Oversized 3ph transformer - will it pop fuses on power-up?

Status
Not open for further replies.
D

DougPLC

Member
Location
Connecticut
I have access to a 60 kva isolation transformer for 480v to 380v. My customer only has 30A at 480v. Can we use this transformer for loads that don't exceed the 30A at 480 / approx 38A at 380v?
Is there a way I can reduce inrush current? Maybe wire a small 3ph 380v motor (with proper OL and OC protection)to the secondary to be energized only at startup? I have heard that a loaded secondary actually causes a lower inrush.
 
DougPLC said:
I have access to a 60 kva isolation transformer for 480v to 380v. My customer only has 30A at 480v. Can we use this transformer for loads that don't exceed the 30A at 480 / approx 38A at 380v?
Is there a way I can reduce inrush current? Maybe wire a small 3ph 380v motor (with proper OL and OC protection)to the secondary to be energized only at startup? I have heard that a loaded secondary actually causes a lower inrush.
Click to expand...

If your transformer were any larger I'd say the risk of blowing fuses upon energizing are pretty good.

This one is just small enough, might depend on other installation factors, if there is enough impedance in the supply and trip curve for the fuses is high enough it may just work. May be border line enough that sometimes it trips and other times it doesn't.

The fact that your source (I don't know if you only have a 30 amp source, or just a 30 amp circuit from a larger source) is limited already probably does help some as well.
 
DougPLC said:
I have access to a 60 kva isolation transformer for 480v to 380v. My customer only has 30A at 480v. Can we use this transformer for loads that don't exceed the 30A at 480 / approx 38A at 380v?
Is there a way I can reduce inrush current? Maybe wire a small 3ph 380v motor (with proper OL and OC protection)to the secondary to be energized only at startup? I have heard that a loaded secondary actually causes a lower inrush.
Click to expand...

use fnq fuses on the primary side.
 
190726-0817 EDT

DougPLC:
I have heard that a loaded secondary actually causes a lower inrush.
Click to expand...
On average this is probably a correct statement.

In an AC circuit turn off of a circuit generally occurs around a current zero crossing. Even with a mechanical switch, and a random turn off time of the switch, the current in the external circuit, as seen at the switch, will continue to flow to a time of something near zero current thru the switch. This results because of inductance in the circuit.

In an unloaded transformer the circuit inductance looks large compared to its resistance. Thus, current lags applied voltage by almost 90 deg.

In a transformer heavily loaded with resistance the input current does not lag applied voltage by much.

Residual flux in a transformer is very much related to applied voltage at the time of turn off. With no load at turn off the residual flux is high, and heavily loaded it is much lower.

At turn on of a transformer, if the residual flux is high and the phasing of the applied voltage is such as to further increase this flux, then the transformer is driven into major saturation and high inrush current occurs.

If residual flux is lower or of opposite polarity at turn on, then less severe saturation occurs, and less inrush current occurs.

See transformer inrush current plots for two different residual flux levels for the same transformer in my plots P6 and P7 at http://beta-a2.com/EE-photos.html .

.
 
petersonra said:
Actually in your case that would not work since they only go up to 30 A.

I don't recall if anyone makes a fuse with higher inrush capability in ratings over 30 A for transformer protection.
Click to expand...
Tried Ferraz?
 
What is your reason for wanting 380V? Is it for foreign equipment? If so, you may not WANT a transformer if it is motor driven, because you will end up with 380V 60Hz, not 380V 50Hz.
 
DougPLC said:
I have access to a 60 kva isolation transformer for 480v to 380v. My customer only has 30A at 480v. Can we use this transformer for loads that don't exceed the 30A at 480 / approx 38A at 380v?
Is there a way I can reduce inrush current? Maybe wire a small 3ph 380v motor (with proper OL and OC protection)to the secondary to be energized only at startup? I have heard that a loaded secondary actually causes a lower inrush.
Click to expand...

Doug -
I didn't see any reference in your OP to the feeder being fuses, so I'll assume the feeder is a 30A molded case, thermal magnetic circuit breaker.

Is this a permanent install, or temporary?
If temporary - go for it. If it trips, try 4, 5, 6, 10 times. If/when it holds, do your work.

If permanent, I would say, No.
Inrush on a 60 kva xfm ~ FLA x 10 = 720A, well into the instantaneous range of a 30A CB.
GE says the max inrush will occur 1 time in six energizations.

I have seen transformers ooze on-line, but not often.

As for a secondary load reducing the inrush, I have also heard that. I have never seen any ieee papers or any other test data supporting this. Gar's example is interesting, however I don't know how extensive the test data is.

As for a transformer with a small feeder, oozing on-line, I have seen a few examples. all were temporary power and In each case, I fully expected a trip. They didn't and I didn't try a second time.

And, no, 30A time delay fuses are not significantly better at transformer energization.

About the best you can do with a 30A, 480V, circuit, will be about 20KVA. Nope never seen a 20KVA, 3ph xfm. Then again, I have never heard of a 60KVA, 3 ph xfm. Normal sizes are:
15 kva
25
37.5
50
75
100 kva​

If the goal is to reliably energize a 60kva transformer, normal inrush characteristics:
Feed with an FLA x 1.25 CB >> 72 x 1.24 = 90.2A, use a 100A CB, #2 CU.

Good luck. Let us know haw it goes

the worm
 
iceworm said:
About the best you can do with a 30A, 480V, circuit, will be about 20KVA. Nope never seen a 20KVA, 3ph xfm. Then again, I have never heard of a 60KVA, 3 ph xfm. Normal sizes are:
15 kva
25
37.5
50
75
100 kva​
Click to expand...
how many of those normal sized units have 380 volts rating on either side?
 
kwired said:
how many of those normal sized units have 380 volts rating on either side?
Click to expand...
Disclaimer: Transformer design is not in my area of expertise. Wizards are requested to jump in and educate.

Don't know

Just looking at the physics: (yes, I know this is extremely simplistic)
Transformer capacity is proportional to the cross sectional area of the core. Winding turns are set by the allowable volts-per-turn. So until the insulation gets in the way, the secondary voltage can be about what ever the designer wants.

So the point was (and is):
With a 480V primary it is not strictly European.
With a 380V secondary, it is not strictly North American.
And 60kva is not a standard size
So, since it is an oddball - be wary of inrush

And I re-read the OP. And he does say "fuses" in the subject line.
Whoops - missed that
 
Status
Not open for further replies.
Top