Transformer blows primary fuses with no load.

[RogerMM]

We have a 5000 kva transformer wired 480 volt on the primary and 230 volt on the secondary. This is a single phase transformer. When you power it up three to four times it blows the both primary fuses. What do you think is the problem.

Thanks,
RogerMM

 

[Skogsgurra]

Inrush current. No doubt.

Can you change primary taps so you get less V/turns? That usually means changing to a lower secondary voltage and improves the situation a lot.

Did you say 5000 kVA? Single phase?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[wbd]

What size and type of fuses are installed? My hunch is that the fuse size is very close to the inrush of the transformer and maybe starting to melt (ie weaken) the fuse and then it finally blows.

 

[Hoxton]

Could the core be magnetised? I seem to remember that vacuum contactors gave this problem at one time, since they could "chop" the current, rather than interupting at current zero. This could magnetise the core permanently, gicing rise to high inrush currents, sometimes making it difficult to reclose breakers. This system is fused, but it could have been caused by an upstream problem. Have there been any power supply outages?

 

[dpc]

5000 kVA 480 V to 230 V single-phase??? This I would like to see. Maybe no one makes large enough fuses

Transformer inrush seems the likely culprit, I agree with the others.

Inrush current can vary significantly depending on instantaneous value of the voltage (voltage phase angle) when the transformer is energized. So often, this type of fuse blowing occurs maybe one time out of 5 or one time out of 10.

If you can confirm the transformer size, maybe we can make some suggestions on the fuse size.

 

[RogerMM]

Your right! The transformer is 5000 VA or 5 KVA.

 

[dpc]

So how big is the fuse that is blowing?

 

[RogerMM]

The fuses we are using is rated slow blow 20 amp. We have taken all the load off the secondary. Powering the transformer two to five times blows the fuses.

 

[Skogsgurra]

Thanks - I thought so.

Are you trying to protect anything thermally with the fuses? Don't. Use fuses only as a short circuit protection. And use slow blow fuses.

If you are using fuses like 16 or 20 A, then they shall blow occasionally. Use something like 35 A.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[RogerMM]

The fuses are Amp Trap ATDR20, 600V, AC

THANKS,
ROGERMM

 

[rbulsara]

Also cycling the transformer in quick succession, not allowing fuse to cool off will add to the woes.

 

[Skogsgurra]

Hey! We are cross-posting. Use 35 A. Or are your wires too small for that?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[rbulsara]

Also for what it is worth, the load on transformer generally do not have the bearing on the inrush current.

 

[Skogsgurra]

Actually, a loaded transformer has less inrush. The reason? Simply because there is some voltage drop in the primary so that the core doesn't get saturated as much as with no load. It may be only a few percent's worth of difference. But that may make the difference between saturated or not.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[RogerMM]

The wire size on the primary is #8 and the secondary is #10.

 

[Skogsgurra]

Sorry, I don't do AWG - only sqmm. Anyone else?

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[rbulsara]

#8 is good for 40A max.

#10 is good for 30A max.

 

[rbulsara]

gunnar:

Thanks for your thoughts on loaded transformer.

 

[dpc]

As skogsgurra has mentioned, try a larger fuse. But you'll have to comply with NEC requirements for transformer protection, if you are in the US.

 

[LionelHutz]

NEC says primary fuses can be 2.5X rated current when using secondary fuses and you can round up. So, you could use a 30A fuse if you're in the USA.