buck - boost

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Mike01

Senior Member
I am trying to understand buck boost transformers in more detail and have some questions and know this is the place so here goes correct me if I am wrong: A buck boost transformer is basically an auto transformer, a transformer with a voltage primary winding and secondary are connect togerther to provide a buck or boost in voltage, but here is the part I am trying to understand when I look in catalogs it indicates the primary voltage as 120x240 and the secondary at 12/24, 16/32, or 24/48 so how by connecting one of the phases on the primary and the secondary togerther do you get say 208 up to 230 volts? It is my understanding that the transformer comes shipped as a insulating transformer 120x240 and a secondary of 12/24 or 16/32 but when you apply 208 volts to the primary and connect the windings you can end up with 230V? how is the kva rating determined? this is the part that stumps me and I was hoping someone could help or point me in the direction of some good literature. thanks.

winnie

Senior Member
There are quite a few good discussions of the issue on this site; just search for them.

In a nutshell: The transformers sold as 'buck-boost' transformers may also be used as low voltage supply transformers.

The only thing that might make a 'buck-boost' different from a 'step-down' transformer is that the 'secondary coil' of a 'buck-boost' must have good enough insulation to tolerate the 'primary' voltage. A transformer with a 240V primary and a 16V secondary might be designed with low voltage insulation on the secondary coil, in which case it couldn't be used as a 'buck-boost'.

As you note, when used as a 'buck-boost' transformer, the 'secondary' is connected in series with the 'primary', in an autotransformer configuration. There are two possible connections, one in which the secondary voltage adds to the primary voltage, and one in which it subtracts. Note that even for 'buck' applications, the _additive_ connection is usually used. For both buck and boost operation, the higher voltage is connected so that you have the maximum number of turns in series, and the lower voltage connected to just the primary coil. It is _possible_ to connect in the 'subtractive' connection for buck applications, however this is usually less efficient.

As far as calculating the KVA rating, the trick is to understand that the KVA rating of the transformer applies to the _boost_, not the total output. A way to think of this is to convert the KVA rating of the secondary into an amp rating. This amp rating doesn't change, even when you are in the boost configuration. Take this amp rating, multiply by the total output voltage, and you can figure the KVA that the boost transformer can serve.

-Jon

Mike01

Senior Member
turns ratio

turns ratio

thanks thats great information after thinking it starts to make more sense, however after looking at some model numbers of a manufacturer it indicates two different connections (see attached images) but my question is if the secondary voltage ends up the same (230v) in this application is there an advantage to one setup over the other?

Mike01

Senior Member
getting closer

getting closer

120x24o - 12/24 or 16/32, because you are adding a connection from the primary to the secondary coils you are creating an autotransformer with series windings, therefor bascilly adding the voltages so if you supplied a 120x240 xfmr with 208 you would have a 24 secondary added to the 208 volts in a series connection would give you approx. 232V if you utilized a 32v secondary you would achieve your 240V secondary all about the turns and putting the secondary coils in series with the primaries and making it additive to the line. but how does the current work i see that if the 24volts gives you the 232 final voltage the xfrm only has to supply the difference 24*load of 15A / 1000 would result in a .36kva xfmr. but why does the xfmr only need to supply the excess? i'am stil stuck on this one....thanks a ton again. next up is the 208-230V-3phase 3 wire open delta....until then.

hurk27

Senior Member
120x24o - 12/24 or 16/32, because you are adding a connection from the primary to the secondary coils you are creating an autotransformer with series windings, therefor bascilly adding the voltages so if you supplied a 120x240 xfmr with 208 you would have a 24 secondary added to the 208 volts in a series connection would give you approx. 232V if you utilized a 32v secondary you would achieve your 240V secondary all about the turns and putting the secondary coils in series with the primaries and making it additive to the line. but how does the current work i see that if the 24volts gives you the 232 final voltage the xfrm only has to supply the difference 24*load of 15A / 1000 would result in a .36kva xfmr. but why does the xfmr only need to supply the excess? i'am stil stuck on this one....thanks a ton again. next up is the 208-230V-3phase 3 wire open delta....until then.

If you look at the secondary windings, you will find they are quite large, a 32v 3kva transformer will have a secondary that is rated at 93.75 amps, which is what the load will see, so the kva is only in the boost portion of the transformer and sized for the load.

I have found buck boost transformers with no secondary fuses, used for LV lighting, but with the currents available with them, it is one of the most dangerous things I have seen a person do. and the fact it is totally against code.

