motor starting simulation

[schan05]

I am using SKM sofoware to plot motor starting curve for TCC with an closed-transition autotransfomer starter @ 65% tap. At around 5", there is a current spike all the way approaching LRC level, and then graduately drops down to FLC level in 6 next seconds. The spike might be shorting contacts closing, but i am wondering the duration of the spike (it doesn't seem to drop expoentially within a fraction of a second).

I am using following setting:

voltage: 4160V
Rated size: 1100HP
FLA: 122Amp (as per manufacturer)
PF: 85% (as per manufacturer)
Eff: 95% (as per manufacturer)
ANSI contribution: 0.256 (1/inrsh factor), X/R=10 (assumed)
starting time: 13 seconds (as per manufacturer)
inrush factor: 3.9 (as per manufacturer)
Load library: motor>500HP (assumed)
Load model: typical based on the hp and load type. (assumed)

Anything wrong with my setting? Thanks.

 

[aolalde]

I think the autotransformer starter is closing the full voltage contactors after 5 seconds, instead of the 13 sec calculated. This condition does not allow full acceleration of the load before the full voltage is applied, resulting in a surge of current similar to LRC.

 

[schan05]

Thanks.

 

[jraef]

schan05,
Aolalde beat me to it, but essentially if the motor has not accelerated beyond at least 90% speed before transitioning, you will get a transition spike even in a Korndorfer starter design (closed transition). Take a look at this paper, done by another member of this forum, Marke. If you scroll down to the section on Autotransformer starters you will see that in his example, at 50% voltage taps the motor does not fully accelerate and the current and torque spike to essentially the same levels as they would with Across-the-Line (DOL) starting. With the same motor and load at 65% taps, it doesn't happen. The problem may be that the 65% taps may be more than you need.

http://www.lmphotonics.com/m_start.htm

This by the way, is one of the reasons to select a Solid State starter over Autotransformer. They are much more flexible.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla