No load losses - normal vs reverse fed

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GoldDigger

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electrofelon said:
Phil,
Are you sure? I thought I have heard frequently that a reverse fed transformer can have a higher than normal inrush as it may not be designed to be fed from that end. You disagree?
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The explanation I have heard is that the inrush (as a multiple of full load amps) will vary depending on whether the winding being energized is wound closest to or farthest from the core. I have not seen a good theoretical explanation for this though.
 
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Phil Corso

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electrofelon said:
Phil,
Are you sure? I thought I have heard frequently that a reverse fed transformer can have a higher than normal inrush as it may not be designed to be fed from that end. You disagree?
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ElectroFelon… You were too impatient.

The answer is yes… but only if it’s an emergency, AND, the Xfmr mfgr agrees! Many factors are involved. For simplicity consider an unloaded 2-winding, single-phase xfmr! Then, some of the factors to consider are:

1) Condition of the supply-side source! Should be sinusoidal, and not have significant harmonics!

2) Number of terms of the winding being energized, i.e., Np or Ns, and not turns-ratio, Np/Ns!

3) Effective impedance of the connection between the source and the winding!

4) Effective resistance of the winding being energized should not be too large relative to the induced voltage expected in that winding!

GoldDigger…

Maybe… but, still depends on the Xfmr’s parameters! Not some Rule-of-Thumb (RoT)!

Phil
 
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iwire

iwire

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Square D seems to think reverse feeding will result in higher inrush currents

Cause:
Customers often desire a voltage change that is reversed from the designed orientation of the primary and secondary windings

Resolution:
1. The practice of backfeeding a general purpose transformer is NOT recommended, especially in transformers smaller than 3Kva. Backfeeding is not allowed for any Industrial Control Transformers of any size, because windings are compensated and 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.
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http://www.schneider-electric.us/en/faqs/FA91962/
 
Besoeker

Besoeker

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electrofelon said:
...if I reverse feed a transformer, will the no load losses typically be the same? I often hear about inrush being higher but I am unsure if that translates to higher steady state no load losses.
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There's another thing to think about. Say, for simplicity, the transformer is designed to step down from 240V primary to 120V secondary the turns ratio is likely to lower than 2:1.
This because the 120V would be designed to be the on-load voltage so it would be higher off-load. If you feed it with 120V you will most likely get less than 240V out.
 
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GoldDigger

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Besoeker said:
There's another thing to think about. Say, for simplicity, the transformer is designed to step down from 240V primary to 120V secondary the turns ratio is likely to lower than 2:1.
This because the 120V would be designed to be the on-load voltage so it would be higher off-load. If you feed it with 120V you will most likely get less than 240V out.
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In the US, that kind of fudged turns ratio is often seen in control transformers, but rarely in power transformers.
One should be particularly careful before reverse feeding a control transformer.

Sent from my XT1585 using Tapatalk
 
Besoeker

Besoeker

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GoldDigger said:
In the US, that kind of fudged turns ratio is often seen in control transformers, but rarely in power transformers.
One should be particularly careful before reverse feeding a control transformer.

Sent from my XT1585 using Tapatalk
Click to expand...
It has to be to get the right on-load voltage. It isn't a fudge.
 
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GoldDigger

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Besoeker said:
It has to be to get the right on-load voltage. It isn't a fudge.
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AFAIK, that might only be done with transformers with multiple taps.
It does seem like the "right" thing to do, but I am not aware of it being routinely done with transformers for US projects.
And it would, of course, make the no-load voltages wrong.

Any transformer manufacturers represented on the Forum?
 
electrofelon

electrofelon

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GoldDigger said:
AFAIK, that might only be done with transformers with multiple taps.
It does seem like the "right" thing to do, but I am not aware of it being routinely done with transformers for US projects.
And it would, of course, make the no-load voltages wrong.

Any transformer manufacturers represented on the Forum?
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I am also skeptical that they routinely fudge turns ratio, just because I never recall noticing an aberration in transformer voltages after hooking one up and testing.
 
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templdl

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Wisconsin
iwire said:
Square D seems to think reverse feeding will result in higher inrush currents



http://www.schneider-electric.us/en/faqs/FA91962/
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Thinking that brush is proportional from the HV to the LV we indings the LV inrush will be proportionally higher.
But, when you take a common step down transformer and use it as stepup the LV windings were wound over the core first. If they weren't it would be difficult at best to tap the HV windings being underneath the LV windings.
That bring said, with the LV windings wound next to the core the inrush will be greater that the pri. and sec. ratio. So instead of 1:4 for example the inrush for the LV windings with be to some degree greater than 4x. So to stay out of trouble it is best to apply a breaker that has the highest mag trip as allowed keeping in mind the the mag trip calibration is normally 10x the breaker rating +-20% noting the breakers a commonly calibrated on the high side.
Inrush may be 13x the Fla or higher for small kva transformers and less for high kva. It will be different for a great Ben kva for (80,115,and 150degC rise, energy efficient, K-factor etc.
 
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Besoeker

Besoeker

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electrofelon said:
I am also skeptical that they routinely fudge turns ratio, just because I never recall noticing an aberration in transformer voltages after hooking one up and testing.
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That's kinda the point - to get the correct on-load voltage. It isn't a fudge.
Turns ratio test is done at no-load so the voltage is higher than the rated on load voltage. This means that turns ratio for my 240/120V tranny is not exactly 2:1.
 
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iwire

iwire

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Besoeker said:
That's kinda the point - to get the correct on-load voltage. It isn't a fudge.
Turns ratio test is done at no-load so the voltage is higher than the rated on load voltage. This means that turns ratio for my 240/120V tranny is not 2:1.
Click to expand...

Yet the reality is the company I work for does back feed transformers and we are not seeing any difference in voltage. back feed a 480-208Y/120 transformer with 208 and we get 480 out of it.

Even the link I posted from Sq D says that the issue you are bring up only applies to power transformers under 3kva or control transformers.
 
Besoeker

Besoeker

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iwire said:
Yet the reality is the company I work for does back feed transformers and we are not seeing any difference in voltage. back feed a 480-208Y/120 transformer with 208 and we get 480 out of it.

Even the link I posted from Sq D says that the issue you are bring up only applies to power transformers under 3kva or control transformers.
Click to expand...

Actually what the link states is:
"The practice of backfeeding a general purpose transformer is NOT recommended, especially in transformers smaller than 3Kva."
 
electrofelon

electrofelon

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Cherry Valley NY, Seattle, WA
Besoeker said:
That's kinda the point - to get the correct on-load voltage. It isn't a fudge.
Turns ratio test is done at no-load so the voltage is higher than the rated on load voltage. This means that turns ratio for my 240/120V tranny is not exactly 2:1.
Click to expand...

but I test/confirm voltages with no load - kinda a good idea to make sure you didnt screw anything up before connecting to clients equipment! Now I admit I am not specifically comparing input to output voltage, just nominal, but I still think I would notice.....
 
iwire

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