Primary fuses for control transformer 1

[stuuk78]

I have a 1500VA control transformer fitted with 5 amp primary fuses. The incoming voltage is 490V and the control voltage is 120V. The primary fuses will pop when the mains power is turned on, even when no motors nor controls are operational. If i fit 20 amp fuses, these will not pop, so i know i don't have a short circuit. According to the NEC i beleive i can go up to 250% i.e. 7 Amp fuse. Will this help or i am i missing something else.

 

[jbartos]

Comment: If 5A fuse blows when 1500VA 490V single phase transformer is at no load on the secondary side, then there is something wrong with the transformer. It should be properly tested. It may have inter-turn shorts or short over the transformer iron eliminating some turns. Normally, no load transformer current should be negligible, e.g. a small fraction of an amp in your case.

 

[NEWFIEBOY]

Hi stuuk78

What type of fuses are you using? When a transformer initally powers up there is an inrush current which could be blowing your fuse, if they are of the fast acting type. You should try some time delay or slow blow fuses.

 

[jbartos]

Comment: NFPA 70-1996 NEC Article 450-3(b)(1) states that the transformer primary protection should not be more than 125% of the rated primary current of the transformer. There is not mentioned fast or time delay dual element fuse for the protection. In fact, the transformer primary can be protected even by a lower rated fuse than 125% of rated transformer primary current, if the transformer is not fully loaded.

 

[stuuk78]

I am using time delay fuses, and i forgot to mention that i have secondary protection on the transformer also which should allow me to go to 250% on the primary side. But it seems like i shouldn't have to, unless the transformer is faulty.

 

[rbulsara]

A faulty transformer will pop even a 20A fuse. Control transformers can have inrush currents up to 25 times.

NEC is not against good engineering. As long as you have secondary OCPD sized to prevent the overload, the primary protection can be greater than 125% as long as you protect the conductors and vs short circuit. 20A fuse with #12 wires will be perfectly acceptable.

I do not have Code in front of me, I will check on Monday.

 

[rbulsara]

125% requirement is when there is no secondary protection.

 

[busbar]

NEC Table 450.3(B) permits 7-amp hiside fuses where total secondary fusing is 15.6A or less.

As suggested by jbartos, it may be worth checking no-load current on the transfomer to verify that it is otherwise working OK. The Article 450 multipliers have evolved over years of application. The 250% allowance would not be there if code submitters hadn’t learned through experience.

 

[stuuk78]

Yes the primary fuses are blowing while there is no load on the secondary side. So i am understanding this correct that it sounds like a faulty transformer? or is it down to the fuse sizing? Thanks for everybodys help so far.

 

[advidana]

you say you are electical -did you check the current with an ampmeter when you used the 20 amp fuse? did you get any current , how much and for how long.

 

[PUMPDESIGNER]

I know much less on electrical than most of you guys, but I do design logical controls for pumps and have to work with transformers frequently, under UL-508A and NEC.
We always use the transformer manufacturer's formulas and tables. All transformers are not the same, some are coupled more closely primary to secondary, they have different inrush values depending on the type of transformer used (more or less isolation, de-coupling, etc.).

Either way, transformers are heavy inductive loads, require time delay fuses (we prefer circuit breakers for this reason of inductive load).



PUMPDESIGNER

 

[Danfossegypt]

hello stuuk78

i hope the problem is that the transformer was the first use,so the core wasn't magentised so it draw a large current,try again the 5 amp fuses, i think it will work well

 

[stuuk78]

Yes i have checked the amps while there is no load on the secondary side and it is reading approx 1.3 amps steady, it is hard to see the inrush as it is so fast and my ammeter takes some time to display the results. It is as if the transformer is de-magentising once the mains power is switched off. The fuses did not blow on one ocasion but then the next time i turned it on they blow again. I have read on some transformer manufactures web pages and they reccomend a 15 amp fuse for primary protection while using secondary protection also.

 

[busbar]

If possible, compare to no-load current of a similar transformer. 1.3 amperes versus 3.13 amperes calculated full load for a 1500VA 480V control-power transformer sounds like indication of a serious problem.

 

[jbartos]

Comment: It is not unusual to have transformer winding partially shorted. This depends on the quality of transformer manufacturing of transformer windings. Some transformers are manufactured so-so.

 

[jraef]

I second the sentiments of PUMPDESIGNER. No 2 transformers are alike. You sometimes need to experiment with finding the lowest size that meets code but will not blow on inrush of THAT particular transformer. Most xfmr mfrgs have a chart showing what they know works. What is your xfmr?

"Venditori de oleum-vipera non vigere excordis populi"

 

[Sanditech]

In the UK (using BS88 HRC fuses) I have always used the rule of a minimum fuse of 4 x the rated primary current. This has always handled the inrush current on countless control transformers. So for your transformer 4x(1500/490)= 12Amp.

 

[Marke]

Hello stuuk78

Is this a new installation, or is this a problem that has developed after a period of time?

I assume that this is new installation.

1. Transformers draw a very high inrush current at switch on. This inrush current is independent of the secondary circuit. The magnitude of the inrush current is dependent on a number of factors including the amount of flux in the iron. If the transformer is over fluxed, i.e. is wound for 460 volt and used on 490 volt, there will be an appreciable increase in the inrush current. The inrush current of transformers only lasts cycles, but can be in the order of 10 - 20 times the full load primary current of the transformer. If your transformer has been very conservatively rated, and is actually say a 2.5KVA capable transformer, then the inrush will be higher again.

2. Fuses fail when the energy through them is too high. You cna over load a fuse and it fails earlier the failure time tends to reduce with the square of the current, hence we talk about the i squared t of the fuse. (more common terminology with semiconductor fuses) A slow fuse will last longer with the overload, but will still fail. Sometimes, we just have to use a fuse that has an i2t greater than the switch on energy of the transformer in order to prevent failure.

3. If the transformer has a shorted turn or similar insulation fault, it will have a high primary current, will tend to be noisy and the open circuit secondary voltage will be low. It would also heat up reasonably quickly. Temporarily fit oversized fuses and run the transformer unloaded for a period of time. If the transformer primary current is reasonable, the transfomer does not make excessive noise or smells, and does not heat up excessively, then I would suggest that you just have a healthy inrush current.

4. In my part of the world, we do not operate under NEC, and I would fuse for the cable rather than the transformer. I would protect the output of the transformer, and use thermal protection in the transformer. (Usually a thermal fuse built into the primary circuit). I agree with Sanditech, about four times the rated primary current is about right in most cases.

Good luck,
Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jbartos]

Suggestion: Visit

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for:
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