Contactor loading
Contactor loading
(OP)
We have a 25kva transformer that is being switched by a contactor on the primary with a magnet as a load (long story).
It has been in use for 5 years, however to now get it engineered approved we need to upsize the contactor.
What is the reason for this? The contactor was rated according to motor loads rather than a xfrm load. My first thought was that a motor load has approximately 6-8xFLC inrush, where as a xfmr has 12-25xFLC. However, when you do the math the contactor should still hold. Then I thought maybe it is because a xfmr is more inductive than a motor, and would inturn produce more stress on the contacts during parting, and hence limit the life of the contactor?
Any ideas??
It has been in use for 5 years, however to now get it engineered approved we need to upsize the contactor.
What is the reason for this? The contactor was rated according to motor loads rather than a xfrm load. My first thought was that a motor load has approximately 6-8xFLC inrush, where as a xfmr has 12-25xFLC. However, when you do the math the contactor should still hold. Then I thought maybe it is because a xfmr is more inductive than a motor, and would inturn produce more stress on the contacts during parting, and hence limit the life of the contactor?
Any ideas??






RE: Contactor loading
RE: Contactor loading
Maybe the engineer sized it for the transformer and motor coming on at the same time. That would almost double your contactor size.
RE: Contactor loading
NEMA Std ICS2-2000 table 2-4-17 establishes maximum transfomer-kVA ratings for primary-side switching with general-purpose contactors. Is it possible that’s the ‘engineered approved’ hitch?
RE: Contactor loading
RE: Contactor loading
I believe you are right. What I have been told is that regardless of the load, the contactor needed for primary switching of a transformer is alot higher than the contactor needed for switching of an equivalent motor load (same rated current, VA etc).
I was unable to find it in the CE code, however, I have noticed in the AB Industrial controls book it says the contactor in use is good for 45A continous, and;
(1) A 25hp motor load or;
(2) 8.9kva xfmr primary switching!
(Our xfmr is fused at 40amps, 25kva...)
By the book we realistically should have a size three or four contactor in rather than a size 2... but then again it has been running for 5 years, no problems....
Thanks folks.
RE: Contactor loading
tulum, thank you for the confirmation. [I first looked at A-B’s site for their {older} Bulletin 500 contactor ratings for transfomer-/various-load switching, and hit a brick wall searching. Indeed, sometimes paper-based information is not automatically obsolete.] {Incidentally, NEMA ICS2-2000 is currently available as a free download at their site.}
RE: Contactor loading
RE: Contactor loading
http://www.iee.or.jp/honbu/back_number/journal/index_back_number/2002/2002_06b_06.pdf
http://www.iee.or.jp/honbu/back_number/journal/index_back_number/2003/2003_06b_08.pdf
http://www.ipst.org/TechPapers/2003/IPST03Paper9d-4.pdf
(The size of inrush current depends when the transformer is switched.)
etc. for more info