Back wired 3 phase transformer

[davet]

Txtrf.- used - 45 kva - 3 phase -delta/y - primary multiple taps ( lowest tap 400V ) - 480 pri.x 240/120 sec. -secondary side bonding jumper connect to case of trany.
Power source - 3 phase 208V - disconnect -100 amp fuses
Present maint.guy has this trany. wants to connect to new machine that requires 3 phase 480V
Present emt wiring to old machine in place - 3 phase/ground wire, wire size ok.
This was brought to my attention.
The manufacturer said he could hook it up backwards.
I see issues with this but not sure exactly what. Ex. delta 480V no gound.
Any input helpfull

 

[templdl]

Txtrf.- used - 45 kva - 3 phase -delta/y - primary multiple taps ( lowest tap 400V ) - 480 pri.x 240/120 sec. -secondary side bonding jumper connect to case of trany.
Power source - 3 phase 208V - disconnect -100 amp fuses
Present maint.guy has this trany. wants to connect to new machine that requires 3 phase 480V
Present emt wiring to old machine in place - 3 phase/ground wire, wire size ok.
This was brought to my attention.
The manufacturer said he could hook it up backwards.
I see issues with this but not sure exactly what. Ex. delta 480V no gound.
Any input helpfull

A transform realy doesn't care which way it receives it's power. Yes, it is how you configure the newly derived system and what you should do do with that pesky X0 termination which now is on the primary side.
Make sure that the X0 is insulated an isolated from all ground connections. There is no need to connect a neutral from the souce, just the 3 lines.
Then there is that secondary that you pointed out is 3 wire now. The simplest way to ground it is by a corner grounding the Delta which is normally the 'B' phase. If you use a 3p breaker for the OCPD of course one of the lines is at ground potential which is normal. What may be strange is that your EGC will originate at the 'B' phase which you have grounded which makes it the same potential as a line anlike a 1ph3w or 3ph4w system where none of the lines are grounded, just the neutral. Is a grounded 3w ground 'B' phase Delta treat the 'B' phase as a you would a common line conductor we ith the exception that it can never be fused. You can, however, use a multipole breaker that has a common trip.
The EGC connect to the 'B' phase it treated like a normal EGC.
However, there can be a reason as to why you may elect to have an ungrounded system which does open up a whole can of worms. Since there will be no ground there will be no ground fault should one line go to ground. The problem is should a second line goes to ground where you are now dealing with a 480v ground fouild which may be greater than 480v if it is an arcing fault, not good. With an ungrounded system a ground fault detection system of some type must be used.
But the simplest thing to do is to just use a corner grounded 'B' phase Delta.

 

[Ingenieur]

Are you sure that it is delta-wye and not delta-delta center tap?
do the taps go high enough to get 480 out of 208?

 

[Jraef]

Txtrf.- used - 45 kva - 3 phase -delta/y - primary multiple taps ( lowest tap 400V ) - 480 pri.x 240/120 sec. -secondary

No, can't be a Delta-Wye, the Wye has to be 208Y120. This is either a mistake in the description, or it's a Delta-Delta with a center tapped winding to provide for a small amount of 120V. Yes, it's still going to be a 4 wire output, but not a Wye if it really says 240/120V for the secondary. So if you put 208V in, you are going to get only 416V out. That will be on the ragged edge of minimally acceptable to a 460V motor, but ANY slight voltage drop on the utility side will cause severe motor stress and premature failure. Other than that, it's fine...

Buy the right transformer, don't connect the X0 terminal on the Wye side and ground one corner of the 480V side as suggested. But if your machine has a VFD on it, you need to take other precautions on that Delta feed. If there is a VFD, you might want to reconsider and find a Drive isolation Transformer, it will be 208V Delta to 480Y277 and ground the X0 on the 480Y side. They are made, but are not common.

 

[templdl]

No, can't be a Delta-Wye, the Wye has to be 208Y120. This is either a mistake in the description, or it's a Delta-Delta with a center tapped winding to provide for a small amount of 120V. Yes, it's still going to be a 4 wire output, but not a Wye if it really says 240/120V for the secondary. So if you put 208V in, you are going to get only 416V out. That will be on the ragged edge of minimally acceptable to a 460V motor, but ANY slight voltage drop on the utility side will cause severe motor stress and premature failure. Other than that, it's fine...

Buy the right transformer, don't connect the X0 terminal on the Wye side and ground one corner of the 480V side as suggested. But if your machine has a VFD on it, you need to take other precautions on that Delta feed. If there is a VFD, you might want to reconsider and find a Drive isolation Transformer, it will be 208V Delta to 480Y277 and ground the X0 on the 480Y side. They are made, but are not common.

Great point. If you can order a transformer specifically as a step up do so. If a transformer manufacturer makes it correctly they will wind the HV windings on the core first rather than the LV windings and the taps will easily be placed on the LV windings.
This configuration will also reduce the inrush when the transformer is energized since the LV winding are now away from the core.

 

[Jraef]

Great point. If you can order a transformer specifically as a step up do so. If a transformer manufacturer makes it correctly they will wind the HV windings on the core first rather than the LV windings and the taps will easily be placed on the LV windings.
This configuration will also reduce the inrush when the transformer is energized since the LV winding are now away from the core.

Yep, another good point that a lot of people miss (me included).

 

[Phil Corso]

Jraef...

'Taps' are generally installed in the higher-voltage winding because it has a greater number of turns, thereby making it easier to select integer-valued turn-ratio's!

Regards, Phil Corso

 

[templdl]

Jraef...

'Taps' are generally installed in the higher-voltage winding because it has a greater number of turns, thereby making it easier to select integer-valued turn-ratio's!

Regards, Phil Corso

You are correct on the advantage of the taps on the HV windings but whern the HV windings are wound first it is impractical to tasp thern so the LV windings being on the outside are trapped. The exact winding ratios may not be available because a fewer turns but remember that voltages are not an exact science as some would like to think as supply voltage are not precise anyway. There are many compromises that are made.

 

[Phil Corso]

Templdl...

There is no doubt about variation in configuration, but in my experience, LV, the larger-current windings are installed first, closest to the core!

Phil

 

[GoldDigger]

The other reason to put taps on the supply-side winding is that you can adjust the magnetizing voltage to avoid saturation in the case of input voltage on the high side of normal.
You are always hitting the maximum efficiency operating point.