5KVA Transformer nuisance tripping of fuses

[manuelgoodman]

One of my machine has a single phase 5KVA transformer 575/120 . The primary side use 20A time delay fuses. The secondary has 15A fuse and the line is terminated on a 15A rated receptacle. My issue is every time i powered up the machine the 20A fuses on the primary randomly trips, even there is no load connected on the secondary side of the transformer (receptacle).

I followed the manufacturer fuse/CB calculation which is (Load x 250% = Fuse rating)

Any advice on how i can fix the nuisance tripping of fuses on the primary line of the transformer?

 

[jim dungar]

Can you start the transformer loaded? This helps sometimes.

You are experiencing a problem with residual magnetism in the transformer core. There is nothing you can do except go to the maximum time delay fuse. Many people use fast 'time delay' fuses for current limiting, you need something really slow.

 

[MTW]

What type of fuse are you using, I would guess a class CC, a RK5 or a class J would be a better choice. Bigger physical dimensions provide more mass (silica sand) to help absorb and dissipate the heat. I would also bump up the size to a 30A. On the primary side.

 

[manuelgoodman]

What type of fuse are you using, I would guess a class CC, a RK5 or a class J would be a better choice. Bigger physical dimensions provide more mass (silica sand) to help absorb and dissipate the heat. I would also bump up the size to a 30A. On the primary side.

Yes , i am using class CC fuse

 

[winnie]

You are experiencing a problem with residual magnetism in the transformer core.

To add to this, the problem is that the magnetic flux associated with the AC cycle at the moment of 'switch on' doesn't match the residual magnetic flux in the core.
If you get unlucky, you get a bad combination which triggers extra inrush current and leads to the fuse blowing.

It is worth double checking the transformer primary taps. If your nominal 575V is a touch high, that will make inrush problems worse.

There is nothing you can do except go to the maximum time delay fuse. Many people use fast 'time delay' fuses for current limiting, you need something really slow.

For large transformers there are relays/ contactors which match the point of the cycle to turn the transformer on, and minimize inrush current.
There are also 'precharge' circuits which could be considered.
But for such a small transformer, I'd go with @jim dungar and use the largest 'slow blow' fuse allowed.

 

[petersonra]

Yes , i am using class CC fuse

There are fuses made for transformers that have a bit of a different curve.

FNQ-R-20 is a class cc fuse that might just fit the bill and they will fit your existing fuse holders.