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adding bucked voltage to supply

adding bucked voltage to supply

adding bucked voltage to supply

(OP)
I was informed of an installation of a gate operator installation where the installers had run 10AWG wires a little over 1000' to supply a 208-230v singlephase motor. Due to the voltage drop, they installed 500VA buck transformers at the operators to get 30v or so and ran the transformer output in parallel with the supply wires, adding the voltage, compensating for the drop when the motor needed current. Is this a common practice? Is it a good or bad idea? Apparently it was more cost effective than using larger wires.

RE: adding bucked voltage to supply

Something doesn't sound right. First off, "Buck" means Drop, more likely they are Boosting. And parallel? That's odd. Maybe you are just interpreting it that way? The buck/boost transformer is likely an auto transformer and to the untrained eye might appear to be a parallel connection, but electrically, it's more of a series/parallel connection.

And yes, that's a valid solution, weighed against the cost of wire. If the wire was already installed, you add the cost of removing and re-pulling, so it usually ends up less expensive of a fix. It would probably have cost less to use the correct size wire in the first place however.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

RE: adding bucked voltage to supply

(OP)
Yes, they 'bucked' it from the 208v supply down to 30v and "added" it to the same supply wires going to the transformer primary. Attached is how it was explained to me. The polarity may be switched concerning the transformer secondary, but you get the idea.

RE: adding bucked voltage to supply

thekman,
Could you please mark the primary & secondary polarity marks of the transformer and kindly re-upload?

RE: adding bucked voltage to supply

It works very well. Have used that to boost (yes, that is the correct word) a weak grid from a mere 350 V to 400 V in order to make a kiln for ceramic pots work well. The transformer that takes voltage down to 30 V (in your case) is not a "buck" transformer - it is just a transformer.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: adding bucked voltage to supply

There's too many connections. I imagine that it's connected in a boost auto transformer connection to send 240 (208+32) to the load.

RE: adding bucked voltage to supply

(OP)
Sorry, I don't have any more detail concerning the wiring or the transformer, etc...The drawing shows all the information I have been given for the way it was wired, what was used and how it was explained to me. I'm wondering if the information i got was incomplete or inaccurate.
I have no experience with an autotransformer, but am vaguely familiar with the concept. Sounds like that is what one is used for, boosting the voltage...It was explained to me such that the installers 'made up' for the voltage drop by adding it back with the low voltage output of the added transformer.


RE: adding bucked voltage to supply

It is NOT an auto transformer. It is a normal transformer that adds voltage to the existing grid voltage.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: adding bucked voltage to supply

The wiring diagram cannot be correct. It shows the primary and the secondary of the transformer wired in parallel, which would burn-up the transformer. The secondary needs to be wired in series with the load, and with the correct polarity. One polarity will add voltage and the other will subtract voltage. When wired this way I would call it an autotransformer or a buck/boost transformer.

RE: adding bucked voltage to supply

Badly drawn, or perhaps clearly drawn but electrically incorrect.

RE: adding bucked voltage to supply

My understanding of the terminology is;
1 Transformer connected as an auto transformer.
2 There are three common connections of a voltage adjusting auto transformer;
a Step up auto transformer (boost)
b Step down auto transformer
c Step up with the secondary connections reversed to drop the voltage instead of increasing it (buck and not really an auto transformer connection)
With primary and secondary voltages of 100 volts and 10 Volts and 100 Volts applied;
The step down connection will give 90.91 Volts.
The buck connection will give 90 Volts.

The connections shown will result in rapid transformer burnout.
Look at the connections of the 30 Volt secondary winding.
Both connections of the secondary should be to either the upper line to the load or to the lower line to the load, and the jumper between the two connection points must be removed. Then, depending on the polarity, the connection will either add or subtract 30 Volts from the load voltage.
This type of voltage boost varies with the load. It is not allowed under NEC as the feeder voltage drop is more than 3%.
It will serve for fixed loads. It is not good for varying loads.
A better arrangement is to transform the voltage up to 480 Volts or 600 Volts, size the wire for the allowable voltage drop and then transform the voltage back down at the load.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: adding bucked voltage to supply

Right on Keith. (Prepare for a visit from the over unity enforcers!! grin)

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: adding bucked voltage to supply

Clever arrangement. Is the transformer big enough at 500VA? Wouldn't the transformer need to carry the full power to the load?

RE: adding bucked voltage to supply

It is merely a two-winding transformer connected as an autotransformer. The 500 VA transformer is able to carry (208/30)*500 VA.

RE: adding bucked voltage to supply

http://files.engineering.com/download.aspx?folder=...

I get a slightly different connection than Keith. Connect windings in series so that polarity connects to non-polarity in order to step up.

Note that full load current passes through the secondary winding, but the voltage across that winding is not the load voltage. Therefore transformer VA doesn't need to match load VA.

RE: adding bucked voltage to supply

Isn't it 238/30*500VA?

RE: adding bucked voltage to supply

stevenal,
The reduced voltage across the secondary tripped me up. I saw all the current going through the 2nd and jumped to conclusion.
Thanks, John

RE: adding bucked voltage to supply

Of course stevenal's additional details are important ones. That's preventing anti-coil disaster where all the smoke spews forth. Unfortunately most transformers fail to show the 'dots'. I usually wire up the auto connection then check the resulting boost voltage with a meter before allowing any load to be applied. If the voltage is wonky(the smoking arrangement) from what is expected I swap the secondary leads and re-check.

Is there any dependable standard out there like H1 and X1 are always the dotted terminals?

Keith Cress
kcress - http://www.flaminsystems.com

RE: adding bucked voltage to supply

TOTAL OUTPUT VOLTS x SECONDARY CURRENT (RATING) = The transformer's auto-connection capacity
The Secondary Current Rating is the transformer's non-auto rating, 500VA divided by the the secondary voltage 30V.

i.e.

Quote (LionelHutz)

Isn't it 238/30*500VA?

238/30*500VA = 3,967VA is correct.

Of course in this mickey-mouse application described by the OP we don't actually know what the Total voltage is...

Here's a question:
In this application which end of the 10AWG run would be the better place to locate the auto-transformer?

Keith Cress
kcress - http://www.flaminsystems.com

RE: adding bucked voltage to supply

There will be slightly more voltage at the load if the auto-transformer is at the source end rather than at the load end. Higher voltage means lower current and thus less voltage drop.

RE: adding bucked voltage to supply

Quote (itsmoked)

Is there any dependable standard out there like H1 and X1 are always the dotted terminals?

I think the standards are pretty dependable this way; H1/X1 indicates polarity. Transformers don't always match the standards, though. I have a mis-marked CT, one oddball out of a set of three, with strange secondary connections as a result.

RE: adding bucked voltage to supply

Thanks stevenal, I'll wire that way first and it should then greatly improve the odds of not needing to re-wire.

I'm doing an auto-tranformer out of a 40VA transformer just today because the machine is a 230V machine and the motor brake is not releasing because the poor machine is being fed crappy 208V e.g. 203V and the brake demands 205V pulsating DC out of a bridge rectumfryer.

Keith Cress
kcress - http://www.flaminsystems.com

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