Question ..LRC and starting current article 430

[haitham]

Hello Guys ,
I have a question regarding article 430 ,determining the starting current ..according the tables 430.7B , 430.251a,b ..

at first , LRC is the same as starting current of motor when starting direct online ,is that right ?
i want to know how to use the tables mentioned above to get the starting c. value

when could we use each table ?
and for example: 5 hp 415 3 ph motor ,marked A locker rotor at N.p

if i depend on table 430.7B it will be between 1:3.14*Hp
but when using table 430.251b it will be more than this value may around 6*Hp

another note table 430.251a,b related to B,C,and D motors design so ,what about A Design ?

Thanks for ur time

 

[GoldDigger]

Actually there can be a one or two cycle inrush current (with DC offset) which is higher than LRC by a factor of two or more, but it will not be repeatable as is depends on both the remanent magnetization from the last time the motor was run and the voltage phase at which the contactor closes.
The code does not deal with this explicitly, but it has to be tolerated by the instantaneous (magnetic) trip of any breaker involved.
Paradoxically, more efficient motors will have generally have higher LRC and inrush than a less efficient motor of the same HP rating.

 

[mustafa]

[FONT=Calibri,Arial,Helvetica,sans-serif][FONT=Calibri,Arial,Helvetica,sans-serif]
[FONT=Calibri,Arial,Helvetica,sans-serif][FONT=Calibri,Arial,Helvetica,sans-serif]• Motor-starting current. When voltage is first applied to the field winding of an induction motor, only the conductor resistance opposes the flow of current through the motor winding. Because the conductor resistance is so low, the motor will have a very large inrush current (see Figure above).[/FONT][/FONT]• Motor-running current. Once the rotor begins turning, there is an increase in counter-electromotive force that increases the impedance and thus reduces the current from the starting amperage to running amperage.

• Motor locked-rotor current. If the rotating part of the motor winding becomes jammed so it can’t rotate, no counter-electromotive force will be produced in the motor winding. This results in a decrease in conductor impedance to the point that it’s effectively a short circuit. What’s the result? The motor operates at locked-rotor current, often six times the full-load ampere rating, depending on the motor Code Letter rating [430.7(B)]. This will cause the motor winding to overheat and be destroyed if the current isn’t quickly reduced or removed[/FONT][/FONT]

 

[haitham]

so ,according to NEC how to calculate the starting current and inverse time Cb for
5 hp 415 3 ph motor ,marked A locker rotor at N.p ?

 

[haitham]

Actually there can be a one or two cycle inrush current (with DC offset) which is higher than LRC by a factor of two or more, but it will not be repeatable as is depends on both the remanent magnetization from the last time the motor was run and the voltage phase at which the contactor closes.
The code does not deal with this explicitly, but it has to be tolerated by the instantaneous (magnetic) trip of any breaker involved.
Paradoxically, more efficient motors will have generally have higher LRC and inrush than a less efficient motor of the same HP rating.

u mean motors with higher efficiency have higher LRC ? I think it have to be lower

 

[haitham]

• Motor-starting current. When voltage is first applied to the field winding of an induction motor, only the conductor resistance opposes the flow of current through the motor winding. Because the conductor resistance is so low, the motor will have a very large inrush current (see Figure above).• Motor-running current. Once the rotor begins turning, there is an increase in counter-electromotive force that increases the impedance and thus reduces the current from the starting amperage to running amperage.

• Motor locked-rotor current. If the rotating part of the motor winding becomes jammed so it can’t rotate, no counter-electromotive force will be produced in the motor winding. This results in a decrease in conductor impedance to the point that it’s effectively a short circuit. What’s the result? The motor operates at locked-rotor current, often six times the full-load ampere rating, depending on the motor Code Letter rating [430.7(B)]. This will cause the motor winding to overheat and be destroyed if the current isn’t quickly reduced or removed

so ,according to NEC how to calculate the starting current and inverse time Cb for
5 hp 415 3 ph motor ,marked A locker rotor at N.p ?

 

[GoldDigger]

u mean motors with higher efficiency have higher LRC ? I think it have to be lower

Higher efficiency normally means lower resistance in the windings. That in turn leads to higher inrush and LRC since those as primarily limited by the DC resistance.
Once the motor is at full speed the current is limited by the reverse EMF and the inductance and the DC resistance is the primary energy loss mechanism.

 

[haitham]

Higher efficiency normally means lower resistance in the windings. That in turn leads to higher inrush and LRC since those as primarily limited by the DC resistance.
Once the motor is at full speed the current is limited by the reverse EMF and the inductance and the DC resistance is the primary energy loss mechanism.

ok,
so according to NEC how to calculate the starting current ,by the code letter marked on the motor as some people are doing , or by tables 430.251