transformer draw

[iwire]

I missed that.
Is a transformer that feeds one device ( hoist ) a separately derived system?

In my opinion if it is field installed yes.

If it was an integral part of the hoist and / or included in the hoists factory schematics I would say no.

 

[LarryFine]

Larry when reverse feeding this transformer XO should remain isolated. (unconnected to anything)

You would not use the neutral from the 208 supply at all.

Wouldn't that mean that, with uneven phase loading on the secondaries, that the X-0 will deviate from zero volts (relative to system neutral), and thereby allowing the 480v secondaries to alter voltage?

 

[iwire]

Wouldn't that mean that, with uneven phase loading on the secondaries, that the X-0 will deviate from zero volts (relative to system neutral), and thereby allowing the 480v secondaries to alter voltage?

Larry to be honest the theory is a bit beyond my knowledge.

I believe you are correct that relative to ground XO will be constantly changing.

But XO has no influence on the output on H1, H2, H3.

In fact under certain fault conditions the conductor connecting XO to ground could burn up.

Here is what Square D has to say in their FAQs

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.

 

[pfalcon]

Bear with me this will take a few postings.

I have a 30kva 3ph transformer when I ordered it I requested a 208 to 480 step up transformer. When it arrived it was a 480 primary to 208/120 secondary. the instructions clearly state it is suitable for reverse connection.
My question is will my load cal. stay the same 83 amp on the 208 primary and 36 amp on secondary?

Yes, the load calculations stay the same. Remember to use transformer type fuses [50+ :1 inrush] over regular [20:1] fuses.

A little help please. Why the need for a 480V transformer? What is it going to feed?

Better motor efficiency.

Bottom line is that this crane was thrown in at the last minute. The owner was able to by this crane completely refurbished had it delivered and told me by the way we need this crane hooked up.

So we are trying to create an industrial power buss in a residentially wired building. NP. If I could afford it I would buy new motors instead.

But I am unclear on the grounding I know the frame has a grounding lug for the frame but as far as xo is concerned and grounding this crane it is fuzzy. My plan is to feed the transformer from the dist. panel with a cb and feed the buss bars with a fuseable disconnect. I would appreciate some solid ideas and or suggestions

Industrial sites typically use the building steel as the ground. Ibeams have a great current capacity for that EG path. The crane on our site would be thoroughly bonded to our building steel.

That would mean you are running ungrounded delta.

That is fine as long as you have a ground fault indicting means and you actually repair each ground fault as it happens.

We run ungrounded delta or high impedence wye on the primary side. This is done specifically to allow one phase to ground fault without shutting down the factory. And yes it must be a high priority repair. I don't recommend this for a small shop.

 

[pfalcon]

My plan is to feed the transformer from the dist. panel with a cb and feed the buss bars with a fuseable disconnect. I would appreciate some solid ideas and or suggestions

With an 83A/36A load have you calculated your needed fuse sizes yet?

Place the fuseable disconnect before the transformer and provide secondary fuses as well. You can adjust the secondary fuses for the load size (recommended) but do not reduce the primaries or you will have nuisance trips. The primaries are driven by the transformer inrush.