Can Unbalanced load on transformer cause voltage drop?

[.]

I have a 75KVA 600V delta to 120/208V wye dry type aluminum
transformer with a fairly high impedence rating of 7.5%.

There is a load of approx 90A across phase A and B and considerably
smaller loads on across BC and CA. The 90 amp load is one piece of
equipment.

I have 600V on H1 H2 H3 but only 200V across the AB phases.
The starpoint is firmly bonded to the ground, and all connections look
good.

Is there a tendency for imbalanced loads to further increase voltage
drop in transformers? I would suspect that I would have closer to 208V
on the secondary if the load was 90A on all three phases.

doug G

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Chrisd

 

[.]

Thanks for the reply.

How is that calculated?
I'm guessing that you assume that at full load (208A) you get 7.5%
drop and at 90 amps (43% load) you get 43% of the drop expected
with full load?

Crappy transformer. Must be the aluminum windings. Its already on the
585V tap, which would give me 213V no load. I have one more tap at
570V that I can use to give me 219V or 123V to N. Thats the way I is
going to have to go I guess.
Looks like this machine will be getting its own transformer.


doug

I calculated the voltage drop at 7.5% impedance on 75 kVA to be about
3.9 volts on line to ground, and about 6.7 volts on line to line, when
the current is at 90 amps. That's assuming zero resistance/reactance
everywhere else, which would certainly not be the case. Other things
could add a little more voltage drop and bring it down to 200 volts.

You might check to see if your transformer has adjustment taps on the
primary. Most transformers at that high kVA rating do have some taps.
Connect the 600 volts at a -2.5% tap. That should give you a small
boost on the 208Y/120 side, to about 213Y/123 in theory. Then your
voltage drop due to load will be less.

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Chrisd

 

[.]

I am trying to get 208V from a 208V transformer. I was under the
assumption that a 75KVA 600 - 120/208V transformer would deliver 75Kva
at 208V. With 600.00V input they are delivering more like 40 - 50 Kva
at 200V.

I have 2 of the same transformer that are doing this with similar
loads. I took the cover off them and took some current readings. One
has approx 150A on the highest leg and the other transformer has 180A.
Both transformers are running unbalanced because there is a large
single phase load on each 3 phase panel. There is a lot of heat being
generated too.

The loads are basically ballasts for high output UV lamps for curing
inks on screen printing press's, with a few small motors for fans
and conveyors etc. I do not know if these ballasts are electronic or
not. I intend to find out tomorrow.

Here is a link to the technical specifications for these transformers.
I think the answer can be found in the % regulation section.

http://www.bemag.ca/design/english/3PhaseAlStd/BA3075V.pdf

doug

| Thanks for the reply.
|
| How is that calculated?
| I'm guessing that you assume that at full load (208A) you get 7.5%
| drop and at 90 amps (43% load) you get 43% of the drop expected
| with full load?

The full load current is 7.5% of the fault current. You can figure
the impedance in ohms given the current and voltage.


| Crappy transformer. Must be the aluminum windings. Its already on the
| 585V tap, which would give me 213V no load. I have one more tap at
| 570V that I can use to give me 219V or 123V to N. Thats the way I is
| going to have to go I guess.

It's probably designed to be a hot transformer. The high impedance
suggests this possibility.

You're starting to get above nominal voltages. Are you trying to get
up to 240 volts? What is the load on this.

To send me e-mail remove the sevens
from my address.

Chrisd

 

[Dave I]

I am trying to get 208V from a 208V transformer. I was under the
assumption that a 75KVA 600 - 120/208V transformer would deliver 75Kva
at 208V. With 600.00V input they are delivering more like 40 - 50 Kva
at 200V.

I have 2 of the same transformer that are doing this with similar
loads. I took the cover off them and took some current readings. One
has approx 150A on the highest leg and the other transformer has 180A.
Both transformers are running unbalanced because there is a large
single phase load on each 3 phase panel. There is a lot of heat being
generated too.

The loads are basically ballasts for high output UV lamps for curing
inks on screen printing press's, with a few small motors for fans
and conveyors etc. I do not know if these ballasts are electronic or
not. I intend to find out tomorrow.

Here is a link to the technical specifications for these transformers.
I think the answer can be found in the % regulation section.

http://www.bemag.ca/design/english/3PhaseAlStd/BA3075V.pdf

doug










To send me e-mail remove the sevens
from my address.

Chrisd

One thought - discharge lamps can give problems due to 3rd harmonics adding
on the Neutral
to give very large neutral currents (this is a disadvantage of ballasts on
a 3 phase supply).
It may be that you are experiencing excessive volt drop due to this.
Put a good ammeter on your Neutral and see what is going on there.
I would expect that the Neutral connections are rated at full phase current
rating,
but in some cases it is better to use an oversize neutral.
Does the transformer manufacturer have anything to say about harmonic loads?

I hope this helps,
Dave