Mostly in AOV control circuits.
How to wire up a Buck-Boost Transformer?
- Thread starter JasonCo
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kwired
Electron manager
- Location
- NE Nebraska
From the "product description" on page 2 of this Square D Buck & Boost transformer publication:
A Buck & Boost Transformer is an insulating transformer with either a 120 V x 240 V primary with
a 12/24 V or 16/32 V secondary, or a 240 V x 480 V primary with a 24/48 V secondary. A Buck & Boost
Transformer is also useful in applications where the primary and secondary are interconnected for use
as an autotransformer. The equipment enclosure is designed and constructed for indoor or outdoor use
and is NEMA 3R rated.
A Buck & Boost Transformer is an insulating transformer with either a 120 V x 240 V primary with
a 12/24 V or 16/32 V secondary, or a 240 V x 480 V primary with a 24/48 V secondary. A Buck & Boost
Transformer is also useful in applications where the primary and secondary are interconnected for use
as an autotransformer. The equipment enclosure is designed and constructed for indoor or outdoor use
and is NEMA 3R rated.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
160405-1902 EDT
JasonCo:
Your first post has inconsistencies that will not produce 24 V output.
Suppose we have an isolation transformer that has four isolated coils consisting of two 120 V coils and two 12 v coils. How can this transformer be wired to produce 24 V output with 264 V input? When connecting windings of a single transformer together the phasing of the coils is important. A typical way of indicating common phasing of windings is to place a large DOT adjacent to and at one end of each coil. If the non dotted ends of each coil are connected together and the transformer is excited, then relative to the non-dotted ends all of the dotted ends will be positive at the same time.
If you take two coils and connect them together with the dotted end of one coil connected to the non-dotted end of the other coil, then the two coils are additive. Thus, if the two 120 V coils are so connected, then that coil combination can be connected to a 240 V source, or if the transformer core was appropriately excited, then this coil combination would supply 240 V, or 120-0-120 V.
It appears that your question wants 24 V output. To get 24 V out you need to additively combine the two 12 V coils. In other words connect the two 12 V coils in series with the correct phasing. This will provide 24 V from a properly excited transformer core. This takes care of how we get 24 V.
We have two remaining coils, each is 120 V. If the coils are connected in parallel and properly phased, meaning DOT to DOT and non-DOT to non-DOT, and if 120 V is applied to this parallel combination, then the voltage from the two 12 V windings in series is 24 V. Further if no connection is made between the 120 V coils and the 12 V coils, then there is electrical isolation between the two different voltage levels.
If instead of connecting the 120 V coils in parallel we connect the 120 V coils in series with proper phasing, then again with electrical isolation we get 24 V output from the seriesed 12 V coils.
Taking the last combination and with correct phasing connect all the coils in series, then we can apply 120 + 120 + 12+ 12 = 264 V across the total series combination and get 24 V out, but now there is not isolation of the two voltages. This combination will be called an auto-transformer.
You do not have 264 V available as a source. Thus, the inconsistency in the initial problem. Available is 277 V. Thus, a possible output is 24*277/240 = 27.7 V. Because of internal impedance in the transformer your output may be somewhat less under load. But also some transformers may be wound with a slightly different ratio to compensate for loaded voltage drop.
If this series combination is connected from a three phase line to neutral with the 12 + 12 V windings to neutral, then your 24 V output will have one side near ground potential under normal conditions, but not isolated so you can not create your ground reference.
.
JasonCo:
Your first post has inconsistencies that will not produce 24 V output.
Let's not call this a buck boost transformer to avoid confusion.
Suppose we have an isolation transformer that has four isolated coils consisting of two 120 V coils and two 12 v coils. How can this transformer be wired to produce 24 V output with 264 V input? When connecting windings of a single transformer together the phasing of the coils is important. A typical way of indicating common phasing of windings is to place a large DOT adjacent to and at one end of each coil. If the non dotted ends of each coil are connected together and the transformer is excited, then relative to the non-dotted ends all of the dotted ends will be positive at the same time.
If you take two coils and connect them together with the dotted end of one coil connected to the non-dotted end of the other coil, then the two coils are additive. Thus, if the two 120 V coils are so connected, then that coil combination can be connected to a 240 V source, or if the transformer core was appropriately excited, then this coil combination would supply 240 V, or 120-0-120 V.
It appears that your question wants 24 V output. To get 24 V out you need to additively combine the two 12 V coils. In other words connect the two 12 V coils in series with the correct phasing. This will provide 24 V from a properly excited transformer core. This takes care of how we get 24 V.
We have two remaining coils, each is 120 V. If the coils are connected in parallel and properly phased, meaning DOT to DOT and non-DOT to non-DOT, and if 120 V is applied to this parallel combination, then the voltage from the two 12 V windings in series is 24 V. Further if no connection is made between the 120 V coils and the 12 V coils, then there is electrical isolation between the two different voltage levels.
