Transformers in Series

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powerplay

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Hello all !

We are moving 480 volt Equipment from an Building with 347/600 volt Supply to an Building with 120/208 volt supply.

The 75 kva Transformer from 600 volt to 120/208 volt, as well as 75 kva Transformer from 600 volt to 120/208 volt we are hoping will be able to be connected in series from the 208 volt source to produce the 480 volts for the Baler Equipment. We'll be doing more homework on Canadian Electrical Code, but does it sound realistic ?

Thank you for any feedback !
 
I would like to read the logic in what you posted if you could please.

Voltages will not add up like that, and even if they did it would only be 416.

Your new building is 120/208. What are you wanting to do with a 600V primary transformer?
if you connect it to 208 your only going to get 41 volts out the back end.

connect it backwards, you get 600 out, but how will that equate to 480 for the baler?
 
At one time I used an auto transformer connection to go between 600V and 480V, this allowed Candian equipment to be used in the US. However I believe this arrangement is no longer allowed under the NEC.
 
ron said:
If both transformers are 600V primary and 120/208V secondary, how will you get 480V by connecting those same transformers? Something is not adding up.
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Oops.. had to run out for an meeting, but the existing building has 600 volts, with:

75 kva transformer #1 600volt to 120/208
75 kva transformer #2 600 volt to 480

Hooking up three phase 208 to transformer secondary of 120/208 volts would put out 600 volts. Connecting transformer #2 with 600 volt on primary woukd ouyput 480 volts.
 
jim dungar said:
At one time I used an auto transformer connection to go between 600V and 480V, this allowed Candian equipment to be used in the US. However I believe this arrangement is no longer allowed under the
jim dungar said:
At one time I used an auto transformer connection to go between 600V and 480V, this allowed Candian equipment to be used in the US. However I believe this arrangement is no longer allowed under the NEC.
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thanks for the feedback !
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Auto transformer not permitted to prevent higher fault current ?

Utilizing existing equipment creatively we presume would work. For improved fault protection we still plan to use HRC Fuses in the 200 amp 120/208 volt Disconnect if possible, but was wondering if using the two isolation transformers connected directly together, protected by an 208 volt fused disconnect, had some code/theory issues ?
 
This could be made to work, but is likely a poor idea.

1) the 600V to 208/120V transformer used 'in reverse' means that it will be subject to increased inrush current, and the voltage adjustment taps will not be on the primary, so the transformer might be run at an excessive volts/turn if the '208V' supply runs high. This can make OCPD on the primary difficult, because of the excessive inrush current.

If you do this, make sure you _don't_ connect the now 'primary' neutral to anything.

2) The 600V system will now be a delta, and you need to ground it. A 'corner ground' is probably just fine for this. You will need OCPD for this 600V secondary.

3) The 600V to 480V transformer will be used as normal.

4) The combination of the two transformers 'in series' will result in higher losses and poorer voltage regulation than the correct transformer.

-Jon
 
230123-2214 EST

winnie:

I am not quite sure I agree with your comment on saturation.

Typically step down power distribution transformers have a smaller turns ratio than their rated full load voltage ratio. This means that when reverse fed with a voltage equal to the full load rated output voltage of the transformer there is a lower peak magnetic flux level. Thus, the core is driven less into saturation. And this also means that the output voltage at full load is percentage wise less than what the transformer voltage ratio would implied by the turns ratio.

.
 
Gar,

My point was not about the normal turns ratio, but rather the lack of adjustment taps when the secondary is used as the primary.

If the nominal 208V supply runs hot, then there is no way to compensate for this, causing saturation issues.

Jon
 
powerplay said:
Hello all !

We are moving 480 volt Equipment from an Building with 347/600 volt Supply to an Building with 120/208 volt supply.

The 75 kva Transformer from 600 volt to 120/208 volt, as well as 75 kva Transformer from 600 volt to 120/208 volt we are hoping will be able to be connected in series from the 208 volt source to produce the 480 volts for the Baler Equipment. We'll be doing more homework on Canadian Electrical Code, but does it sound realistic ?

Thank you for any feedback !
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Assuming you posted the incorrect ratios there, and meant that you want to use the 600 to 208 to step UP from 208 to 600, then put the other one in series to go from the 600 step back DOWN to 480?

Aside from the problems Winnie already posted, you will not likely really save yourself any money compared to just buying a 208 to 480 transformer. Don’t forget that you will need primary protection for the 208-600 transformer, mounting, conduit, wire and at least primary protection on the 600-480 transformer, then secondary protection on the 480 side. That’s a lot of labor and hardware just to avoid buying the correct single transformer.

And I’ll just re-emphasize the added losses issue. All transformers have losses, probably in the 2-3% range (worse if they are older) under load. But even 1-2% unloaded, meaning that you will be losing an EXTRA 1500W+ of heat, whether you are using the machine or not. Think of it as at least a 6ft baseboard heater running 24/7/365 (unless someone opens the 208V primary disconnect EVERY TIME the bailer is not being used). That energy cost will add up over time.
 
powerplay said:
I am under the impression that motors are ok, but with solid state controls will cause problems with voltage spiking.
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The issue had to do with branch circuits and feeders with grounded conductors being fed from autotransformers. I see that exceptions have been added for industrial applications using 600 and 480V.
 
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