LarryFine

Master Electrician Electric Contractor Richmond VA
Mike, the dual-voltage primary is of less use than the dual-voltage secondary because we rarely buck/boost 120v, but the only difference between the two diagrams is series (for 240v) versus parallel (for 120v) connections.

Keep in mind that, when you connect a 240v primary to 108v, the available secondary voltages from 12/24v windings will actually be 10.4/20.8, due to the 10:1 turns ratio. That's why they make 16/32v secondaries, also.

Added:The seemingly-too-low power of the transformer works because the added or subtracted voltage, while of full load current, is of only the added or subtracted voltage, and remember that P=ExI.

The transformer primary is merely energized by the source for the purpose of developing the secondary voltage. The majority of the load current, and thus power, is still supplied directly by the source.

The transformer primary current is in addition to the equipment load current, so remember that the total power (at the new voltage) still has to be provided by the source (at the old voltage.

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Mike01

Senior Member
Turns

Turns

So how many turns are in a 120x240 12/24 xfmr? The ratio would be 10:1 correct? But how many turns would be in the primary 100, 200? What determines the quantity of primary turns? Also when you connect the winding in series how do you calculate the secondary voltage from the turns when they are connected in series?

LarryFine

Master Electrician Electric Contractor Richmond VA
So how many turns are in a 120x240 12/24 xfmr? The ratio would be 10:1 correct?
Correctamundo. Each 120v winding will have about 10x as many turns as each 12v winding. (Expected losses are compensated for by slight variations from an exact 10:1 ratio.)

But how many turns would be in the primary 100, 200? What determines the quantity of primary turns?
It's based on the desired capacity, which determines core size and the quantity of turns and size of winding conductors. I really don't know how they arrive at the exact numbers.

Also when you connect the winding in series how do you calculate the secondary voltage from the turns when they are connected in series?
The real basic is the volts/turn number. If one winding is supplied a certain voltage per turn ratio, all of the other windings will produce the same voltage per turn.

If you feed only one of the 120v windings with 120v, the transformer will still work, but at only half the capacity. The secondaries will each still produce their 12v.

The other 120v winding will produce 120v, too. 1:1 isolation transformers work that way. This is why we can use transformers as either step-up or step-down.

If you feed 120v into half of a 240v center-tapped winding, the other half will also have 120v across it, doubling the voltage (but at half the ampacity.)

Placing two windings in parallel enables their ampcities to combine; placing them in series enables their voltages to combine, but at the individual ampacity.

That's how a dual-voltage transformer (or motor, for that matter) is able to provide its rated power at either voltage, just like batteries wired in series or parallel.

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Mike01

Senior Member
3-phase

3-phase

Is it possible to take a208Y/120V-3?, 4W panelboard, install a 3-pole circuit breaker, (3?, & Grd.) to two buck/boost transformers to derive a 3?-3W+grd open delta secondary configuration or would you have to provide a 3?-4W to three transformers and create a 230/133 secondary and provide no neutral to the load?

winnie

Senior Member
If you do not need the neutral, then yes, you can use an 'open delta' arrangement of two boost transformers to take 208V up to 240V.

Buck-boost catalogs will have schematics for this arrangement.

-Jon

Mike01

Senior Member
If you do not need the neutral, then yes, you can use an 'open delta' arrangement of two boost transformers to take 208V up to 240V.

Buck-boost catalogs will have schematics for this arrangement.

-Jon

Jon, thanks thats what I thought however some colleges are telling me that you cannont feed a open delta buck boost transformer installation from a 208V-wye system but I do not understand why not? if you take the phase conductors and the ground conductor to a wireway just below the xfmr installation and take the leads from the xfmr. into the wireway and make all the proper terminations why would it matter? you would be connecting it in a delta facshion without the neutral how would this cause a problem?

hurk27

Senior Member
Jon, thanks thats what I thought however some colleges are telling me that you cannont feed a open delta buck boost transformer installation from a 208V-wye system but I do not understand why not? if you take the phase conductors and the ground conductor to a wireway just below the xfmr installation and take the leads from the xfmr. into the wireway and make all the proper terminations why would it matter? you would be connecting it in a delta facshion without the neutral how would this cause a problem?

I have done many tanning beds, or should I say up rights, this way, they have no neutral loads, and using two 3kva, 16/32 BB transformer, work just fine. FLA of the beds was 83 amps/ 3-phase @240v and only had 208v available.

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