Old Pad mount transformer rated in kW

[maxPE]

I have some nuisance tripping of distribution line fuse links feeding a pad mount transformer. I suspect the fuse is not appropriate for the transformer. The transformer is rated at 45 kW, primary voltage is 4800, secondary is 480. The load is a 40hp motor with a soft start/stop so the inrush is zip. The blown link was a 7 amp K fuse. If I assume the transformer P.F is .8, the transformer would be rated at 56kVA? Is that correct? IF that is true, the line fuse link should be about 10 amp and I am thinking a slower fuse link would be better like a type X or?

Thanks for any advice.

 

[dpc]

The transformer isn't a load so doesn't have a power factor, other than the small exciting current. kW and kVA are effectively equivalent in a transformer. I have never seen a transformer nameplate with a kW rating. Where is this located - that's an odd primary voltage. Does the nameplate voltage agree with the actual voltage on the primary?

 

[crshears]

That type of bug could be used on a single-phase tap off an 8 kV feeder; what's got me scratching my head is the secondary voltage, as I've seen 277/480 but not 480 phase to ground . . .

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[maxPE]

Well I guess I should have opened the door. I was sure the rating was in kW-oops. Here is the data plate behind the green door. I still am questioning the 7 amp K fuse on the dist line that blows every few years. Costly to bring out a lineman. Hence my wish to change the fuse links to 10 amp slow blow. The power line is my private line and Edison is feeding it with three 10kVA transformers where I have three 45 kVA pad mounts running on my end. Two close to full load at the same time. Seemed a bit cheap on the Edison side.

 

[Palletjack]

I would put the 10 in there but use a fast fuse. K is fast enough if it coordinates.
That would allow the XF to go to 185% for a while.

 

[maxPE]

Thanks palletjack. I will take your advice. Fortunately I have motor protector that detects the voltage drop due to a locked rotor and shuts the soft start off. Save my bacon several times.

 

[LionelHutz]

40HP is around 40kVA, and the motor would reach 80-160kVA when starting on a soft-starter, so that's not a trivial load on a 45kVA transformer.

 

[bacon4life]

I might not be picturing this correctly. The utility has three 10 kVA transformers and you have three 45 kVA transformers downstream. I know utilities undersize transformers, but this seems very extreme. Replacing your fuse with a 10 amp fuse might mean that the utility fuse/transformer fails instead of your fuse next time.

 

[Estimated Profit]

I'm not familiar with a 7A "K". What I see are increments of 1, 2, 3, 6, 8A and so on. An 8A "K" should technically work unless you have a motor that is really, really eating up inrush current in the 290kVA range after 60 cycles. The fuse that transformer SHOULD be fused at is a 7A KS. I suspect that's what you have. That's the perfect fuse as it's as high as it can be before you start getting into the damage curve of the transformer. Make sure you're using 7A KS fuses and if it's blowing, you have another issue.

This is assuming you have a 45kVA 3ph Tx with a 4800V delta primary (or 4.8kV WYE). I don't see an issue with a 40HP motor. The transformer curve for a 45kVA is 650A @ 0.1 sec, 327A @ 1 sec, 163A @ 10 sec. That means your 480V motor could pull 163A (135kVA) 10 seconds into startup and your transformer and 7A KS would be fine.

With regards to the upstream 10kVA's - I assume you mean that the utility steps down their distribution voltage to 4800V delta with a closed delta bank of 10's, equaling 30kVA. Are their fuses blowing?

 

[cranky108]

Sounds like they uprated the service voltage some time ago, or this industrial had more than one transformer. Or something like cheaper service for primary service, and the customer chose 4800.
Nonetheless, I have seen this where my old company uprated the service voltage.

Common old distribution voltages of 2400 and 4800V. Now I normally see these in power plants, but those sizes are becoming too small for larger plants. But they are good industrial voltages.

I have also seen where a transformer fails, and they replaced it with a larger transformer, because that is what they have.
I see transformer up rating all the time, but more recently it is because of POT grows using residential homes.

 

[Estimated Profit]

Cranky - yeah, I'm sure this building is old and had a 4800V delta svc from the utility. The utility probably upgraded to a 12-25kV WYE distribution system and just stuck a small stepdown where there used to be a primary meter. It sounds like this plant owns some 4800V distribution they pay someone privately to maintain bc the 7A fuse is an Overhead fuse, feeding a dip. Understanding the service details would help me to make sure I'm not missing an important detail.