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#24625 - 04/16/03 05:35 PM sizing control transformer
drillman Offline

Registered: 03/24/03
Posts: 97
Loc: Somewhere in Texas
I have a lighting contactor that needs a control transformer. It is 120v coil and only 277v is avaible.

I cannot find any info on the contactor itself as to what the coil draw is. I also checked all the rest of the contactors we got around the shop of various brands and found no info.

Spent some time on yhe Square D website and got some numbers so I have something to work with.

That took a few hours of hunting and downloading, so I wonder if there a rule of thumb for this sort of thing?

Last but not least, I have to ground one side of the 120 secondary right?

many thanks

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#24626 - 04/16/03 05:53 PM Re: sizing control transformer
caselec Offline

Registered: 04/14/02
Posts: 557
Loc: San Jose, CA
Unless this is this a very old contactor you should be able to get a 277V coil for it. How big of contactor is this? The amount of current the coil draws would depend on the size. You could energize the coil from a temporary 120V source and use a meter to check the draw.


[This message has been edited by caselec (edited 04-16-2003).]
Curt Swartz

#24627 - 04/16/03 06:15 PM Re: sizing control transformer
drillman Offline

Registered: 03/24/03
Posts: 97
Loc: Somewhere in Texas
I probbaly could get a coil for it if I had to.

I should have mentioned that these parts were laying around the shop and I was trying to find out if they would work together.

Theres a 50 va transformer and a 30 amp 3 pole lighting contactor.

#24628 - 04/16/03 06:40 PM Re: sizing control transformer
NJwirenut Offline

Registered: 09/15/01
Posts: 816
Loc: Bergen County, NJ
The contactor manufacturer should provide coil VA ratings, both inrush and sealed. Make sure that your control power source can provide the required inrush current.

Yes, one side of the secondary needs to be grounded, especially if any of the secondary wiring leaves the contactor/transformer cabinet.

#24629 - 04/16/03 09:04 PM Re: sizing control transformer
Bjarney Offline

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
I'll side with NJ. 277V primaries are a little unusual—a 240x480V primary is more common. The problem with "machine-tool control power" transformers is that they need to be able to supply very high inrush currents of contactor/starter coils.

Put a {±½-amp time delay} fuse on the ungrounded leg of the transformer if there’s not one already furnished. A 30-amp rating corresponds to a NEMA size 1 lighting contactor…50-75VA is probably OK.


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