Reverse Fed 240-480 Transformer question

Status
Not open for further replies.
D

Darkcustom2013

Member
Location
PA
Occupation
Electrician
Hi all,

Looking for alittle help on the topic. We had a existing corner ground delta delta 45kva transformer that was hooked up in reverse (240-480) feed a piece of equipment in one part of the plant. It was feeding 240v 3 wire 3phase on the secondary and feeding 480v 3 phase off the primary. This piece of equipment was removed and we had the transformer just sitting around.

Someone in engineering order another piece of equipment for 480v not 240v, so they wanted me to hook the old transformer back up. The current setup was a 100A main breaker, disconnect feeding the 240v primary side fused at 80A, secondary disconnect fused at 50A protecting a motor with a FLA of 28A. My question is, when energized I am getting 480V P-P on the secondary but my P-G is 200v-210v on 2 phases and 315v P-G on another phase. Is this normal? or is there something going on?
 
The secondary is bonded to the transformer case and to building ground. Would it be better to drive a ground rod in?
 
transformer.jpg

This is how all the grounds were connect. Originally, one of the grounds ran out the bottom of the transformer to a ground rod. When I hooked it back up I connect it to a building I beam behind the trans.
 
It might be worth noting that, although it is done, operating a transformer in "reverse" is a Code violation unless so designed by the manufacturer.
 
Augie, I agree with you. Management didn't want to spend the money for a new one and since they already had it hooked up that way before, they didn't want to listen to me about it being a violation.
 
Just a point.
The primary of a transformer is the side where the source is connected, while the secondary is the side with the load. The windings with the H terminals should be the high voltage ones, while the X terminals would be the low voltage. Typically a transformer is built with the secondary windings close to the core and the primary windings on the outside with taps.
 
You state the 480v side is bonded: how ?
(THe strap shown in the picture is a static ground).
As noted, it appears the 480v side is presently operating ungrounded,
 
Last edited:
correct, at first I didn't notice that it was just a static ground. so it seems that it was always running with a ungrounded 480v side. So I would need bond one phase, correct?
 
You can operate it ungrounded if you like (the NEC calls for ground fault indication) or you can corner ground. In either case, if any drives are being used they can suffer damage.
 
what are you guys recommendations as far as corner grounding the 480v side vs just leaving it as is and doing a ground fault indicator. Thanks again for all your help.

Also, this is a piece of equipment that is only used about once a month. Would it be better to leave the transformer energized at all times, or just have them turn the transformer off when not in use.
 
Darkcustom2013 said:
what are you guys recommendations as far as corner grounding the 480v side vs just leaving it as is and doing a ground fault indicator. Thanks again for all your help.

Also, this is a piece of equipment that is only used about once a month. Would it be better to leave the transformer energized at all times, or just have them turn the transformer off when not in use.
Click to expand...
Ground detectors should only be employed were there are qualified individuals that will remediate grounding issues. I would go with a corner ground.

Why pay for monthly transformer losses? If there is no problem energizing the reverse connected transformer I would turn it off when not used.
 
augie47 said:
In an I.M. then OP advised a magnetic motor starter and 3 phase motor.
(I suggested corner grounding but asked he get other opinions)
Click to expand...

For a motor with no drive, I absolutely agree, corner ground is the way to go.

The only thing worth checking is if the motor itself is 'dual voltage', because you might be able to ditch the transformer entirely, simply needing a new higher current starter.

-Jon
 
Darkcustom2013 said:
We had a existing corner ground delta delta 45kva transformer that was hooked up in reverse (240-480) feed a piece of equipment in one part of the plant. It was feeding 240v 3 wire 3phase on the secondary and feeding 480v 3 phase off the primary.
Click to expand...
Darkcustom2013 said:
The secondary is bonded to the transformer case and to building ground.
Click to expand...
Darkcustom2013 said:
... at first I didn't notice that it was just a static ground. so it seems that it was always running with a ungrounded 480v side. So I would need bond one phase, correct?
Click to expand...

There can be some confusion when talking about the primary and secondary when a transformer is fed in reverse. And so when you said the secondary is bonded, I just want to make sure you didn't mean that XO of a reverse fed delta-wye transformer was bonded to ground. Doing so could cause circulating currents in the delta and therefore current through an XO to ground connection when there are even small imbalances between the L-G input voltages.
 
winnie said:
For a motor with no drive, I absolutely agree, corner ground is the way to go.

The only thing worth checking is if the motor itself is 'dual voltage', because you might be able to ditch the transformer entirely, simply needing a new higher current starter.

-Jon
Click to expand...
The motor is dual voltage, originally I wanted to do just that and swap out the starter. The issue was going to the bigger starter, it wouldn't physically fit inside the cabinet that was supplied. The cost for a new cabinet and starter was too much for management to justify, when we already had the transformer sitting there not being used.
 
Status
Not open for further replies.
Top