Can someone please help me draw the line between when I can use an Industrial Control transformer and when I should use a General Purpose transformer. For our control circuits we use a general purpose transformer and I have oftened wondered why. I do not think we get the SCCR calculation advantages with a Control Transformer that we get with the GP encapsulated dry type isolation transformer? Does a Control Transformer only have to have control equipment (relays, timers, SSR's etc) or can they also have motor, heater, and light loads?
Industrial Control Transformer Vs General Purpose
- Thread starter fifty60
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petersonra
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I don't think there is that much real difference.
Industrial control xfmrs supposedly are designed to handle short term surge currents better than a std xfmr (such as when a coil turns on). Whether that is a desirable feature in a specific application is a design decision.
As for SCCR calculations, there is no difference. The numbers are handled the same way.
Industrial control xfmrs supposedly are designed to handle short term surge currents better than a std xfmr (such as when a coil turns on). Whether that is a desirable feature in a specific application is a design decision.
As for SCCR calculations, there is no difference. The numbers are handled the same way.
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I agree. The real difference is in the design of the core and windings such that on a CPT, a high inrush, such as might be caused by a large coil on a contactor solenoid, causes less voltage instability; less voltage drop. Voltage drop can be a big problem with large or multiple contactors being controlled, it can cause chattering and destruction of the contacts. A GP transformer would need to be over sized to accomplish the same thing, so THAT may affect your SCCR rating.
So bottom line, you can use a CPT for anything at it's rating, but you pay more for a CPT of the same VA rating compared to a GP transformer, so if you are not operating solenoids, it's kind of a waste of money. But if controlling mainly contactors and other solenoids, you will have to buy a larger GP transformer to get the same performance, which means more waste heat inside the box, more weight, more panel space etc. Generally, CPTs make a better choice for control panels because of all of those reasons. But if you need a 5kVA transformer to supply lighting, panel heat etc., and only 500VA worth of contactor coils, I would use a 5kVA GP transformer rather than a 5kVA CPT.
I have read somewhere that you cannot use the primary OCPD in you SCCR calculations when you are using an Industrial Control transformer, that it had to be a power transformer otherwise you have to take the lowest SCCR of any component on the secondary. What differentiates a power transformer from any other transformer?
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That is the cheap and easy way out.
The other, better, way to do it is to build a transformer with a lower impedance and exactly the same no-load output voltage. That is what has been described in earlier posts.
You might find transformers that do both, and be able to identify them as CPT by that characteristic.
I reviewed my post and I didn't see anywhere that I referred to cost. So you suggest that a custom transformer be built with a lower impedance be designed and that cost is not an issue. From my perspective this is what the manufactures are offering. These transformers have been available for years and transformer manufacturers most certainly will after new designs if warranted. These transformers for the most part are small and may go up a 1500va or so as I recall without consulting a catalog.
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- Retired PV System Designer
Not exactly. What I am saying is that you can reduce the bad effects of surges just by running the output voltage up higher (to the extent that the devices can tolerate the higher voltage) without changing the transformer design in any way but the turn count. This would leave the spread between normal and surge voltages the same but move both of them higher by the same percentage,
OR you can specify/design a transformer with the same turn ratio and larger cross section windings and a lower loss core to reduce the spread between the no-load and surge-load voltages directly.
That second approach, using either a custom transformer or one sold specifically for that purpose (that is a control transformer), will give the best results, but will inevitably cost more for the same nominal power handling capacity.
petersonra
Senior Member
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- Northern illinois
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- engineer
for purposes of SCCR calculations according to UL508a, there is no difference.
A power transformer is one that has power loads such as motors or heaters. A control xfmr is one that has only control circuits.
The second approach using a larger cross sectional area is basically Over sizing the transformer is just another way of saying higher va than necessary which is a greater expense. How would that cost compare to a CPT with a greater turns ratio. I am not aware of any "bad affects" of using a CPT.
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- Retired PV System Designer
Can you point me to a reference that states that a typical CPT is wound to deliver a higher than nominal no-load voltage? I have not found any such indications so far.
