As you may have noted the previous message was sent unitentionally prior to being completed, reviewed, and eddited.
A transformer is simply made up of two sets of windings where each set of windings may have the same number of turns on each winding may have a different number of turns. A transformer really doesn't care which winding it is fed from.
It is us who define a step down transformer as being reverse feed when is is energized from the LV windings when it could be simply stated is being applied as a step up.
Transformers that are applied as step down will require a given amount is of inrush, the magnetizing current that is require to magnetize the core, when it is first energized.
In very general terms the inrush will be in the following ballpark:
X FLA KVA
23X Up To KVA7-1/2
19X To 10
14X To 15
11X To 50
10X To 75
9X To 150
8X To 225
As you can see the trend is that as the KVA decreases the inrush increases. What you have to pay attention to are smaller KVA transformers and sizing the breaker which feeds the transformer.
Residential breakers may be magnetically calibrated at 6-7 times their FLA where industrial/commercial breakers at 10X. One must us caution not to undersize the breakers because the transformer is unforgiving and will draw the same inrush and trip the breaker as you energize the transformer.
Now, using that same transformer as a step up compounds the problem because the inrush multiplier increases. Instead ao 15X it may be 220X the LV full loss current which making it ever more important not to undersize the breaker.
Why is the LV inrush current ratio so much greater? The LV windings happen to be wound first next to the core and the HV windings over then. The LV windings are closer to the core and the effect of magnetizing the core is much greater.
A LV transformer that is specifically designed, as a step up will have its HV winding wound first and the LV wound last. Is addition a good transformer design will be designed with a lower flux density that also aids in reducing the LV inrush.
Remember the taps? If transformer is designed for step down the taps are located on the HV windings. When designed as a step up, the taps will be located of the LV windings.
A transformer is simply made up of two sets of windings where each set of windings may have the same number of turns on each winding may have a different number of turns. A transformer really doesn't care which winding it is fed from.
It is us who define a step down transformer as being reverse feed when is is energized from the LV windings when it could be simply stated is being applied as a step up.
Transformers that are applied as step down will require a given amount is of inrush, the magnetizing current that is require to magnetize the core, when it is first energized.
In very general terms the inrush will be in the following ballpark:
X FLA KVA
23X Up To KVA7-1/2
19X To 10
14X To 15
11X To 50
10X To 75
9X To 150
8X To 225
As you can see the trend is that as the KVA decreases the inrush increases. What you have to pay attention to are smaller KVA transformers and sizing the breaker which feeds the transformer.
Residential breakers may be magnetically calibrated at 6-7 times their FLA where industrial/commercial breakers at 10X. One must us caution not to undersize the breakers because the transformer is unforgiving and will draw the same inrush and trip the breaker as you energize the transformer.
Now, using that same transformer as a step up compounds the problem because the inrush multiplier increases. Instead ao 15X it may be 220X the LV full loss current which making it ever more important not to undersize the breaker.
Why is the LV inrush current ratio so much greater? The LV windings happen to be wound first next to the core and the HV windings over then. The LV windings are closer to the core and the effect of magnetizing the core is much greater.
A LV transformer that is specifically designed, as a step up will have its HV winding wound first and the LV wound last. Is addition a good transformer design will be designed with a lower flux density that also aids in reducing the LV inrush.
Remember the taps? If transformer is designed for step down the taps are located on the HV windings. When designed as a step up, the taps will be located of the LV windings.