REVERSING A TRANSFORMER FOR STEP-UP

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TRANSMAN

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WHAT WOULD THE CALCULATIONS BE FOR LINE AND LOAD IF WE HAVE 240V 3-PHASE AND STEP-UP TO 480V IF THE 480V EQUIP. IS RATED FOR 3-PHASE 40 AMPS.I AM USING A 45KVA 3-PHASE TRANSFORMER 480-240V AND REVERSE THE CONNECTION.WHAT SIZE BREAKERS WOULD I NEED ON THE LINE AND LOAD SIDE.THANKS
 

tom baker

First Chief Moderator
Staff member
Re: REVERSING A TRANSFORMER FOR STEP-UP

tURN oFF yOUR cAPS lOCK.
This has come up before and its my statement its a violation to use a step down as a step up unless its listed for that use.
Heres another consideration;
A step down is Delta-Wye, grounded secondary by 250.20
A reversed step down is a wye delta. You end up with an ungrounded secondary, a dangerous but allowed installation.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Re: REVERSING A TRANSFORMER FOR STEP-UP

It is my opinion that reverse feeding a transformer is not a violation when following manufacturers published directions. For example Square D no longer uses the terms primary and secondary on the nameplates of their general purpose transformers.

The transformer OCPD sizing requirements of the article 450 do not change when reverse feeding. But beware that the inrush of the low voltage windings will be about 10 to 12 times FLA so your breaker will need to have a magnetic (instantaneous) setting that high. Also I know of no manufacturer that recommends (or allows) the X0 terminal to be connected when reverse feeding.
 

tom baker

First Chief Moderator
Staff member
Re: REVERSING A TRANSFORMER FOR STEP-UP

Jim I found that Sq D makes both a step up and step down transformer. I installed 15 KVA step up 208 to 480 and it was $1500 and a regular step down was $1,000.
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: REVERSING A TRANSFORMER FOR STEP-UP

From Square D FAQ enter backfeed transformer

Question Can Transformers be backfed (used in reverse)?

Answer

1. Backfeeding is not recommended for general purpose transformers smaller than 3kVA, and is not allowed for any Industrial Control Transformers of all sizes, because windings are compensated. Backfeeding will result in lower than expected output voltage.

2. Backfeeding causes very high excitation inrush, making coordination to breakers or fuses difficult without exceeding the limits set by the National Electrical Code. Avoid backfeeding wherever possible for this reason.

3. If a Delta-Wye transformer is to be backfed so that the Wye side is the input, do not connect the neutral terminal to the primary system neutral, nor should the neutral terminal be connected to ground.

4. If the transformer has taps, when backfed the taps do not help compensate for poor (other than nominal) source voltage to provide the appropriate magnetic flux levels in the core as they are designed to do.
Question Does back-feeding a low voltage dry type distribution transformer 15kVA or larger void the transformer`s UL label?

Answer Square D does not label our low voltage dry type distribution transformer`s terminals as primary or secondary. They are labeled as high voltage or low voltage. In the event that the line-to-line voltage of the designed primary is identical to that of the designed secondary, then the designed primary is marked high voltage (HV) and the designed secondary is marked low voltage (LV.) This does not prevent the transformer from being backfed. Square D does not recommend backfeeding transformers.
It seems backfeeding is poor design choice even though allowed.
 

jtester

Senior Member
Location
Las Cruces N.M.
Re: REVERSING A TRANSFORMER FOR STEP-UP

If the transformer is properly installed, and the manufacturer does not forbid it, there is no reason why a transformer in the larger kva's can't be fed backwards. I don't know how one side could be labeled in and the other out.

I suggest that the reason that some manufacturers don't recommend backfeeding is that they make more money selling a 208 delta to 480 wye, when the 480 neutral is not needed.

Jim T

Jim T
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Re: REVERSING A TRANSFORMER FOR STEP-UP

Jim T,

I think the reason most manufacturers do not recommend back feeding transformers is the high likely hood of a dissatisfied customer due to all of the potential problems. Read the information iwire posted from Square D, all five points are potential "gotchas".
 

kthbrwn

Member
Re: REVERSING A TRANSFORMER FOR STEP-UP

1. ...because windings are compensated. Backfeeding will result in lower than expected output voltage.

