# Control transformer sizing



## FH1982 (Sep 14, 2016)

Need some help. I do not have much experience with this. I need to size a control transformer for a 3 phase 480v motor control system. Other than the 120v coil of the 480v motor starter, the only other things in the the control circuit are an emergency stop button and start/stop buttons. I know that I need a 480-120 transformer. With reference to sizing the VA for the transformer, what all must I consider?


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## frenchelectrican (Mar 15, 2007)

FH1982 said:


> Need some help. I do not have much experience with this. I need to size a control transformer for a 3 phase 480v motor control system. Other than the 120v coil of the 480v motor starter, the only other things in the the control circuit are an emergency stop button and start/stop buttons. I know that I need a 480-120 transformer. With reference to sizing the VA for the transformer, what all must I consider?


Very basic question to ask you .,

What size starter you are refering to this ?

IIRC you dont need very big control transfomer but before I can sling out the correct size I need to know the starter size and most case useally 300 Va size depending on starter size.


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## FH1982 (Sep 14, 2016)

the starter is labeled:

Siemens Cat. No. 14BS+32A* 
Series C
Size 00
Max Amps 9
600 VAC
Max HP @460/575 VAC = 2 HP


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## frenchelectrican (Mar 15, 2007)

FH1982 said:


> the starter is labeled:
> 
> Siemens Cat. No. 14BS+32A*
> Series C
> ...


Bingo .,,

75 or 100 Va will do it very nice I know the 50 va can get by but I suggest stay with common 75 va control transfomer and add a fuse on primary side on it.,

You should have room in that encloseure., the control transformer is not very big at all


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## FH1982 (Sep 14, 2016)

frenchelectrican said:


> Bingo .,,
> 
> 75 or 100 Va will do it very nice I know the 50 va can get by but I suggest stay with common 75 va control transfomer and add a fuse on primary side on it.,



Thank you. :thumbsup:


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## JRaef (Mar 23, 2009)

70VA would be a little overkill if it is an actual Machine Tool Transformer (Control Power Transformer). A Size 00 Furnas (now Siemens) starter requires a 45 or 50VA CPT. Over sizing without an additional load can lead to high voltage on the coil, which was not good for those old Furnas coil designs.


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## frenchelectrican (Mar 15, 2007)

JRaef said:


> 70VA would be a little overkill if it is an actual Machine Tool Transformer (Control Power Transformer). A Size 00 Furnas (now Siemens) starter requires a 45 or 50VA CPT. Over sizing without an additional load can lead to high voltage on the coil, which was not good for those old Furnas coil designs.


Thanks for the heads up on that .,,

I have ran into some cpt with multi taps so I can able dail down the voltage down a little on short runs but very long runs just notch it up a little.


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## FH1982 (Sep 14, 2016)

JRaef said:


> 70VA would be a little overkill if it is an actual Machine Tool Transformer (Control Power Transformer). A Size 00 Furnas (now Siemens) starter requires a 45 or 50VA CPT. Over sizing without an additional load can lead to high voltage on the coil, which was not good for those old Furnas coil designs.


So you are saying that a 50VA transformer would be sufficient for the 120 VAC motor starter coil along with the start/stop buttons and emergency stop?


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## micromind (Aug 11, 2007)

There's a serious difference between a normal power transformer and a control power transformer. 

If you compare the two, you'll note that the CPT has way more iron and is thus heavier. This is because it is designed to handle the large inrush of current that a coil takes. The impedance of a CPT is much less than a power transformer so there's less voltage drop during a momentary overload.


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## micromind (Aug 11, 2007)

FH1982 said:


> So you are saying that a 50VA transformer would be sufficient for the 120 VAC motor starter coil along with the start/stop buttons and emergency stop?


For a size 00 starter it almost certainly will.


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## frenchelectrican (Mar 15, 2007)

FH1982 said:


> So you are saying that a 50VA transformer would be sufficient for the 120 VAC motor starter coil along with the start/stop buttons and emergency stop?


Yuh it will be the single contractor it is fine for 50 va transfomer for size 00 and 0 and 1 .,

but when you get more than one contractor it will be depending on the set up so that answer will varies a little.

Majorty of the time 50 Va will do it just fine.

I useally stock couple 75 va's because I useally run into size 2 starter from time to time.


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## B-Nabs (Jun 4, 2014)

FH1982 said:


> So you are saying that a 50VA transformer would be sufficient for the 120 VAC motor starter coil along with the start/stop buttons and emergency stop?


I feel it is important for someone to point out that start/stop and emergency stop buttons are not loads, they are switches. They have no bearing on the rating of the transformer you need to select.


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## Big John (May 23, 2010)

micromind said:


> ...If you compare the two, you'll note that the CPT has way more iron and is thus heavier. This is because it is designed to handle the large inrush of current that a coil takes. The impedance of a CPT is much less than a power transformer so there's less voltage drop during a momentary overload....


 You'll also find CPTs often have screwball turns ratios, and unexpectedly high secondary voltages that way when they are under nameplate load the voltage drop ensures they are actually still delivering nameplate voltage. 

I'm foggy this morning, but I believe it's called "compensated."


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## JRaef (Mar 23, 2009)

FH1982 said:


> So you are saying that a 50VA transformer would be sufficient for the 120 VAC motor starter coil along with the start/stop buttons and emergency stop?





micromind said:


> For a size 00 starter it almost certainly will.


In those Furnas / Siemens starters, they use the exact same coil for all starters from Size 00 to size 2-1/2 (They are one of the few who offer "half size" NEMA starters). So a 50VA CPT is all that is necessary for that entire range. 

But to micromind's other point, a CPT *is *different. So if you buy a 50VA bell transformer, it may not work. The inrush on that coil is something like 225VA. A 50VA _*CPT*_ can deliver that momentarily, a standard little transformer will likely collapse and drop voltage so much that the coil chatters.


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## FH1982 (Sep 14, 2016)

So, would a Sola HD E050 be sufficient?


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## FH1982 (Sep 14, 2016)

JRaef said:


> In those Furnas / Siemens starters, they use the exact same coil for all starters from Size 00 to size 2-1/2 (They are one of the few who offer "half size" NEMA starters). So a 50VA CPT is all that is necessary for that entire range.
> 
> But to micromind's other point, a CPT *is *different. So if you buy a 50VA bell transformer, it may not work. The inrush on that coil is something like 225VA. A 50VA _*CPT*_ can deliver that momentarily, a standard little transformer will likely collapse and drop voltage so much that the coil chatters.


So would a Sola HD E050 be sufficient? Thanks for your input.


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## JRaef (Mar 23, 2009)

FH1982 said:


> So, would a Sola HD E050 be sufficient?


Yes, absolutely. Those are one of the best in my opinion.


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## Tom the lecrician (Sep 23, 2016)

You should check the amp draw of the coil. If it's not listed, read your coil redistance and do the calculation on it...resistance devised by voltage will give you your amps...waalaa


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## Tom the lecrician (Sep 23, 2016)

Or your voltage devised by your resistance...whatever. anyway are there lights on your buttons? I would go with a 75va, or just a little play rom. You measuring the coil so we can know for sure what it's gonna pull?


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## Tom the lecrician (Sep 23, 2016)

Ok, good talk


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## MechanicalDVR (Dec 29, 2007)

Tom the lecrician said:


> Ok, good talk


You live alone? You seem well versed in talking to yourself.


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