficiency' 45kW, 415V, 50Hz asynchronous induction motor which is normally softstarted via a Telemecanique softstarter type Altistart ATS48 C14Q which applies a constant accelerating torque to the motor during start.  The motor drives a conveyor belt.
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

I'd say your calculation looks pretty reasonable, if conservative.  As a point of comparison, the US National Electrical Code (NEC) allows the instantaneous trip to be set up to 17 times the motor FLA for energy-efficient motors.  

As to Question 2 - for a standard Motor Circuit Protector, it will respond to the dc offset current, hence the allowable NEC settings.  

 

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