One thing that I do see is that for a general purpose transformer, you select the size of the transformer based on the continuous load, while for a CPT you select the size of the transformer based on tables of the inrush load and the allowable voltage drop, rather than looking at the nominal continuous wattage. Either way the transformer is oversized.
One factor about CPTs that I have seen mentioned in product brochures is that the windings and insulation are designed mechanically to be more tolerant of repeated inrush current loads with their corresponding increased vibration and magnetic forces between turns of the wires, while a GPT will see inrush loads less frequently.
Do you have a 480v-120v CPT? If so apply 480v to the primary and measure the secondary voltage and report back what the results are. Also, you can apply 120v to the secondary and advise what the results are.
The winding insulation temperature is not unique to CPTs and I am intrigued with your reference to repeated inrush and vibration. I maintain a 2" thick binder on application references and notes from the manufacture that I previously worked for as well as a distribution dry type transformer manufacture and this is something that has never been a feature.
Performance requirements are to equal or exceed ANSI/NEMA ST1 and regulation to exceed ANSI/NEMA requirements.
The are CPTs that where the regulation exceeds ANSI/NEMA standard by 10%-200%.
I have information from Eaton Electrical (Cutler-Hammer), Acme, SqD, and Jefferson and there are no references to the vibration/magnetic force issues. The important factor wit CPTs is "regulation". To properly select the VA of a CPT you must take an inventory of the loads on the CPT. Contactor's and relays have both sealed VA and inrush VA. Timers and ILs just VA. Take the square of the total sealed VA + the square of the total of the inrush VA. Then compute the square root of the total. Then you have to decide what your allowable voltage drop is usually 95, 90, or 85%. Based upon the inrush in Amps at 20% power factor.
It is desirable to keep you regulation above 85%.
But your brochure that you have referee to intrigues me. Do you have a reference to it?
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Well, some of what I mentioned can be found in the description of this medium voltage CPT from ABB: http://www05.abb.com/global/scot/scot235.nsf/veritydisplay/af157f28b05dde0d85256d9c005dbc70/$file/cpt_flyer.pdf
I also suspect that because of their use inside control panels for up to 480V systems whose overall power consumption is high, a CPT is more likely that a GP transformer of the same size to be available with optional integrated fuse blocks. http://ab.rockwellautomation.com/Power-Supplies/Control-Power-Transformer#/tab5
A short overview of the selection process for a CPT can be found at http://www.ab.com/en/epub/catalogs/12768/229240/1151309/229413/6488323/Product-Selection.html
I do not have any CPTs lying around to perform voltage ratio tests on, so I will take your word for that.
I will say that unlike a CPT, a GP will often be used in reverse rather than forward configuration, making it undesirable to fudge the turns ratios to allow for more droop under load. A CPT is pretty clearly intended to be used in a forward direction only.
Ah, a medium voltage CPT which is an entirely a different subject than CPTs in the LV class such as 480v-120v CPTs. It's about talking about apples and oranges.
Gold digger, I finally had the time to refer to the links that you included. This one is excellent and answers the OPs question.
http://www.ab.com/en/epub/catalogs/12768/229240/1151309/229413/6488323/Product-Selection.html
One of my previous response was based upon it.
http://www.ab.com/en/epub/catalogs/12768/229240/1151309/229413/6488323/Product-Selection.html
One of my previous response was based upon it.
That make sense.
Thanks
Thanks for the information. I am still reading the posts and trying to process everything. I am revisiting this issue. There are control transformers with dual voltages available across the secondary (120/240). Can I use a 115V tap of the Control Transformer for my control circuit (solenoids, coils, and a small 1/6HP motor), and then use the 240V secondary taps for a 1/4HP 240 Volt rated motor?
Also, one thing that I have found is the Control Transformers are more expensive than General purpose, but are much smaller. I have to buy CE Marked transformers and the GP's get very large even at 1KVA. I have to find a smaller solution.
Also, one thing that I have found is the Control Transformers are more expensive than General purpose, but are much smaller. I have to buy CE Marked transformers and the GP's get very large even at 1KVA. I have to find a smaller solution.
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