If instead of connecting the 120 V coils in parallel we connect the 120 V coils in series with proper phasing, then again with electrical isolation we get 24 V output from the seriesed 12 V coils.
Taking the last combination and with correct phasing connect all the coils in series, then we can apply 120 + 120 + 12+ 12 = 264 V across the total series combination and get 24 V out, but now there is not isolation of the two voltages. This combination will be called an auto-transformer.
You do not have 264 V available as a source. Thus, the inconsistency in the initial problem. Available is 277 V. Thus, a possible output is 24*277/240 = 27.7 V. Because of internal impedance in the transformer your output may be somewhat less under load. But also some transformers may be wound with a slightly different ratio to compensate for loaded voltage drop.
If this series combination is connected from a three phase line to neutral with the 12 + 12 V windings to neutral, then your 24 V output will have one side near ground potential under normal conditions, but not isolated so you can not create your ground reference.
.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
160406-1912 EDT
To add to my previous post.
Unloaded, and therefore also loaded, transformers have some nominal coil voltage. By design this voltage is as high as resonably possible. Fundamentally the problem is core saturation and the resulting magnetizing current.
Different core materials have different saturation curves and in turn have different overvoltage limitations.
A rather standard transformer I have (Signal Transformer A14-175-16) has the following unloaded input characteristics:
Voltage ... Current ... Power ... VA
100 .......... 0.06 ........ 2.2 ........ 6.3
110 .......... 0.10 ........ 2.8 ...... 11.4
120 .......... 0.16 ........ 3.6 ...... 19.2
130 .......... 0.23 ........ 4.6 ...... 30.9
140 .......... 0.36 ........ 6.3 ...... 50.5
This transformer is rated at 175 VA at 120 V input.
At nominal 120 V the magnitizing current plus some losses is using 11% of the transformers VA rating. Obviously the real VA rating is 175 + 19 = 194. Magnitizing is thus (19.2/194)*100 = 10% of the actual rating.
With 17% overvoltage (140 V) the magnetizing current is requiring 29% of the transformer rating, or (50.5/194)*100 = 26% of the true rating.
Increase the overvoltage more and the magnetizing current goes up rapidly.
In the problem presented in post #1 277 V nominal is already 5% overvoltage for a 264 V primary. To the 277 you need to add whatever might be the actual overvoltage that could be applied to the 264 V primary.
Minor overvoltage to a transformer primary will probably shorten the transformer life. Substantial overvoltage will destroy the transformer quickly. Double voltage will ptrobably create destruction in seconds.
.
To add to my previous post.
Unloaded, and therefore also loaded, transformers have some nominal coil voltage. By design this voltage is as high as resonably possible. Fundamentally the problem is core saturation and the resulting magnetizing current.
Different core materials have different saturation curves and in turn have different overvoltage limitations.
A rather standard transformer I have (Signal Transformer A14-175-16) has the following unloaded input characteristics:
Voltage ... Current ... Power ... VA
100 .......... 0.06 ........ 2.2 ........ 6.3
110 .......... 0.10 ........ 2.8 ...... 11.4
120 .......... 0.16 ........ 3.6 ...... 19.2
130 .......... 0.23 ........ 4.6 ...... 30.9
140 .......... 0.36 ........ 6.3 ...... 50.5
This transformer is rated at 175 VA at 120 V input.
At nominal 120 V the magnitizing current plus some losses is using 11% of the transformers VA rating. Obviously the real VA rating is 175 + 19 = 194. Magnitizing is thus (19.2/194)*100 = 10% of the actual rating.
With 17% overvoltage (140 V) the magnetizing current is requiring 29% of the transformer rating, or (50.5/194)*100 = 26% of the true rating.
Increase the overvoltage more and the magnetizing current goes up rapidly.
In the problem presented in post #1 277 V nominal is already 5% overvoltage for a 264 V primary. To the 277 you need to add whatever might be the actual overvoltage that could be applied to the 264 V primary.
Minor overvoltage to a transformer primary will probably shorten the transformer life. Substantial overvoltage will destroy the transformer quickly. Double voltage will ptrobably create destruction in seconds.
.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
The kVA capacity of the transformer is based on useful current to the loads and allows for the heating (copper and iron) of the magnetizing current.
K8MHZ
Senior Member
- Occupation
- Electrician
Thanks. When you said 'instrumentation' I was thinking 0-5v / 4-20mA instruments, not vent motors. I have only worked on one AOV and it was 120 volts.
Well, I was being lazy at the time. Even considered writing I&C, but I don't believe a majority of patrons know that is short for instrumentation and control... and the 48V is more on the control side than the instrumentation side of it. The only place I've run across 48V is in power plants.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
160407-2353 EST
My interpretation of the first post was that the transformer had two 120 V windings, and two 12 V windings with all four windings being isolated from one another.