2. Backfeeding causes very high excitation inrush

Can anyone expound on these two statements from Square D, which IWIRE posted for us? I'm lacking in the theory behind them.
 

rattus

Senior Member
Re: REVERSING A TRANSFORMER FOR STEP-UP

Originally posted by kthbrwn:
1. ...because windings are compensated. Backfeeding will result in lower than expected output voltage.

2. Backfeeding causes very high excitation inrush

Can anyone expound on these two statements from Square D, which IWIRE posted for us? I'm lacking in the theory behind them.
Let me make educated guesses:

1. I believe the turns ratio is fudged a bit to compensate for internal voltage drops. This fudge factor works the wrong way when backfeeding.

2. Magnetizing inductance is proportional to the square of the number of primary turns. If we have a step-down of 20:1 say, then the magnetizing inductance seen in a backfed transformer is 1/400 instead of 1/20 as we might expect. The magnetizing current would be 20x what we would expect and the inrush current would be much greater as well.

Let's hear some comment on these ideas.
 

hardworkingstiff

Senior Member
Location
Wilmington, NC
Re: REVERSING A TRANSFORMER FOR STEP-UP

Well crap.

I just quoted a job stepping up a 120/240-volt service to 600-volts, then stepping it back down due to the distance involved and the fact that the power company will not provide a 240/480-volt single phase service.

Is it the consensus of the more educated people (aka engineers) on this board that I'm asking for trouble? How about the more experienced (aka old contractors) people on this board?

Thanks,
Lou
 

templdl

Senior Member
Location
Wisconsin
Re: REVERSING A TRANSFORMER FOR STEP-UP

Yes, inrush is proportional. Is it's 20x the HV FLA it will be 20x the LV FLA. No magic. However, in the lower KVA 150degC transformers the inrush can approach 23x with transformers up to about 7kva decreasing to about 10x at 75kva and so on.
115, and 80degC, and K-factor transformers can increase the inrush 20-25% and more.
 

rattus

Senior Member
Re: REVERSING A TRANSFORMER FOR STEP-UP

"2. Magnetizing inductance is proportional to the square of the number of primary turns. If we have a step-down of 20:1 say, then the magnetizing inductance seen in a backfed transformer is 1/400 instead of 1/20 as we might expect. The magnetizing current would be 20x what we would expect and the inrush current would be much greater as well."

Let me correct this statement. If we do the arithmetic, we see that the magnetizing current would be 20x the primary magnetizing current in a 20:1 backfed stepdown transformer--as we might expect.

Don't take this as gospel truth, I am merely trying to understand it myself. Listen to the guys who have done it!
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Re: REVERSING A TRANSFORMER FOR STEP-UP

Hardworkingstiff,

I have done this several times without an issue. The compensated winding causing voltage problems is only on transformers up to 3kVA (well maybe 7.5kVA).

The major concern is the inrush, be prepared to use the full 250% rating for your primary protection. Also, I have experienced that the inrush of a moderately loaded "600V" transformer is less than that of an unloaded one.

Finally, do not use the taps on the step up transformer (it is okay to use them on the step down one).
 

sandsnow

Senior Member
Re: REVERSING A TRANSFORMER FOR STEP-UP

I am in no way qualified from an theory standpoint to comment on this. I only pass on what I've read.
Eaton-Cutler Hammer has a handout on this. It states that on a step down XFMR that the H windings on on the outside and the X windings are on the inside. The high inrush comes from magnetizing the core from the inside out. They do not recommend backfeeding above 75kVA. I think this relates to Rattus' comment #2.
I can post an exact exerpt back at work on Monday.
 

brian john

Senior Member
Location
Leesburg, VA
Re: REVERSING A TRANSFORMER FOR STEP-UP

I always buy the right transformer for the job at hand. I prefer to have a delta primary with a wye secondary that permits grounding of the XO/Neutral/grounded conductor. Step up, step down I buy delta/wye

The major problem I see with a delta grounded secondary is. Other electricians will freak out when they find "C" phase at "0" volts to ground and "A" and "B" phases at 480 VAC to ground and no 277 VAC for lighting.

For testing purposes, such as deriving the proper voltage on the output of a UPS set for a foreign voltage ( testing UPS's and Generators in embassies) I have utilized two transformers stepping voltage up and down adjusting taps as necessary to achieve a voltage my load banks operate accurately at. I also have to feed the fans from a different source as these UPS's/generators are normally 50hz.
 
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