Transformer...almost there

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Bosco

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I'm sorry for starting another thread but I need to hook this up tomorrow. As I stated in the previous post is I'm using a 9 KVA 3 phase 480 to 240 transformer in reverse. My input is 208...hooked it up and got around 420 volts. The band saw has a name plate of 440 volts so I think it will work.
The coils of the transformer are on a floating metal frame and the frame has a large strap bonding to to the shell of the transformer. My concern is that I ground this transformer correctly. Should I remove the strap? I was going to corner ground the H2 leg of the high side and leave XO floating on the low side. Well there is no XO only an X6 (looks like it is the same as an XO). Then I read this in the manual


Grounding: "All core and coils assemblies are solidly grounded to the enclosure internally to ensure that all conductive metal parts have the same
potential."
"The transformer in turn should be securely and effectively grounded as a safety precaution."


Please see attatched image (sorry I made it quick and dirty) of the diagram from the cover. Is it showing that H1 is already grounded to the shell via the strap? I don't quite understand what the dotted lines going into the center of both deltas is representing. Does it seem OK to ground H2?
 
Usually the dashed lines are field installed connections, an ohmmeter check can quickly verify this. X6 should be insulated, not grounded since you are backfeeding this. I would also ground H2 as you planned.
 
After looking at the diagram again, the manufacture is suggesting H1 to be grounded, which will work also, they are using H1 instead of H2 as the grounded conductor. Physically it does not matter as long as only one of them is grounded.
 
hillbilly1 said:
Usually the dashed lines are field installed connections, an ohmmeter check can quickly verify this. X6 should be insulated, not grounded since you are backfeeding this. I would also ground H1 as you planned.
Click to expand...

I agree. Check with a meter to be sure, but my opinion would be to ground H1 also.
If it smokes, blame hillbilly :D
 
On a side note some transformers have adjustable primary taps which could be used to kick it on up to 440, but from the looks of the diagram, yours is not one of them. (keep in mind you will have 420 to ground from the other two phases):grin:
 
Thank you I'll check with a meter. I tried changing the taps it only takes the the voltage lower. So the strap should stay?

What does this mean?

Grounding: "All core and coils assemblies are solidly grounded to the enclosure internally to ensure that all conductive metal parts have the same
potential."

Is it telling me that both the primary and secondary are already bonded to the shell via that strap? If it is is it ok?
 
The strap is usually connected to the core laminations and the enclosure, it just ensures a fault path if coil insulation fails. As for the taps, do they stop at 480? or do they go higher? If they go higher, which I think is 3% or 5% increments, it should increase your output voltage.
 
Bosco said:
Please see attatched image (sorry I made it quick and dirty) of the diagram from the cover. Is it showing that H1 is already grounded to the shell via the strap? I don't quite understand what the dotted lines going into the center of both deltas is representing. Does it seem OK to ground H2?
Click to expand...

Your transformer is a "high leg" design. The low voltage side is rated 240/120 3-phase 4-wire. The high voltage side is 480V 3-phase 3-wire. As others have said, the dotted lines represent possible field connections.

Do not connect anything to the X6 terminal, let it float (just like you do with the primary of a normal step-down transformer). If there is a factory installed bonding strap on this terminal it must be disconnected but the core and enclosure still must be bonded to a building grounding conductor. You will now have a transformer with both primary and secondary isolated from ground.

Your only choice for the 480V side is to leave it ungrounded or ground one of the corners of the delta. In the old days, I recommended floating the system when it fed a dedicated piece of equipment but now, with recent NEC changes, I suggest grounding the B-phase. Just remember, a grounded conductor is colored white or gray and cannot be fused.
 
Hillblly: Thanks I have it set to the highest tap.

Jim: thanks for the reply I will ground the delta with at H2. If H1 is not already tied to the frame (as discussed earlier in the post). Now when you say the grounded conductor must be white or grey. Do you mean the "B" phase wire from the Xformer to the machine will be white (or grey) and not be orange as in a normal 480 feed? I have no neutral it's a straight 3 phase 480 machine.
 
Note the Code says GROUNDED conductor (as opposed to neutral)

200.6 Means of Identifying Grounded Conductors.
(A) Sizes 6 AWG or Smaller. An insulated grounded conductor of 6 AWG or smaller shall be identified by a continuous white or gray outer finish or by three continuous white stripes on other than green insulation along its entire length.
 
Thaks Gus, So the wire going into the machine from the transformer should be gray not orange even though it has potential to the other phase conductors?


On a side note. Now that the grounded "B" phase is going into machine it will have no potential difference to the machines grounded frame theoretically the "B" could short to the frame of the machine and it wouldn't cause a ground fault am I correct? Only a phase to phase short would cause a trip?
 
Bosco said:
I have no neutral it's a straight 3 phase 480 machine.
Click to expand...

This machine will be fed by a corner grounded Delta (if you do what's been suggested). It will work (the machine will not know the difference).

I would install a sign to plainly state it's a corner grounded delta (in case you're not around later).
 
I think I got it. I will pull a grey to the equipment (instead of an orange). I'm grouding the H2 leg to building steel and a ground rod. What would happen if say someone would be touching the frame of the machine and say a part of the building that has a slightly higher impedance to ground (say building steel with a loose bolt). Could there be a potential difference between the machine's frame and the building steel? Would the be a shock hazard there?

Thanks for all responses
 
Bosco said:
On a side note. Now that the grounded "B" phase is going into machine it will have no potential difference to the machines grounded frame theoretically the "B" could short to the frame of the machine and it wouldn't cause a ground fault am I correct? Only a phase to phase short would cause a trip?
Click to expand...
You are correct. A line to ground fault would also be a phase to phase fault.

I assume you are not using 480V breakers, if you were, they would need to be rated for use on a grounded-B phase system (not common).
 
Thanks Jim, I'm not using secondary OCP since I've protected the primary (208 volt at 125%)

I think I'm getting it although I think there could be a shock hazard if certain conditions were met. As in my post above. I hope I'm wrong.
 
Your equipment grounding means (conductor or conduit or both) should also keep everything at the same potential.
 
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