One transformer can not be supplied from 480, but two of these transformers could be wired to provide 24 V from from 480 V.
.
My interpretation of the first post was that the transformer had two 120 V windings, and two 12 V windings with all four windings being isolated from one another.
One transformer can not be supplied from 480, but two of these transformers could be wired to provide 24 V from from 480 V.
.
sparkselec
Member
- Location
- staunton, va
How to wire up a Buck-Boost transformer
How to wire up a Buck-Boost transformer
The poster obviosly doesn't understand what a Buck-Boost transformer is. A Buck-Boost transformer is a autotransformer that raises or lowers the supply voltage to the required utalization voltage. A connection diagram for the the required voltage combination can be found on the website of Acme, Square D, Jefferson, FPE or several others. Good luck.
How to wire up a Buck-Boost transformer
The poster obviosly doesn't understand what a Buck-Boost transformer is. A Buck-Boost transformer is a autotransformer that raises or lowers the supply voltage to the required utalization voltage. A connection diagram for the the required voltage combination can be found on the website of Acme, Square D, Jefferson, FPE or several others. Good luck.
SMARTERCARTER
Member
- Location
- Oceanside CA
Buck Boost Simple Solution
Buck Boost Simple Solution
Since there is already lots of good tech info here...
For future quick problem solving tip if have a situation you are not familiar with- Go to the source/manu. of what you are installing.
I don't work there but call ACME transformer Co. or the like and they have an excellent tech support, because that is what they do....every day! and they will send you via email the exact diagrams that come with Transformers so your not guessing and they have already figured out all the solutions its all on paper. They will tell you which diagram to use and sometimes help with the cheapest option or trend or solution if you are not sure if you want one big transformer or two smaller buck boosts etc. I did a 208 to 236 volt boost and it runs hot when not in use=normal, because a transformer is a load remember (Power factor!). the smaller and cheaper the transformer sometime the lower the power factor maybe. If you remeber even landscape transformers do that. 108 degrees if I remember my infrared test correctly. The diagrams are easy to follow and apply to all the brands usually. I had some 240 volt motor from china and we bumped up the 208 to 236 volt using 2 Size 3 (used ) transformers. If you want to be an engineer go to school to do that... other wise just be an electrician and just install the thing...LOL. There is too much transformer info to fit in one persons head anyway unless you don't want a life or you work at ACME I guess they might know it all.
I tell my customers why would you hire a handyman to do something that other men have entire trucks and business dedicated to do that one trade and they pull up in a minivan with a ladder on the luggage rack and they can do it all???. Leave the engineering to the engineers they can save you lots of headaches and give some family time! :dunce:
TIP: Don't let your customers make you do everything and everyone else's job. I know it's easy to do. I sometime as owner operator get into those sticky situations no plans no design etc. how do you charge for that engineering and design if your are not qualified to do it. You don't usually because your a nice guy.
Hope this helps someone know what to do but not exactly how to do it. :blink:
Always go to the source and you find the cleanest water. ;-)
Buck Boost Simple Solution
Since there is already lots of good tech info here...
For future quick problem solving tip if have a situation you are not familiar with- Go to the source/manu. of what you are installing.
I don't work there but call ACME transformer Co. or the like and they have an excellent tech support, because that is what they do....every day! and they will send you via email the exact diagrams that come with Transformers so your not guessing and they have already figured out all the solutions its all on paper. They will tell you which diagram to use and sometimes help with the cheapest option or trend or solution if you are not sure if you want one big transformer or two smaller buck boosts etc. I did a 208 to 236 volt boost and it runs hot when not in use=normal, because a transformer is a load remember (Power factor!). the smaller and cheaper the transformer sometime the lower the power factor maybe. If you remeber even landscape transformers do that. 108 degrees if I remember my infrared test correctly. The diagrams are easy to follow and apply to all the brands usually. I had some 240 volt motor from china and we bumped up the 208 to 236 volt using 2 Size 3 (used ) transformers. If you want to be an engineer go to school to do that... other wise just be an electrician and just install the thing...LOL. There is too much transformer info to fit in one persons head anyway unless you don't want a life or you work at ACME I guess they might know it all.
I tell my customers why would you hire a handyman to do something that other men have entire trucks and business dedicated to do that one trade and they pull up in a minivan with a ladder on the luggage rack and they can do it all???. Leave the engineering to the engineers they can save you lots of headaches and give some family time! :dunce:
TIP: Don't let your customers make you do everything and everyone else's job. I know it's easy to do. I sometime as owner operator get into those sticky situations no plans no design etc. how do you charge for that engineering and design if your are not qualified to do it. You don't usually because your a nice guy.
Hope this helps someone know what to do but not exactly how to do it. :blink:Always go to the source and you find the cleanest water. ;-)
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