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coolyaar
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Control Transformer Sizing

01/04/2013 5:28 PM

Control transformers are used to suppy power to control ciruit i.e relay coils of a ciruit.now every transformer has a kva rating along with voltage rating.so how are we gonna find the VA rating of the transformer used to supply power to the control circuit containing relay coils and pilot lamps as main loads.
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Fredski
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#1

Re: Control transformer sizing

01/04/2013 5:41 PM

this is a Wiki question thats rather elementary
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JPool
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#2
In reply to #1

Re: Control transformer sizing

01/04/2013 5:56 PM

Yes it is, but it's not as fun posting them here and winding Lyn up...

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Fredski
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#4
In reply to #2

Re: Control transformer sizing

01/04/2013 6:47 PM

some Ops are just plain lazy! I enjoy sharing knowledge and experience, and I try to help genuine people when I can. I started typing 1.67...and erased it!!!
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lyn
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#3

Re: Control Transformer Sizing

01/04/2013 6:44 PM

"so how are we gonna find the VA rating of the transformer used to supply power to the control circuit containing relay coils and pilot lamps as main loads."?

Pay somebody to care.
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TonyS
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#8
In reply to #3

Re: Control Transformer Sizing

01/05/2013 5:30 PM

Coolyaar pay? When he can come here? Good God man, get a grasp.

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Tornado
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#5

Re: Control Transformer Sizing

01/04/2013 10:06 PM

Add up the va of all the stuff that might be simultaneously energized, and then multiply by a safety factor of say 1.25. In unusual cases, this safety factor might vary. The worst case is if many of the loads switch from off to on at the same time, which could happen on recovery from a power failure.

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jmueller
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#11
In reply to #5

Re: Control Transformer Sizing

01/07/2013 5:04 PM

Coolyaar & Tornado,

Actually, you should add up all the VA for the loads that are on and then separately add the inrush VA for the relays and contactors that are being closed simultaneously. The inrush VA can be as much as 3-5 times the sealed VA, so the multiplier of 1.25 may not be adequate. The manufacturers of the relays and contactors all list the inrush and sealed VA's for their equipment. After doing this, then add a safety margin for future expansion of loads. At one time I designed panels for a custom shop making them with U/L listing, and we had to go this way.

--JMM
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Tornado
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#12
In reply to #11

Re: Control Transformer Sizing

01/07/2013 9:44 PM

Agreed. The latter part of my post implied similarly, though not in detail.

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MagicWand
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#6

Re: Control Transformer Sizing

01/04/2013 10:46 PM

Take your pick:

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roykav
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#7

Re: Control Transformer Sizing

01/04/2013 11:58 PM

Calculate the total SEALED (steady state) VA load of your control circuit. This is done by adding the continuous VA requirements of the maximum number of "omponents that will be-sagrgized at any given time, including non-inductive as well as inductive com­ponents.

Calculate the PEAK INRUSH VA of your control circuit.

First, analyze the sequence of operation of all com­ponents. Then add together the inrush VA ratings of the components that will be energized simultaneously. Next, determine the peak - or maximum simultaneous - inrush VA load that the transformer will "see." The VA requirements :or indicating lights, timers and othernon-iriductive components which do not have an inrush VA also should be included

calculate the TRANSFORMER SELECTION INRUSH

VA. Use the following formula:

Selection Inrush VA = root of (VA Seaied)2 + (VA Inrush)2

see any control transformer manufacturers data
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PWSlack
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#9

Re: Control Transformer Sizing

01/05/2013 6:18 PM

Er, by using simple arithmetic, perhaps?

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Phaddy
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#10

Re: Control Transformer Sizing

01/06/2013 2:09 AM

I assume first that you have the transformer already. Try one or all of these:

1. Use a clamp meter to measure AC current, when control circuits powered up.

2. Look up the circuit diagram, see what exists, get current rating of each item, add.

3. Measure the cross-section dimensions of the steel through the copper windings. Then guesstimate.

These represent guesstimates only, because not all control circuits are likely to be operating at the same time.

If you don't have the transformer, try 2. above.
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Users who posted comments:

Fredski (2); jmueller (1); JPool (1); lyn (1); MagicWand (1); Phaddy (1); PWSlack (1); roykav (1); TonyS (1); Tornado (2)

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