Motor short circuit contribution 1

[Homerjs78]

How do you calculatie motor short circuit contributions? I know that C37.010 and 013 give impedance multipliers. For a 100hp motor the first cycle calc is 1.2*Z" but how does this translate into a short circuit current? Doesn't the motor kV come into play?

 

[dpc]

Same way you calculate it for any other source - it just doesn't last as long.

I = V/Z

The trick is figuring out which Z to use. Also these calulations are generally done in per unit or percent.

 

[JensenDrive]

Here is a tough motor contribution analysis I have wondered about occasionally:

What is the contribution from a lightly loaded motor during a slow clearing line to ground fault, while the utility and motor are still both feeding the fault, and hence the motor sees "good" voltage on two unfaulted phases? The motor contributes no ground current, but it remains in the positive and negative sequence networks. I think you have to model the induction machine like it is a synchronous generator. The motor internal positive sequence voltage driving the fault stays in sync with the power system and I would guess is maybe 1pu late in the fault, but I may be all wrong on that. I believe X1 starts low (1/Xd") and then goes to maybe 1pu late in the fault. I believe X2 stays low throughout the fault. Also, I believe the motor internal rotor voltage will lag the power system voltage, so the motor contribution to the fault is not in phase with the utility contribution.

Does any of the fault analysis software on the market handle this analysis?

 

[ijl]

JensenDrive, see

http://pp.kpnet.fi/ijl

Not literally on the market, but able to calculate the motor contribution in different cases.

 

[WDeanN]

I would appreciate any additional input on a separate thread about motor short circuit contribution to an arcing fault. It can be found here under the title "Arc Flash Motor Contribution", also known as thread238-192141.

I have been trying to calculate the motor contribution to an arcing fault for some time, but have been unable to resolve it to my satisfaction. Everything I've been told seems to be approaching it, but does not account for everything.

 

[nightfox1925]

The maximum short circuit contribution of a motor is its associated locked rotor current.

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[JensenDrive]

ijl
Interesting. I downloaded it and it is going to take a few minutes to work through it; will do it this weekend. I will send a comment/message to you via the email in the software.