Short circuit and Arc Flash motor's load factor contribution?
Short circuit and Arc Flash motor's load factor contribution?
(OP)
I am studing short circuit and arc flash analysis and I am using Etap to do some experiments. I noticed that when determining the short circuit current value whether it was 3 phase bolt, L-G, arc flash, etc, a motor's load factor did not contribute to the fault current. In other words, if the motor is loaded to 40% or 90% it does not make any difference for short cicuit analysis.
From my reasoning, the explaination for this is:
1) The purpose of the short circuit analysis is to calculate the maximum fault current, thus motors are assumed to have a 100% LF (ie, fully energized.)
2) In most cases only the motor's rotor contributes to interia/momentum. Generally speaking, a typical load does not have significant inertia that drives the motor into a generatative state. Typical non momentium loads are pumps and fans. (Regenerative systems such as conveyors going down hill, etc are an exception.)
Am I correct? I'm in learning mode, and always interested in reading your comments.
Thanks :)
From my reasoning, the explaination for this is:
1) The purpose of the short circuit analysis is to calculate the maximum fault current, thus motors are assumed to have a 100% LF (ie, fully energized.)
2) In most cases only the motor's rotor contributes to interia/momentum. Generally speaking, a typical load does not have significant inertia that drives the motor into a generatative state. Typical non momentium loads are pumps and fans. (Regenerative systems such as conveyors going down hill, etc are an exception.)
Am I correct? I'm in learning mode, and always interested in reading your comments.
Thanks :)






RE: Short circuit and Arc Flash motor's load factor contribution?
RE: Short circuit and Arc Flash motor's load factor contribution?
I'm just curious on the reasoning why the mechanical load on the motor does not contribute to short circuit? I gave an explaination conducted from my reasoning (I pondered it over lunch), I just want to know if I am right or wrong?
Granted DPC, I'm not trying to sound sour. Always a million thanks for taking the time to write back. :)
RE: Short circuit and Arc Flash motor's load factor contribution?
Fault contributions from a generator work the same - the fault contribution from a generator is not a function of the actual load on the generator at the time of the fault.
RE: Short circuit and Arc Flash motor's load factor contribution?
When the voltage falls to 0V, it will regenerate until its magnetizing flux decays. In motor theory: the magnetizing current establishes the magnetic field so that the motor can spin. Magnetizing current is constant and the amount of magnetizing current that a motor draws depends on how the motor was made and does not vary with load.
In case anyone reading this needs a visual illustration:
http
In addition, I did take some time this morning to review my reference material, in particular I enjoyed the explaination from HANDBOOK OF ELECTRICAL ENGINEERING by Alan L on pg 294:
"Induction motors react as sub-transient generators during the fault. The magnitude of the subtransient
current is normally taken as the starting current or, more specifically, determined by the air-gap emf and the sub-transient impedance of the induction motor. Since
the induction motor has no external excitation system to create flux, then during a disturbance the flux in the machine is that which is ‘trapped’ in it."
Thanks again DPC :)
RE: Short circuit and Arc Flash motor's load factor contribution?
RE: Short circuit and Arc Flash motor's load factor contribution?
If you get a chance, I am still trying to confirm the length of time that a motor would contribute to an arc fault.
If you have any further thoughts or confirmation, please take a look and post at:
http://www
Thanks,