Transformer taps when reverse feeding

Sounds like nonsense. (At least as a general statement without consulting specific manufacturer instructions.)

What exactly do we mean by reverse feeding here?
 
Reverse fed transformer taps do not function like they do when they are on the incoming side.

Taps are designed and intended to compensate for changes in the supply voltage while trying to maintain the nominal output voltage.

I know many people instead use the taps to purposely tweak the secondary voltage even with the proper nominal supply voltage. Using the output side taps on a reverse fed transformer would be a similar action.
 
jaggedben said:
Sounds like nonsense. (At least as a general statement without consulting specific manufacturer instructions.)

What exactly do we mean by reverse feeding here?
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Discussion was general, as in your typical dry type 480 delta to 208Y. This was the statement:

"When you reverse feed a transformer you will lose the ability to use the voltage taps on the unit"
 
jim dungar said:
Reverse fed transformer taps do not function like they do when they are on the incoming side.

Taps are intended to compensate for changes in the supply voltage while trying to maintain the nominal output voltage.

I know many people instead use the taps to purposely tweak the secondary voltage even with the proper nominal supply voltage. Using the output side taps on a reverse fed transformer would be a similar action.
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Ok but regardless of what they are "intended" for, what is the issue if adjusting the TAPS in a
reverse fed application provides me the voltage I want for whatever reason?
 
electrofelon said:
Ok but regardless of what they are "intended" for, what is the issue if adjusting the TAPS in a
reverse fed application provides me the voltage I want for whatever reason?
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For some reason transformers scare a lot of people (not talking about you), they are very simple when you know how they work. Changing the taps only add or subtract a couple of windings, the voltage just follows in response. Corner grounded systems really scare a lot of electricians. You may have a greater inrush current raising the output voltage, that’s probably what they are concerned about.
 
hillbilly1 said:
For some reason transformers scare a lot of people (not talking about you), they are very simple when you know how they work. Changing the taps only add or subtract a couple of windings, the voltage just follows in response. Corner grounded systems really scare a lot of electricians. You may have a greater inrush current raising the output voltage, that’s probably what they are concerned about.
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Yeah just curious if there was something I'm missing such as the taps being on the primary instead of the secondary coil or vice versa causing some disadvantage or problem I never knew about. I have several 2400-240 transformers at the homestead here that are reverse fed for step up operation and tapped off nominal and I am not aware of any issues.
 
Maybe it's just that all the labeling for the taps is geared at what jim described in post 4. So if you go the other way you're on your own. (Possibly a 110.3(B) issue, but again, without specific documentation to look at who can say.)
 
When normally fed, the taps allow you to adjust the system to get proper volts per turn based on the actual supply voltage.

When reverse fed, the taps are now on the secondary side, and will allow you to adjust the output voltage, but won't change the transformer volts per turn.

This might be a problem if the supply voltage is higher than nominal, because the volts per turn will be higher than the design point and you will see increased core saturation, possibly with greater losses or inrush problems.

The taps do not become physically unavailable, and will still function to adjust the number of turns in the (now) secondary circuit, and thus to change the output voltage. The taps simply no longer effect the primary circuit.

FWIW in the lab I support we have a drive isolation transformer which was designed with the taps on the secondary side. The core must have been designed to tolerate the highest allowed volts/turn. This probably makes the transformer slightly heavier than it might otherwise need to be, but the manufacturer had their reasons.

-Jon
 
Transformers are designed for a nominal volts-per-turn on the primary. The magnetic flux density in the core is proportional to the volts-per turn applied at the primary. And so applying too many volts-per-turn can bring the core closer to saturation when the magnetic flux is at its maximum value during an AC cycle. A higher applied voltage can be corrected by using a tap that adds more turns.

Having taps on the secondary side can change the transformer turns ratio just the same as it would with taps on the primary side. But taps on the secondary side will not change the flux density in the core created by the primary excitation.

I just saw that winnie posted some similar comments, but I may as well post this since I've already typed it.
 
The video from Maddox is OK as far as it goes, except for omitting grounding issues as Ethan mentioned. However, the presenter says during 0:28-0:35 that "the power is technically being reversed sixty times a second". Instead, the direction of power flow from source to load remains the same over a full AC cycle if we are talking about real power from resistive loads. Perhaps he is just using "power" in a generic sense, but this still might cause some confusion.
 
synchro said:
The video from Maddox is OK as far as it goes, except for omitting grounding issues as Ethan mentioned. However, the presenter says during 0:28-0:35 that "the power is technically being reversed sixty times a second". Instead, the direction of power flow from source to load remains the same over a full AC cycle if we are talking about real power from resistive loads. Perhaps he is just using "power" in a generic sense, but this still might cause some confusion.
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Yeah I considered the omission of the system grounding issue rather significant. They did talk about the increased inrush and the new labeling requirement which is good.
 
Well, they are selling transformers, and they sell the proper step up transformers. Probably getting someone that’s on the edge of buying a new transformer, instead of using one they have lying around, to pull the trigger.
 

From Maddox​

​

3. Reverse feeding​

Step-down transformers are occasionally used as a step-up transformer by “reverse feeding” or “back feeding” the transformer.

Reverse feeding is simply a matter of having the power supply enter at the low voltage windings.

Given the greater availability of step-down transformers, reverse feeding is a fairly common practice. With that said, there are some important things you should consider before making the decision to reverse feed.

There are four main issues with reverse feeding:

Code compliance​

NEC 450.11 (B) permits a transformer to be fed from the secondary side of the transformer as long as the installation complies with the manufacturer's instructions. The code reads as follows: "A transformer shall be permitted to be supplied at the marked secondary voltage, provided that the installation is in accordance with the manufacturer's instructions."

This is typically the reference that is cited for reverse feeding transformers by inspectors to ensure an installation complies.

So while there’s nothing in the NEC that directly prohibits reverse-feeding, your local electrical inspector may decide they want to see language on the transformer nameplate like “suitable for step-up operation” or “rated for bi-directional use” before approving the installation.

Voltage Taps​

Transformer voltage taps are included by default on the primary windings. When you reverse feed a step-down transformer, you lose the ability to adjust the primary voltage rating to accommodate for small discrepancies in the supply voltage. And if there is more than a 5% variance, the windings will over-excite causing excess heat and energy loss. This can result in either saturation of the windings, or a failure to produce secondary voltage.

Inrush​

Inrush current (the amount of current the transformer draws when initially energized) can be up to 16 times greater when reverse-feeding, which could result in nuisance tripping of fuses and circuit breakers.

This greater inrush occurs because normally the output winding is wound first and is therefore closest to the transformer core. When a transformer is back-fed, the inner winding is energized first.

Sizing a larger breaker for the excess load may violate code. The greater the kVA the greater the inrush, which is why some manufacturers recommend you only reverse feed transformers 75 kVA and below.

Vector grouping / No neutral​

The most common transformer vector group is DELTA-WYE, with DELTA being the configuration on the primary side, and WYE on the secondary side. Therefore, reverse feeding a transformer originally designed for step-down operation will result in a DELTA secondary connection, which lacks a neutral.
 
synchro said:
However, the presenter says during 0:28-0:35 that "the power is technically being reversed sixty times a second". Instead, the direction of power flow from source to load remains the same over a full AC cycle if we are talking about real power from resistive loads. Perhaps he is just using "power" in a generic sense, but this still might cause some confusion.
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Plus, it's actually 120 times per second, but who's counting? ;)
 
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