Transformer

[scott501st]

I'm working on a circuit design problem and need to power a 50hp, 480vac motor from a 208/120 panel. I'm thinking that I could backfeed a transformer but am unsure what i would need to do from that point

 

[hillbilly1]

Keep in mind your new "primary" current will almost double that of the secondary. Will your exsisting 208 feed or service be large enough?

 

[scott501st]

this is a theoretical design problem not a realworld design problem, so for the purpose of the problem it's assumed that the feed is of sufficient size

 

[kwired]

I'm working on a circuit design problem and need to power a 50hp, 480vac motor from a 208/120 panel. I'm thinking that I could backfeed a transformer but am unsure what i would need to do from that point

You do exactly what you would do if feeding a 208 load from a 480 source, you just have different numbers to work with. You still have a feed to a transformer, a feeder tap on the secondary, a secondary overcurrent device, and a motor circuit. The only problem you will have that is different than transforming the other way is that your separately derived system will not have a neutral point and you will need to set up a proper ungrounded system with ground monitor or ground a phase conductor and properly install a grounded phase system. You will also not want to connect the 120 volt neutral to the XO terminal, you do need to connect the equipment grounding conductor from the 120/208 supply to the transformer case.

 

[iceworm]

---. You will also not want to connect the 120 volt neutral to the XO terminal, you do need to connect the equipment grounding conductor from the 120/208 supply to the transformer case.

New poster - long time engineer, but I still need regular doses of enlightment.

What happens if you connect the transformer 208Y primary neutral (Xo) to the system 208V neutral?

I've back fed a few Delta/Wye transformers, feeding the wye side for the primary. And as you said, I've never connected the primary wye point.

The utilities regularly use wye primary transformers connected to an MGN and they seem okay with it.

So, while I agree I wouldn't connect the wye point to the 208V neutral, I don't know why it would cause a problem, or what problems it would cause.

 

[hurk27]

and the fact that the inrush to the transformer will be much higher, and if the listing for the transformer is not for the transformer being back fed, it can be a 110.3 issue.

 

[kwired]

New poster - long time engineer, but I still need regular doses of enlightment.

What happens if you connect the transformer 208Y primary neutral (Xo) to the system 208V neutral?

I've back fed a few Delta/Wye transformers, feeding the wye side for the primary. And as you said, I've never connected the primary wye point.

The utilities regularly use wye primary transformers connected to an MGN and they seem okay with it.

So, while I agree I wouldn't connect the wye point to the 208V neutral, I don't know why it would cause a problem, or what problems it would cause.

I don't have experience with what happens myself, but if you search this site there have been a few threads more recently addressing this, many of them have had overheating problems with the transformer if they had the 208 volt neutral connected to XO terminal.

 

[weressl]

I'm working on a circuit design problem and need to power a 50hp, 480vac motor from a 208/120 panel. I'm thinking that I could backfeed a transformer but am unsure what i would need to do from that point

If this is all the data you have it is insufficient to decide the size of the transformer. An across-the-line start, depending on the load and motor characteristic and the transformer impedance, the transformer should be anywhere between 2.5-8 times the motor kVA. On a 1-to-1 sized transformer the inrush would collapse the voltage so the motor would not be able to come up to speed.

 

[cripple]

I don't have experience with what happens myself, but if you search this site there have been a few threads more recently addressing this, many of them have had overheating problems with the transformer if they had the 208 volt neutral connected to XO terminal.

When the secondary (WYE) of a DELTA-WYE transformer is energized, instead of the primary (DELTA), the neutral should not be connected. An unbalanced source might circulate current in the transformer DELTA and cause over-heating. Also the impedance to ground of the transformer might be lower than the system ground impedance and excessive current to ground might over-heat the transformer. Unbalanced conditions could cause a voltage to ground to appear on the enclosure if it was connected to the neutral. But since you are feeding a single three phase load I would think the unbalanced condition could not occur.