Contactor loading 2

[tulum]

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??

 

[GOTWW]

Do you also need to consider the inductance of the electromagnet. This will have an associated inrush as well.

 

[Jerry123]

you say the contactor was sized for the motor load only.
Maybe the engineer sized it for the transformer and motor coming on at the same time. That would almost double your contactor size.

 

[busbar]

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?

 

[DanDel]

It sounds like the 'engineering upsize' is more for a loading issue than an inrush issue.

 

[tulum]

Busbar,

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.

 

[busbar]

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.}

 

[tulum]

Thanks for the link busbar... It definitly compounds the arguement!

 

[jbartos]

Comment: The transformer inrush current is somewhat dependent on a type of transformer, i.e. core type or shell type. Visit

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