Intermittent Transformer Trips on the Primary

[fifty60]

I have a 1KVA control transformer that has 240V primary and 120V secondary. My primary fusing is 10A class CC fuse. I can cycle power to the machine 4 or 5 times in a row without any issue, but on the 6 or 7th power cycle I blow the 10A fuse on the primary. 10A is the maximum size fuse I can go, and I find it hard to believe that a CC fuse sized at just under 250% cannot hold.

On the secondary of the transformer I have a GFCI receptacle and a 60W 24VDC power supply. The 1KVA is overkill for this, but it is standard. I know that the inrush on the transformer is going to be worse depending on where the 60Hz cycle is at when power is applied, and I am also kind of suspect of the GFCI's unique relationship with ground that most loads do not have.

The wiring all looks correct on this transformer, any ideas from what I described above could be causing me intermittent trips? Could spikes on the 240V power supply to the primary cause this?

 

[broadgage]

I strongly suspect inrush current is blowing the fuse, transformers can have surprisingly large inrush currents. I doubt that much is wrong with the transformer or the wiring thereto as it works correctly most of the time.
An intermittent fault in the transformer is a remote possibility, but unlikely in practice.

Here under UK conditions, I have been known to simply double the size of the fuse and ALSO to use a thermal overload relay to protect against modest but long continued overloads.
Would a 20 amp fuse and a 6 amp thermal overload relay be acceptable ?

 

[Jraef]

A) There is no “special relationship to ground” in a GFCI. The GFCI has nothing to do with this.

B) Are you using a Time Delay fuse? If not, ie a KTK fuse, you should be. KTK fuses, although still Class CC, are not Time Delay and even though people regularly refer to them as being good for transformer primaries, they are prone to blowing on control transformers, which have different characteristics compared to distribution transformers. Use an equivalent that is Time Delay.

If you are using a T-D fuse something like an FNM or FNQ (Bussman designators, any equivalent is the same) and it is still blowing, you can get a slightly longer time delay by using a Branch rated fuse like an FNM or LPN. They will not fit in the fuse clips that often come mounted on the transformer, you will need a separate fuse block, but that has worked for me in a situation like this. It was recommended to me by Bussman in a similar situation and it worked.

 

[steve66]

No fusing on the secondary? That would help to narrow down the problem.

But what is the power supply like? A standard rectifier input to a filter cap and a regulator? If so, a quality power supply would include a fractional ohm resistor in series with the caps to limit the inrush current, but cheap ones might omit this resistor.

My guess is that the caps are drawing a large surge if you turn on the power at just the right time during a cycle. A smaller transformer would probably limit the current more and might eliminate the problem.

If that's the case, a few extra feet or turns of wire on the secondary might even be enough to help.