Effect of Motor SC Contribution to Main Fault Current

[vincec]

I just watched one of Mike's listed video's "Available Fault Current Label". In the video Mike used the example of a 112.5 kva/208 v transformer, in his example with 3.2 % Z the fault current was close to 9,700 amps. Someone in the video brought up motor contribution and brought up changing out a motor with a 100 hp motor. The fault contribution of a motor is 5 X FLA, for a 100 amp motor it is 5 X 273, worse case would be 1365 amps. However this is impractical for a 112.5 kva transformer, 10 hp would be much more practical. 5 X FLA, for a 10 hp motor would be 5 X 30.8 = 154 amps. This would be 1.5 % of the utility contribution, and keep in mind that the motor contribution decays rapidly. I believe that Mike is right about leaving motor contribution out of the calculation of fault current, however it can be extremely important for calculating available fault current for internal customer equipment

 

[paulengr]

The key is all about time. The ANSI method recommends ignoring all motors 25 HP and less, adding the horsepowers from there to 160 HP, and treating all loads above that individually. It has to do with the transient time. Under 25 HP the transient lasts about 1 cycle so the contribution is too quick to matter. On 3 cycle UL 489 equipment (panelboards, MCCs) motor contribution certainly matters. On UL 1077 switchgear with a 30 cycle withstand I would lean more towards the idea that the transient contribution doesn’t matter except for the largest motors. That is in line with the IEEE/ANSI as well as IEC methods. This is for short circuit withstand purposes.

You jumped over to starting though. That’s where you absolutely must consider motor contribution. It is the entire reason in many cases that reduced voltage starting is done. Adding 500% of FLA is correct procedure on medium size NEMA B/C curve starting. But it is not correct at 10 HP or less or with crusher duty or synchronous motors where starting current can be up to 10x FLA. Even with reduced voltage starting a lot of equipment such as ammonia compressors and hammer mills often require 450-600% starting FLA.

I recognize that this is out of line with what many “experts” will say. I am the engineer for a large regional motor shop in the mid-Atlantic area. I deal with motors in the hundreds to thousands of HP all the time in demanding applications.

Your way of doing things is a little odd. The way I do it is to figure up all my current contributions then if I have a stiff primary bus just use the infinite bus assumption and ratio the short circuit current with the starting current of the largest load only to find the VD. If the primary bus is weak I just translate the impedance to the primary bus then calculate SC/VD then again go through the transformer calculations knowing primary VD. Anything more complicated and just load it into EMTP. SKM just really isn’t good for transient analysis. But you will find what you are doing is trying to over-optimize something that is a marginal application which always eventually comes back to haunt me when I do it.